A method for preparing chromium-doped anhydrous manganese spinel single crystal under high temperature and high pressure

Abstract

The invention discloses a method for preparing chromium-doped anhydrous manganese spinel single crystals under high temperature and high pressure. The method comprises: using solid rose-colored triangular crystal system rhombohedral manganese carbonate crystals, solid aluminum isopropoxide powder, solid oxalic acid powder, solid chromium (III) acetylacetonate crystalline powder and liquid dilute nitric acid as starting raw materials, preparing a manganese spinel powder sample according to the stoichiometric ratio of manganese spinel; cold-pressing the manganese spinel powder sample into discs, stacking the discs and placing them in a graphite crucible; placing the graphite crucible in a high-temperature oxygen atmosphere furnace for high-temperature calcination to prepare a cylindrical manganese spinel sample; and subjecting the cylindrical manganese spinel sample to a high-temperature and high-pressure reaction to obtain a chromium-doped anhydrous manganese spinel single crystal. The method solves the technical gap in the preparation of large-particle chromium-doped anhydrous manganese spinel single crystals under high temperature and high pressure conditions, thereby obtaining an experimental sample of large-particle chromium-doped anhydrous manganese spinel single crystals.

Description

Technical Field

[0001] The invention belongs to the technical field of mineral single crystal sample synthesis, and in particular relates to a method for preparing chromium-doped anhydrous manganese spinel single crystal under high temperature and high pressure. Background Art

[0002] As an oxide mineral spinel with the general chemical formula AB2X4, the spinel structure can be divided into three main types according to the differences in the specific lattice arrangements of the A-site cations and the B-site cations in the unit cell: (1) positive spinel type: its general chemical formula can be expressed as A[B2]X4, that is, the tetrahedral lattice coordination occupying the unit cell in the spinel mineral is composed of 8 A-sites with positive divalence, and the octahedral lattice coordination occupying the unit cell is composed of 16 B-sites with positive trivalence; (2) inverse spinel type: its general chemical formula can be expressed as B[AB]X4, that is, the tetrahedral vacancy occupying the unit cell in the spinel mineral is composed of 8 B-site cations, and the octahedral lattice coordination occupying the unit cell is composed of 8 A-site cations and 8 B-site cations; (3) mixed spinel type: its general chemical formula can be expressed as A[B2]X4, that is, the tetrahedral vacancy occupying the unit cell in the spinel mineral is composed of 8 B-site cations, and the octahedral lattice coordination occupying the unit cell is composed of 8 A-site cations and 8 B-site cations; (1-γ) B γ [A γ B (2-γ) ]X4, that is, the tetrahedral lattice and octahedral lattice coordination occupying the unit cell in the spinel mineral is a mixture of positive spinel type and inverse spinel type in different proportions.

[0003] Manganese spinel, with the chemical formula MnAl2O4, is an important manganese- and aluminum-rich oxide mineral and a key end-member in the spinel family of minerals with an equiaxed crystal system. The mineralogy of manganese spinel, expressed as oxide percentages, can be expressed as: MnO / (MnO+Al2O3) = 41.0% and Al2O3 / (MnO+Al2O3) = 59.0%. Manganese spinel is a typical mixed-type spinel mineral found in nature, with inverse spinel accounting for ~0.29% of the corresponding unit cell. In nature, manganese spinel is generally small, less than 1.0 mm in size, and often appears as rounded particles, octahedral, or vesicular exsolutions. The spinel twinning pattern in equiaxed hexahedral crystals along the {111} direction is observed. Naturally occurring manganese spinel, ranging from opaque to thinly translucent, appears light red, red, or black under single polarized light, and golden yellow, brownish orange, mahogany red, deep red, and reddish black under crossed polarized or transmitted light. Geological data from confirmed field sites confirms that manganese spinel from vein-forming manganese ore deposits at Bald Knob in North America coexists with a series of manganese-containing silicate minerals, including olivine, rhodonite, and chondroitinite. Manganese spinel from manganese deposits along fault-replaced veins in flint layers in East Asia coexists with manganese-rich oxide and silicate minerals, such as hausmannite and chondroitinite.

[0004] In the manganese spinel crystal structure, the transition metal element chromium easily occupies the octahedral position, thereby forming an isomorphous substitution of the trivalent cation at the B position. Since the valence of the replaced aluminum element and the doped chromium element in manganese spinel are both positive trivalent, this isomorphous substitution is an equivalent substitution. Chromium (Cr), located in the 4th period and group VIB of the periodic table, has an atomic number of 24 and its outermost electron configuration is 3d 5 4s 1 . Chromium is the hardest metallic element discovered in nature so far. The common valence states of its compounds are +6, +5, +4, +3 (mainly), +2, +1, 0, -1 and -2. In different geological regional backgrounds and field geological structural units, the element content and mineral resource distribution of transition metal chromium vary significantly. Its average abundance in the earth's crust is about 0.01%, ranking 17th. In nature, chromium is mainly enriched in chromite, while free metallic chromium is relatively rare. In addition, the average content of chromium in the surface seawater of the solar system and the Pacific Ocean is: ~20ppm and ~0.15ppb, respectively.

[0005] As a typical nominally anhydrous mineral, manganese spinel contains no water or hydroxyl groups in its molecular structure. It is a high-pressure oxide mineral ubiquitous in the Earth's lower crust and upper mantle. Existing laboratory high-temperature and high-pressure experimental simulations and theoretical mineral physics simulations indicate that the widespread anomalies in electrical properties and elastic wave propagation velocities in the mantle transition zone, at depths ranging from 410 km to 660 km, corresponding to pressures and temperatures of 16.0-23.0 GPa and 1450-1800°C, are caused by a phase transition between spinel and post-spinel. The main methods used to synthesize manganese spinel in laboratories in materials science, both domestically and internationally, include metal alkoxide sol-gel, microemulsion, high-pressure hydrothermal synthesis, carbonate chemical coprecipitation, and high-temperature solid-state sintering. Most of these existing synthesis techniques rely on simple solution chemical reactions or direct physical grinding of sample powders followed by high-temperature sintering, making them more suitable for producing nanosized manganese spinel crystals. Since single crystal mineral experimental samples with micron-sized or larger particles are usually required in the field of high-temperature and high-pressure experimental geoscience research, it is obvious that the nano-scale manganese spinel samples obtained by material synthesis in the past have failed to meet the minimum particle size requirements of the samples, and there is no effective synthesis method to date. In the past, more geoscience researchers also usually used natural manganese spinel samples instead of artificially synthesized samples to meet the needs of high-temperature and high-pressure experimental geoscience research. However, these natural samples have the obvious disadvantage of uneven distribution of trace element chromium. Therefore, it is particularly urgent to effectively synthesize a large-particle chromium-doped anhydrous manganese spinel single crystal that meets the needs of various high-temperature and high-pressure laboratory simulations of geoscience research, especially the study of the preferred lattice orientation and crystal axis anisotropy of manganese spinel single crystal minerals under high pressure. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for preparing chromium-doped anhydrous manganese spinel single crystals under high temperature and high pressure, so as to solve the technical gap in the preparation of large-grained chromium-doped anhydrous manganese spinel single crystals under high temperature and high pressure conditions, and to obtain experimental samples of large-grained chromium-doped anhydrous manganese spinel single crystals.

[0007] The technical solution of the present invention is:

[0008] A method for preparing chromium-doped anhydrous manganese spinel single crystals under high temperature and high pressure comprises: using solid rose-colored trigonal rhombohedral manganese carbonate crystals, solid aluminum isopropoxide powder, solid oxalic acid powder, solid chromium (III) acetylacetonate crystalline powder, and liquid dilute nitric acid as starting raw materials, preparing a manganese spinel powder sample according to the stoichiometric ratio of manganese spinel; cold-pressing the manganese spinel powder sample into discs, stacking the discs, and placing the discs in a graphite crucible; placing the graphite crucible in a high-temperature oxygen atmosphere furnace for high-temperature calcination to prepare a cylindrical manganese spinel sample; and subjecting the cylindrical manganese spinel sample to a high-temperature and high-pressure reaction to obtain a chromium-doped anhydrous manganese spinel single crystal.

[0009] The purity of solid rose-colored trigonal rhombic manganese carbonate crystals is >99.99%, the purity of solid aluminum isopropoxide powder is >99.99%, the purity of solid oxalic acid powder is >99.99%, the purity of solid chromium (III) acetylacetonate crystalline powder is >99.99%, and the concentration of liquid dilute nitric acid is 10%.

[0010] The method for preparing a manganese spinel powder sample using solid rose-colored trigonal rhombohedral manganese carbonate crystals, solid aluminum isopropoxide powder, solid oxalic acid powder, solid chromium (III) acetylacetonate crystalline powder, and liquid dilute nitric acid as starting materials and according to the stoichiometric ratio of manganese spinel comprises:

[0011] Step 1. Weigh out 60 ml of 10% dilute nitric acid and place a glass pipette in a 500 ml notched beaker.

[0012] Step 2: Weigh out 5.0 g of rose-colored trigonal rhombohedral manganese carbonate crystals and add them to a notched beaker containing 10% dilute nitric acid solution, and place a magnetic stirring rotor in it;

[0013] Step 3: Cover the notched beaker with a glass watch glass, place it on a ventilated high-temperature magnetic stirring hot plate, and react at room temperature and 700 rpm for 72 hours;

[0014] Step 4: Weigh out 17.7677 g of solid aluminum isopropoxide powder and 180 mg of solid chromium (III) acetylacetonate crystalline powder, and add them to the dilute nitric acid solution containing manganese carbonate;

[0015] Step 5: Cover the notched beaker with a glass watch glass.

[0016] Step 6: Place the notched beaker on a ventilated high-temperature magnetic stirring hot plate and stir at room temperature and 800 rpm for 48 hours;

[0017] Step 7: Weigh out 2 grams of solid oxalic acid powder and add it to the notched beaker;

[0018] Step 8. Place the notched beaker of the mixture back on a ventilated high-temperature magnetic stirring hot plate, cover with a glass watch glass, and stir at 80°C and 1000 rpm for 36 hours.

[0019] Step 9: Remove the glass watch glass from the beaker and increase the temperature of the high-temperature magnetic stirring hot plate to 110°C until the mixed solution in the notched beaker is completely evaporated.

[0020] Step 10: Take out the magnetic stirring rotor in the notched beaker, take out all the sample powder and place it in a graphite crucible;

[0021] Step 11: Place the graphite crucible in a muffle furnace, increase the temperature to 1100° C. at a heating rate of 300° C. / hour, and keep the temperature constant for 5 hours;

[0022] Step 12: Cool the sample powder in the muffle furnace to room temperature at a cooling rate of 200°C / hour, and take out the sample powder;

[0023] Step 13: Grind the sample powder in a corundum mortar for 1 hour to obtain a fine-grained and homogenized manganese spinel powder sample.

[0024] The method of preparing a cylindrical manganese spinel sample by cold-pressing a manganese spinel powder sample into discs, stacking the discs, and placing the discs in a graphite crucible, and then placing the graphite crucible in a high-temperature oxygen atmosphere furnace for high-temperature calcination comprises:

[0025] Step 14: cold-press the manganese spinel powder sample into three sample discs of Φ10.0 mm × 3.0 mm using a tungsten carbide grinder on a stainless steel tablet press. Vertically stack the three cold-pressed sample mixtures and place them in a graphite crucible.

[0026] Step 15: Drill two symmetrical circular holes with a diameter of 1.0 mm symmetrically on the wall of the graphite crucible using an electric drill; pass a 0.5 mm platinum-rhodium alloy wire through the two symmetrical 1.0 mm circular holes in the graphite crucible wall, so that the graphite crucible is suspended in the center of the high-temperature oxygen atmosphere furnace; fix the ends of the platinum-rhodium wire connecting the graphite crucible to a vertical four-hole alumina tube with a diameter of 0.6 mm; and fix the upper end of the four-hole alumina tube to the center of the round lid that can be inserted into and pulled out of the furnace body;

[0027] Step 16: Place a container filled with secondary deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace;

[0028] Step 17: The top of the high-temperature oxygen atmosphere furnace body is connected to an argon inert gas cylinder and carbon monoxide and carbon dioxide cylinders with adjustable ratios;

[0029] Step 18: After opening the argon inert gas valve and continuously charging for 30 minutes, the sample is calcined at a high temperature to 800° C. at a heating rate of 400° C. / hour under the protection of argon inert gas;

[0030] Step 19: After the temperature inside the furnace reaches 800°C, switch the carbon monoxide cylinder and the carbon dioxide gas control valves so that the volume ratio of carbon monoxide to carbon dioxide in the oxygen atmosphere furnace reaches 4:1;

[0031] Step 20: After the mixed gas flow of carbon monoxide and carbon dioxide in a volume ratio of 4:1 to control the oxygen fugacity in the sample chamber reaches a stable state, the temperature of the sample chamber in the furnace is increased to 1450° C. at a heating rate of 200° C. / hour, and the mixture is calcined at a constant temperature for 15 minutes;

[0032] Step 21: After the sample is calcined at a constant temperature of 1450° C. for 15 minutes, the graphite crucible containing the sample, the four-hole alumina tube, and the round cover on the furnace body are pulled out of the furnace body, and directly immersed in secondary deionized pure cold water for quenching to obtain manganese spinel glass;

[0033] Step 22: taking the quenched manganese spinel glass out of the graphite crucible and grinding it in a corundum mortar; drying the glassy manganese spinel powder in a vacuum drying oven at 200° C. for 12 hours;

[0034] Step 23: cold-press the dried glassy manganese spinel powder using a tungsten carbide grinding tool on a cold isostatic press to form a cylindrical manganese spinel sample of Φ4.0 mm×4.0 mm.

[0035] The method for obtaining a chromium-doped anhydrous manganese spinel single crystal by subjecting a cylindrical manganese spinel sample to a high-temperature and high-pressure reaction comprises:

[0036] Step 24: seal the cylindrical manganese spinel sample in a graphite tube with a diameter of 4.0 mm (inner diameter) × 4.4 mm and a wall thickness of 0.2 mm, and encapsulate the upper and lower ends of the sample tube with graphite sheets with a diameter of 4.0 mm (diameter) × 0.2 mm (height);

[0037] Step 25: Place the graphite tube containing the sample on a typical 6-8 type multi-faceted large cavity high temperature and high pressure equipment in the laboratory (Kawai-1000t). Set the pressure and temperature increase rates to 0.5 GPa / hour and 10°C / minute, respectively. Raise the pressure and temperature to 3.0 GPa and 1050°C, respectively, and perform hot pressing sintering. The reaction time is 72 hours at constant temperature and pressure.

[0038] Step 26: After maintaining constant temperature and pressure at 3.0 GPa and 1050° C. for 72 hours, the temperature in the sample chamber is reduced from 1050° C. to 800° C. at a cooling rate of 3° C. / min and maintained at this temperature for 1 hour; then, the temperature in the sample chamber is reduced from 800° C. to room temperature at a cooling rate of 5° C. / min;

[0039] Step 27: After the temperature in the sample chamber drops to room temperature, the pressure in the sample chamber is reduced from 3.0 GPa to normal pressure at a pressure reduction rate of 0.5 GPa / hour;

[0040] Step 28: Take the sample out from the Kawai-1000t typical 6-8 type multi-faceted large cavity high temperature and high pressure equipment, remove the graphite tube wrapping the sample, and select the chromium-doped anhydrous manganese spinel single crystal.

[0041] During high temperature and high pressure reactions, the temperature is calibrated using two sets of high temperature resistant tungsten-rhenium thermocouples. Each set of tungsten-rhenium thermocouples is made of two different tungsten-rhenium alloys, with a chemical composition of W. 95％ Re 5％ and W 74％ Re 26％ Each set of tungsten-rhenium thermocouples is symmetrically placed at the upper and lower ends of the graphite tube sample cavity.

[0042] By changing the amount of the added chemical reagent of chromium (III) acetylacetonate crystalline powder from 151.9617 mg to 212.7464 mg, the corresponding chromium content in the chromium-doped anhydrous manganese spinel single crystal sample was obtained from 5000 ppm wt % to 7000 ppm wt %.

[0043] Beneficial effects of the present invention:

[0044] The present invention organically combines experimental earth science, crystallography and mineralogy, gemology, rare earth element geochemistry, rock-forming mineral beneficiation, mineralogy, crystal chemistry, advanced geochemistry, field geology, mining geology, structural physical chemistry, deep earth material science, magmatic petrology, sedimentary petrology, metamorphic rock petrology, cosmochemistry, astrogeochemistry, planetary geology and other related earth science disciplines, and uses the laboratory Kawai-1000t typical 6-8 type multi-faceted large cavity high temperature and high pressure equipment to simulate the formation process of chromium-doped anhydrous manganese spinel single crystal under high temperature and high pressure conditions. The present invention involves the following main chemical reaction equation:

[0045] MnCO3+2HNO3→Mn(NO3)2+CO2+H2O

[0046] Mn(NO3)2+2C9H 21 AlO3→MnAl2O4+2(NH3·H2O)+6C2H2+6CO+10H2

[0047] MnAl2O4+2Cr(C5H7O2)3→Mn(Al,Cr)2O4+12CO+8CH4+5C2H2

[0048] In the present invention, under high temperature and high pressure conditions, the selected initial raw material, manganese carbonate [chemical formula: MnCO3, also known as manganese (II) carbonate, manganese white or rhodochrosite], is a solid substance of rose-colored triangular crystal system rhombus crystals, which has stable chemical properties, is soluble in dilute acid, and is insoluble in solvents such as water and ethanol. In the research and development of soft ferrites for telecommunications equipment, manganese carbonate is an essential raw material for synthesizing manganese dioxide and producing other manganese salts; as a desulfurization catalyst, manganese carbonate is widely used in the pigment industry of porcelain glazes, coatings and varnishes, as well as an additive for fertilizers and feeds; as an important raw material for the production of electrolytic manganese metal, it is widely used in the fields of medicine, welding rod auxiliary materials, etc. Rose-colored triangular crystal system rhombus manganese carbonate crystals are selected because of their excellent properties of stable performance and easy solubility in dilute acid, making them an excellent raw material for providing manganese elements in artificially synthesized manganese spinel. The initial raw material aluminum isopropoxide [chemical formula: C9H 21 AlO3] is a white and tetrameric powdery solid substance with strong hygroscopicity, strong chemical reactivity, and is easily decomposed when exposed to water. Aluminum isopropoxide powder is selected because of its superior properties of being easily decomposed and highly chemically reactive when exposed to dilute acid solutions, and therefore is an excellent raw material for providing aluminum elements in artificially synthesized manganese spinel. The initial raw material chromium (III) acetylacetonate [chemical formula: Cr(C5H7O2)3, also known as vinyl copper chromium or chromium (III) acetylacetonate] is a purple crystalline powder substance that is insoluble in water but soluble in toluene, acetic acid, etc. As an important chemical industry production intermediate, chromium (III) acetylacetonate has unique physical and chemical properties and is widely used in resin cross-linking agents, chromium nitride films, metal chromium preparation, olefin polymerization catalysts, resin curing agents, chemical adsorbents, etc. In the present invention, the purple chromium (III) acetylacetonate crystalline powder is selected because it is soluble in dilute nitric acid solution, and therefore is an excellent raw material for providing trace element chromium in artificially synthesized manganese spinel. Among the chemical reaction products involved in the present invention, the obtained NH3·H2O, CH4, C2H2, CO2, CO and H2 are all high-temperature volatile substances.

[0049] The present invention requires the synthesis of large-grained anhydrous manganese spinel single crystals with a high chromium content. The synthesized sample contains chromium-doped manganese spinel single crystals that match the development and comprehensive utilization of manganese mineral resources, and is widely used in experimental simulation studies of rock formation and mineralization of mineral rocks under high temperature and high pressure conditions. Compared with natural manganese spinel samples exposed in nature, which may be substituted by impurity ions such as magnesium ions, iron ions, and vanadium ions, in the preparation process of the chromium-doped anhydrous manganese spinel single crystals of the present invention, the laboratory environment is pure, the sample is in a sealed environment, and is not in contact with impurities. The obtained chromium-doped anhydrous manganese spinel single crystals are pure substances with good chemical stability, which provide important experimental sample guarantees for the measurement of physical property parameters of chromium-doped anhydrous manganese spinel single crystals, especially for the study of crystal axis anisotropy and lattice preferred orientation of spinel single crystal minerals under high pressure.

[0050] Compared to previous synthetic methods for manganese spinel single crystals, which employed methods such as metal alkoxide sol-gel, microemulsion, high-pressure hydrothermal synthesis, carbonate chemical co-precipitation, and high-temperature solid-state sintering, the preparation method of the present invention offers significant advantages, including a simpler process and shorter reaction time. The resulting manganese spinel single crystals exhibit superior physical and chemical properties, including high purity, large size, and stable chemical properties. Most importantly, the synthesized manganese spinel product has a high chromium content (5000-7000 ppm wt%), and this chromium content is fully controllable. The large particle size of manganese spinel single crystals can fully meet the sample requirements of single crystal mineral properties and spectroscopic experimental simulations under high temperature and high pressure conditions, such as electrical conductivity, synchrotron radiation X-ray diffraction, confocal Raman spectroscopy, and vacuum Fourier transform infrared spectroscopy on diamond pressure cell high-pressure equipment. This method provides important experimental sample guarantees for the measurement of physical property parameters of chromium-doped anhydrous manganese spinel single crystals, especially for the study of the preferred lattice orientation and crystal axis anisotropy of single crystal minerals under high pressure, breaking through the existing technical bottleneck of manganese spinel single crystal synthesis. DETAILED DESCRIPTION

[0051] A method for preparing chromium-doped anhydrous manganese spinel single crystals under high temperature and high pressure, comprising:

[0052] The present invention uses solid rose-colored trigonal rhombohedral manganese carbonate crystals (purity: >99.99%), solid aluminum isopropoxide powder (purity: >99.99%), solid oxalic acid powder (purity: >99.99%), solid chromium (III) acetylacetonate crystalline powder (purity: >99.99%) and liquid dilute nitric acid (concentration: 10%) as starting materials.

[0053] The high-purity solid manganese carbonate used as the starting material in the present invention is a rose-colored, trigonal, rhombohedral crystal. It is chemically stable, soluble in dilute acid, and insoluble in solvents such as water and ethanol. In the research and development of soft ferrites for telecommunications equipment, manganese carbonate is an essential raw material for synthesizing manganese dioxide and producing other manganese salts. As a desulfurization catalyst, manganese carbonate is widely used in the pigment industry for enamels, coatings, and varnishes, as well as as an additive for fertilizers and feed. As a key raw material for the production of electrolytic manganese metal, it is widely used in the pharmaceutical and welding rod industries.

[0054] The present invention selects rose-colored triangular rhombic manganese carbonate crystals, which are excellent raw materials for providing manganese elements in artificially synthesized manganese spinel due to their stable performance and excellent properties of being easily soluble in dilute acid.

[0055] The high-purity solid aluminum isopropoxide powder used as the starting material in the present invention is a white, tetrameric substance with strong hygroscopicity and chemical reactivity, and is easily decomposed in contact with water. Aluminum isopropoxide powder is selected because of its superior properties of easily decomposing in dilute acid solutions and its strong chemical reactivity, making it an excellent raw material for providing aluminum in synthetic manganese spinel.

[0056] The high-purity chromium (III) acetylacetonate used as the starting material is a purple crystalline powder that is insoluble in water but soluble in toluene, acetic acid, and the like. As an important intermediate in the chemical industry, chromium (III) acetylacetonate has unique physical and chemical properties and is widely used in resin crosslinking agents, chromium nitride films, metallic chromium preparation, olefin polymerization catalysts, resin curing agents, and chemical adsorbents. The purple chromium (III) acetylacetonate crystalline powder selected in the present invention is soluble in dilute nitric acid solution and is therefore an excellent raw material for providing trace element chromium in the synthetic manganese spinel. The high-purity solid oxalic acid selected as the starting material of the present invention is a chelating agent for metal substances. The purpose is that oxalic acid powder has a significant impact on the bioavailability of minerals and has a strong coordination effect. When oxalic acid combines with divalent manganese ions, it can greatly reduce their solubility, thereby forming a complex sol of divalent manganese ions in a dilute nitric acid solution. At the same time, when oxalic acid combines with transition metal cation chromium, due to its coordination effect, a soluble transition metal cation complex is formed, which significantly enhances the solubility of the metal cation containing trivalent chromium in acid solution, allowing it to fully dissolve in the dilute nitric acid solution. If the dilute nitric acid (concentration: 10%) selected as the starting material of the present invention is too low, due to its limited solubility, it may cause residues of manganese carbonate, aluminum isopropoxide, chromium (III) acetylacetonate, and oxalic acid powder. If the nitric acid concentration is too high, due to its enhanced oxidizing property, the manganese subcarbonate in the sample will directly undergo a rapid oxidation reaction or decompose directly, and generate thick smoke, which may bring certain risks to the preparation.

[0057] Step 1. Open the chemical fume hood, select a standard volume 100 ml volumetric flask, accurately weigh out 60 ml of 10% dilute nitric acid, place the glass pipette rod in a 500 ml notched beaker, and carefully transfer all the liquid dilute nitric acid into the beaker along the pipette rod. The notched beaker is chosen as the reaction container mainly because the beaker is not completely sealed after the glass watch glass is covered, and the gas generated can easily evaporate in the fume hood.

[0058] Step 2: Accurately weigh 5.0 g of high-purity rose-colored trigonal rhombohedral manganese carbonate crystals on a 10 μg high-precision analytical balance, carefully add them to a notched beaker containing 10% dilute nitric acid solution, and place a magnetic stirring rotor.

[0059] Step 3: Using a glass watch glass, cover the notched beaker containing the dilute nitric acid solution containing solid manganese carbonate crystals. Place the beaker on a high-temperature magnetic stirring plate in a fume hood. To fully dissolve the solid manganese carbonate crystals in the dilute nitric acid solution and simultaneously cause hydrolysis and acidification, the reaction conditions are room temperature, 700 rpm, and 72 hours.

[0060] Step 4: According to the stoichiometric ratio of manganese spinel Mn(Al,Cr)2O4, 17.7677 g of high-purity solid aluminum isopropoxide powder and 180 mg of high-purity solid chromium (III) acetylacetonate crystalline powder were accurately weighed on a high-precision analytical balance and carefully added to the dilute nitric acid solution containing manganese carbonate.

[0061] Step 5: Place the dilute nitric acid solution containing solid manganese carbonate crystals, solid aluminum isopropoxide powder, and solid chromium (III) acetylacetonate crystalline powder in a beaker and cover it with a glass watch glass to ensure that the gas generated by the reaction evaporates from the gap in the beaker and to prevent the dilute nitric acid solution of the initial material in the beaker from splashing out during the high-speed stirring process, which would cause danger and affect the accuracy of manganese spinel synthesis.

[0062] Step 6: The beaker containing the sealed initial dilute nitric acid mixture and the magnetic stirring rotor is placed on a high-temperature magnetic stirring hot plate in a fume hood. At room temperature, a rotating speed of 800 rev / min, and a stirring time of 48 hours, the solid-state chromium (III) acetylacetonate powder of the initial material is completely dissolved in the mixed solution of the dilute nitric acid solution without any residue. At the same time, volatile substances such as NH3·H2O, CH4, C2H2, CO2, CO, and H2 are more easily volatilized in the fume hood.

[0063] Step 7: On a high-precision analytical balance, 2 grams of high-purity solid oxalic acid powder are accurately weighed and added to a dilute nitric acid solution containing solid manganese carbonate crystals, solid aluminum isopropoxide powder, and solid chromium (III) acetylacetonate crystalline powder. The high-purity oxalic acid powder is an important metal chelating agent. The purpose is that the oxalic acid powder has a great influence on the bioavailability of minerals and has a strong coordination effect. When oxalic acid combines with positive divalent manganese ions, its solubility can be greatly reduced, thereby forming a complex sol of positive divalent manganese ions in the dilute nitric acid solution. At the same time, when oxalic acid combines with transition metal trivalent cation chromium, due to its coordination effect, a soluble transition metal trivalent cation chromium complex is formed. The solubility of the metal cation having positive trivalent chromium in the acid solution will be significantly enhanced, so that it is fully dissolved in the dilute nitric acid solution.

[0064] Step 8. Place the notched beaker of the mixed solution back on the high-temperature magnetic stirring hot plate in the fume hood, cover it with a glass watch glass, and set the conditions of the high-temperature magnetic stirring hot plate to 80°C, 1000 rpm, and stirring time for 36 hours, so that all the initial reagents form a uniform sol under the action of the mixed solution of dilute nitric acid and oxalic acid.

[0065] Step 9: Remove the glass watch glass from the beaker and raise the temperature of the high-temperature magnetic stirring hot plate to 110°C until the mixed solution in the notched beaker is completely evaporated.

[0066] Step 10: Remove the magnetic stirring rotor from the notched beaker on the high-temperature magnetic stirring hot plate and remove all powder sample adhered to its surface into the beaker. Use a spatula to carefully remove all the mixed powder from the notched beaker and place it in a graphite crucible. The purpose of using a graphite crucible is that the carbon that makes up the graphite crucible inevitably produces a certain concentration of carbon monoxide and carbon dioxide during the high-temperature calcination process, thereby controlling the oxygen fugacity within the manganese spinel graphite crucible and ultimately achieving the goal of constraining the valence state of the variable-valence metal cations manganese and chromium in the manganese spinel sample.

[0067] Step 11: The graphite crucible containing the powder mixture was heated in a muffle furnace at atmospheric pressure and high temperature at a relatively slow heating rate of 300°C / hour to 1100°C and held at this temperature for 5 hours. The relatively slow high-temperature calcination rate and longer hold time facilitated the control of the oxygen atmosphere within the graphite sample chamber and facilitated the complete removal of residual nitric acid, oxalic acid, and other organic matter from the powder mixture.

[0068] Step 12: Cool the mixed sample powder in the graphite crucible in the muffle furnace to room temperature at a cooling rate of 200°C / hour. Compared with the heating rate, a slower cooling rate is selected to more easily form a honeycomb-shaped loose sample powder. Carefully remove the mixed sample powder.

[0069] Step 13: Place the honeycomb-shaped loose sample powder in an ultra-hard thickened corundum mortar and grind it thoroughly for 1 hour to obtain a fine-grained and homogenized experimental powder sample.

[0070] Step 14: Cold-press the uniform and fine-grained manganese spinel powder sample mixture into three 10.0 mm x 3.0 mm sample discs using a stainless steel tablet press with a high-precision tungsten carbide grinding tool measuring 10.0 mm x 10.0 mm. The three cold-pressed sample mixtures were stacked vertically and placed in a graphite crucible.

[0071] Step 15. Use a high-speed electric drill to symmetrically drill two 1.0 mm diameter circular holes in the graphite crucible containing the three stacked samples. Carefully thread a 0.5 mm platinum-rhodium alloy wire through the two 1.0 mm symmetrical circular holes in the graphite crucible wall and suspend it in the center of the high-temperature oxygen atmosphere furnace. The ends of the platinum-rhodium wire connecting the graphite crucible are fixed to a vertical 0.6 mm diameter four-hole alumina tube with an outer diameter of 5.0 mm and a length of 40 cm. The upper end of the four-hole alumina tube is fixed in the center of the round lid, which can be inserted and removed from the furnace at any time.

[0072] Step 16. Place a stainless steel container containing 3 liters of secondary deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace in advance. The purpose is to pull the graphite crucible containing the sample directly out of the high-temperature oxygen atmosphere furnace at an extremely high temperature and quickly immerse it in the 3 liters of secondary deionized water in the cold stainless steel container to quickly cool it down. The main purpose is to avoid the re-oxidation / reduction of the variable valence metal elements manganese and chromium during the slow cooling process of the furnace body, to achieve rapid quenching of the sample, and to completely preserve the glassy manganese spinel sample.

[0073] Step 17: At the top of the high-temperature oxygen atmosphere furnace, a cylinder of argon inert gas and cylinders of carbon monoxide and carbon dioxide with adjustable ratios are connected. A barometer controls the amount of gas introduced into the sample chamber. During the high-temperature calcination of the sample, each gas can be switched and adjusted at any time via valves. The present invention uses argon inert gas to provide a completely reducing oxygen atmosphere when the furnace temperature is below 800°C.

[0074] The present invention uses carbon monoxide and carbon dioxide in an adjustable ratio to effectively control the oxygen fugacity of samples during high-temperature calcination when the furnace temperature is above 800°C. If argon inert gas is continuously introduced when the furnace temperature is above 800°C, the sample chamber will be exposed to an over-reduced oxygen atmosphere, which may reduce the variable valence elements manganese and chromium to metallic manganese and metallic chromium. Therefore, when the temperature is above 800°C, we use a mixed gas of carbon monoxide and carbon dioxide in an adjustable ratio to control the oxygen fugacity of samples in the high-temperature oxygen atmosphere furnace chamber.

[0075] The reaction principle is

[0076] The method can well realize the adjustment of any oxygen partial pressure in the sample cavity, thereby realizing the valence states of variable-valence manganese elements and chromium elements in the chromium-doped anhydrous manganese spinel single crystal.

[0077] The maximum rated temperature of a high-temperature oxygen atmosphere furnace is 1800°C. Turn on the circulating cooling water in the high-temperature oxygen atmosphere furnace to lower the upper and lower temperatures of the furnace body to prevent the entire furnace body from overheating, which may cause carbon monoxide and carbon dioxide to leak, thus causing danger.

[0078] Turn on the highly sensitive argon, carbon monoxide and carbon dioxide concentration monitoring alarms to avoid gas leakage during high-temperature calcination in the oxygen atmosphere furnace and ensure the safety of operators.

[0079] Step 18: Open the argon inert gas valve and rotate the pointer button controlled by the gas pressure gauge to continue inflating for 30 minutes to properly expel excess air from the sample chamber. Under the protection of argon inert gas, calcine the sample at a high temperature of 800°C at a heating rate of 400°C / hour.

[0080] Step 19. After the temperature inside the furnace reaches 800°C, quickly switch the carbon monoxide cylinder and the carbon dioxide gas control valve, and rotate the pointer button of the gas pressure gauge control to make the volume ratio of carbon monoxide and carbon dioxide passing through the sample oxygen atmosphere furnace reach 4:1. The purpose is to achieve a carbon monoxide and carbon dioxide mixed gas with this volume ratio during the high-temperature calcination process, which can well regulate the oxygen fugacity in the sample chamber.

[0081] Step 20: After the flow of the mixed gas of carbon monoxide and carbon dioxide (CO2) in a 4:1 volume ratio to control the oxygen fugacity in the sample chamber reaches a stable state (this step takes approximately 3–5 minutes), the temperature of the sample chamber in the furnace is then raised to 1450°C at a heating rate of 200°C / hour. The sample chamber is then calcined at this temperature for 15 minutes to melt the manganese spinel into a glassy state. During the heating process in the high-temperature oxygen atmosphere furnace, the sample chamber is heated at two different heating rates: 400°C / hour and 200°C / hour, respectively, from room temperature to 800°C and from 800°C to 1450°C. The present invention has multiple purposes, including applying a relatively slow heating rate as the temperature of the sample chamber in the high-temperature oxygen atmosphere furnace increases, which is more conducive to the formation of stronger ionic bonds such as Mn-O, Al-O, and Cr-O in chromium-doped manganese spinel; more accurately controlling the temperature of the sample chamber in the high-temperature oxygen atmosphere furnace; and completely avoiding excessive temperature in a local area of ​​the furnace body due to unbalanced heat transfer from the sample chamber, which can easily damage the heating element of the oxygen atmosphere furnace.

[0082] The present invention uses a high-temperature calcination process in which a mixed gas of carbon monoxide and carbon dioxide controls an oxygen atmosphere, with the purpose of achieving the synthesis of large-particle chromium-doped anhydrous manganese spinel single crystals and providing a purer manganese spinel glassy material. The high-temperature calcination in an oxygen atmosphere can better control the valence states of the variable-valence metal elements manganese and chromium in the product. The relatively high calcination temperature of 1450°C ensures that any small amount of volatiles, nitric acid, oxalic acid, organic matter, and other substances that may affect sample preparation after high-temperature calcination in a muffle furnace are completely volatilized.

[0083] The 15-minute constant-temperature calcination is a relatively short time because manganese spinel powder melts rapidly at temperatures above 1400°C. If the calcination time is too short, some residual starting powder may remain in the manganese spinel melt, seriously affecting the chemical composition of the prepared manganese spinel sample. If the calcination time is too short, it is not conducive to sufficient ion exchange and chemical diffusion between cations such as metallic manganese ions, aluminum ions, and chromium ions, nor is it conducive to the formation of strong chemical bonds such as Mn–O, Al–O, and Cr–O in manganese spinel. If the calcination time is too short, the doped chromium element will undergo stratification and differentiation in the manganese spinel, resulting in uneven element distribution, which seriously affects the preparation effect. If the calcination time is too short, the density of the product will be reduced, and it may be difficult to form high-density manganese spinel glass. However, calcination times exceeding 15 minutes may lead to excessive melting, causing the sample to adhere firmly to the wall of the graphite crucible, making it difficult to clean, and also increase the sample preparation cost.

[0084] Step 21. After the sample is calcined at a constant temperature of 1450°C for 15 minutes, the graphite crucible containing the sample, the four-hole alumina tube and the round cover on the furnace body are pulled out of the furnace body and directly immersed in a stainless steel container containing 3 liters of secondary deionized pure cold water to be rapidly quenched into manganese spinel glass. The purpose of rapid quenching is to well preserve the glassy manganese spinel sample with uniform composition at high temperature.

[0085] Step 22: Carefully remove the quenched glassy manganese spinel sample from the graphite crucible and thoroughly grind it in a corundum mortar to form a fine-grained and uniform sample powder. The glassy manganese spinel powder is placed in a vacuum drying oven at 200°C for 12 hours.

[0086] Step 23: On a cold isostatic press, the manganese spinel glass powder is cold pressed using a high-precision Φ4.0 mm (diameter) × 10.0 mm tungsten carbide grinding tool to form a Φ4.0 mm × 4.0 mm cylindrical sample.

[0087] Step 24: The cylindrical manganese spinel sample is sealed in a graphite tube with a diameter of 4.0 mm (inner diameter) × 4.4 mm and a wall thickness of 0.2 mm. Graphite sheets with a diameter of 4.0 mm (diameter) × 0.2 mm (height) are used at the upper and lower ends of the sample tube. Graphite is used as a sealing material. The main purpose is to control the oxygen fugacity values ​​of carbon monoxide and carbon dioxide in the sample chamber to be within the control range, and ultimately to achieve the valence state of the variable valence metal elements manganese and chromium in the manganese spinel sample.

[0088] Step 25. Manganese spinel is one of the important manganese- and aluminum-rich oxide minerals in the lower crust and upper mantle regions of the Earth and other terrestrial planets. To realistically simulate the growth environment of manganese spinel at depths within the lower crust of the Earth and other terrestrial planets, and to invert the temperature and pressure conditions for the stable existence of the manganese spinel mineral phase, the graphite tube containing the sample was placed on a typical Kawai-1000t laboratory 6-8 type multi-faceted large cavity high-temperature and high-pressure equipment. The pressure and temperature were set to 0.5 GPa / hour and 10°C / minute, respectively. The pressure and temperature were raised to 3.0 GPa and 1050°C, respectively, and hot pressing sintering was performed. The reaction time was kept at constant temperature and pressure for 72 hours.

[0089] The preparation process of the high pressure of 3.0GPa and the sintering temperature of 1050℃ selected by the present invention is designed entirely based on the physical and chemical properties of manganese spinel itself. The specific main purposes are as follows: First, the preparation process of high temperature and high pressure conditions, relatively slow pressure and temperature increase rate and long constant temperature and pressure reaction time can fully guarantee the complete mineral phase transformation from the initial manganese spinel glass phase powder to the manganese spinel crystal phase, and the final product manganese spinel mineral phase can stably exist under the temperature and pressure conditions; secondly, the preparation process of high temperature and high pressure conditions, relatively slow pressure and temperature increase rate and long constant temperature and pressure reaction time can significantly increase the self-diffusion and chemical diffusion coefficients of metal cations such as manganese ions, aluminum ions and chromium ions, thereby realizing the isomorphic replacement of chromium ions for metal aluminum ions in manganese spinel crystals, and the reaction is complete and no free chromium element remains, thereby forming a perfect rare earth element chromium-doped manganese spinel single crystal sample; secondly, the high temperature The preparation process of high-pressure conditions, relatively slow pressure and heating rates, and long constant temperature and pressure reaction times can fully ensure the formation of stable chemical bonds such as Mn–O, Al–O, and Cr–O, thereby avoiding the uneven distribution of the doped chromium element in the manganese spinel, such as stratification and differentiation, and thus achieving a uniform equiaxed chromium-doped manganese spinel single crystal sample; finally, the preparation process of high-temperature and high-pressure conditions, relatively slow pressure and heating rates, and long constant temperature and pressure reaction times makes the chromium element distribution of the final prepared manganese spinel more uniform, while increasing the density, strength, and particle size of the product, thereby preparing a large-grain equiaxed chromium-doped manganese spinel single crystal sample with excellent physical and chemical properties such as uniform element distribution, high mechanical strength, and high density.

[0090] The temperature is precisely calibrated using two sets of high-temperature resistant tungsten-rhenium thermocouples. Tungsten-rhenium thermocouples have the advantages of good temperature-potential linearity, reliable thermal stability, and low price. They can achieve a temperature calibration range of 0-2300°C and are widely used in high-pressure mineral physics experiments, high-tech metallurgical industry, high-temperature electronic thermoelectric system structure engineering, space vehicles, nuclear reactors and other fields for ultra-high temperature calibration. Each set of tungsten-rhenium thermocouples is composed of two different tungsten-rhenium alloys, with a chemical composition of W. 95％ Re 5％ and W 74％ Re 26％. Put together one end of tungsten-rhenium thermocouple wires of different materials with a diameter of 0.1 mm, and hang them into a twisted shape with a vise; connect the other end of the tungsten-rhenium thermocouple wires of different materials with a diameter of 0.1 mm to the positive and negative poles of a high-power welding stabilized DC power supply. Adjust the output current control knob of the high-power welding stabilized DC power supply so that the twisted tungsten-rhenium high-temperature thermocouple wire is completely immersed in a saturated sodium chloride solution, melt it, and weld it into a sphere, and remove the oxide layer of the spherical thermocouple wire. Using the same technical solution as above, two groups of hot tungsten-rhenium thermocouples are prepared respectively, and each group of tungsten-rhenium thermocouples is symmetrically placed at the upper and lower ends of the graphite tube sample cavity. The present invention adopts the method of placing tungsten-rhenium dual thermocouples at the upper and lower ends respectively. This technology can realize the accurate calibration of the temperature in the sample cavity, and can also accurately indicate the temperature gradient at the upper and lower ends of the sample chamber, ensuring that the sample is in a stable constant temperature zone during the synthesis process of manganese spinel sample.

[0091] Step 26: After maintaining constant temperature and pressure at 3.0 GPa and 1050°C for 72 hours, the temperature in the sample chamber is reduced from 1050°C to 800°C at a cooling rate of 3°C / min and maintained at this temperature for 1 hour; the temperature in the sample chamber is then reduced from 800°C to room temperature at a cooling rate of 5°C / min. By adopting a step-wise cooling method and a slower constant pressure cooling rate relative to the sample preparation heating rate (10°C / min), the superior physical and chemical properties of the chromium-doped manganese spinel single crystal sample with uniform chromium distribution, high mechanical strength, and high density are further enhanced, completely avoiding the uneven stress in the sample caused by an excessively fast cooling rate, which in turn causes cracks and damage to the manganese spinel crystal. Furthermore, this preparation process is more conducive to the growth of large-grained manganese spinel single crystals, thereby achieving the preparation of large-grained manganese spinel single crystal samples of 100 micrometers.

[0092] Step 27: After the temperature in the sample chamber has dropped to room temperature, the pressure in the sample chamber is reduced from 3.0 GPa to atmospheric pressure at a rate of 0.5 GPa / hour. Furthermore, the present invention utilizes a hot-pressing sintering process to prepare chromium-doped anhydrous manganese spinel samples. The preparation process is pure, without the introduction of any potential water-derived substances from the sample itself or during high-pressure sample assembly.

[0093] Step 28: After the high-temperature, high-pressure preparation reaction is complete, the sample is removed from the Kawai-1000t, a typical 6-8-inch multi-faceted, large-cavity, high-temperature, high-pressure apparatus. The graphite tube encasing the sample is carefully removed, and the cylindrical sample is cut down the middle using a high-precision diamond wire cutter. The manganese spinel single crystal is selected under a high-precision Olympus microscope at 20x magnification.

[0094] The manganese spinel single crystal obtained by the present invention is a single phase without any other impurity phases; electron probe microanalysis (EPMA) detection results show that the molecular formula of the obtained manganese spinel single crystal is MnAl2O4; multifunctional ion mass spectrometer (ICP-MS) detection results show that the chromium content in the obtained manganese spinel single crystal is 5924 ppm by weight; vacuum Fourier transform infrared spectroscopy (FT-IR) detection results show that the water content of the obtained manganese spinel sample is less than 2 ppm by weight, which has a low water content and is an anhydrous oxide mineral.

[0095] The chromium-doped anhydrous manganese spinel single crystal obtained in the present invention is a cubic crystal system with a space group of Fd3m (no.227) and a lattice parameter of α=β=γ=90°, the unit cell volume is The average particle size was 125 microns and the maximum particle size was 319 microns.

[0096] The chromium-doped anhydrous manganese spinel single crystal obtained by the present invention has superior properties such as high purity, large particle size, stable chemical properties, and high mechanical strength. More importantly, it has a high chromium content (5924ppm wt%), and the chromium content in the manganese spinel can be fully controlled. By changing the amount of chemical reagent added as the initial solid-state chromium (III) acetylacetonate crystalline powder from 151.9617 mg to 212.7464 mg, the corresponding chromium content in the chromium-doped anhydrous manganese spinel single crystal sample is finally achieved from 5000ppm wt% to 7000ppm wt%. The obtained chromium-doped anhydrous manganese spinel single crystal can fully meet the needs of physical simulation of minerals in the lower crust and upper mantle regions of the Earth and other terrestrial planets under high temperature and high pressure conditions, breaking through the technical bottleneck of existing manganese spinel single crystal synthesis, and providing important experimental sample support for the study of lattice preferred orientation and crystal axis anisotropy of single crystal minerals in the lower crust and upper mantle regions of the Earth and other terrestrial planets under high temperature and high pressure conditions.

Claims

1. A method for preparing chromium-doped anhydrous manganese spinel single crystals under high temperature and high pressure, characterized by: The method comprises: using solid rose-colored trigonal rhombohedral manganese carbonate crystals, solid aluminum isopropoxide powder, solid oxalic acid powder, solid chromium (III) acetylacetonate crystalline powder and liquid dilute nitric acid as starting materials, preparing a manganese spinel powder sample according to the stoichiometric ratio of manganese spinel; cold-pressing the manganese spinel powder sample into discs, stacking the discs and placing them in a graphite crucible; placing the graphite crucible in a high-temperature oxygen atmosphere furnace for high-temperature calcination to prepare a cylindrical manganese spinel sample; and subjecting the cylindrical manganese spinel sample to a high-temperature and high-pressure reaction to obtain a chromium-doped anhydrous manganese spinel single crystal. The method for preparing the cylindrical manganese spinel sample comprises: Step 14: cold-press the manganese spinel powder sample into three sample discs of Φ 10.0 mm × 3.0 mm using a tungsten carbide die of a stainless steel tablet press. Vertically stack the three cold-pressed sample mixtures and place them in a graphite crucible. Step 15: Drill two symmetrical circular holes with a diameter of 1.0 mm symmetrically on the wall of the graphite crucible using an electric drill; pass a 0.5 mm platinum-rhodium alloy wire through the two symmetrical 1.0 mm circular holes in the graphite crucible wall, so that the graphite crucible is suspended in the center of the high-temperature oxygen atmosphere furnace; fix the ends of the platinum-rhodium wire connecting the graphite crucible to a vertical four-hole alumina tube with a diameter of 0.6 mm; and fix the upper end of the four-hole alumina tube to the center of the round lid that can be inserted into and pulled out of the furnace body; Step 16: Place a container filled with secondary deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace; Step 17: The top of the high-temperature oxygen atmosphere furnace body is connected to an argon inert gas cylinder and carbon monoxide and carbon dioxide cylinders with adjustable ratios; Step 18: After opening the argon inert gas valve and continuously charging for 30 minutes, the sample was calcined at a high temperature of 800 °C at a heating rate of 400 °C / hour under the protection of argon inert gas; Step 19: After the furnace temperature reaches 800°C, switch the carbon monoxide cylinder and carbon dioxide gas control valves so that the volume ratio of carbon monoxide to carbon dioxide in the oxygen atmosphere furnace reaches 4:1; Step 20: After the mixed gas flow of carbon monoxide and carbon dioxide in a volume ratio of 4:1 to control the oxygen fugacity in the sample chamber reaches a stable state, the temperature of the sample chamber in the furnace is increased to 1450°C at a heating rate of 200°C / hour and calcined at a constant temperature for 15 minutes; Step 21: After the sample is calcined at a constant temperature of 1450°C for 15 minutes, the graphite crucible containing the sample, the four-hole alumina tube, and the round cover on the furnace body are pulled out of the furnace body and directly immersed in secondary deionized pure cold water for quenching to obtain manganese spinel glass; Step 22: remove the quenched manganese spinel glass from the graphite crucible and grind it in a corundum mortar; place the glassy manganese spinel powder in a vacuum drying oven at 200° C. for 12 hours; Step 23: Cold-press the dried glassy manganese spinel powder using a tungsten carbide mold on a cold isostatic press to form a cylindrical manganese spinel sample with a size of Φ 4.0 mm × 4.0 mm.

2. The method for preparing a chromium-doped anhydrous manganese spinel single crystal under high temperature and high pressure according to claim 1, characterized in that: Solid rose-colored trigonal rhombic manganese carbonate crystals with a purity >99.99%, solid aluminum isopropoxide powder with a purity >99.99%, solid oxalic acid powder with a purity >99.99%, solid chromium (III) acetylacetonate crystalline powder with a purity >99.99%, and liquid dilute nitric acid with a concentration of 10%.

3. The method for preparing a chromium-doped anhydrous manganese spinel single crystal under high temperature and high pressure according to claim 1, characterized in that: The method for preparing a manganese spinel powder sample using solid rose-colored trigonal rhombohedral manganese carbonate crystals, solid aluminum isopropoxide powder, solid oxalic acid powder, solid chromium (III) acetylacetonate crystalline powder, and liquid dilute nitric acid as starting materials and according to the stoichiometric ratio of manganese spinel comprises: Step 1. Weigh out 60 ml of 10% dilute nitric acid and place a glass pipette in a 500 ml notched beaker. Step 2: Weigh out 5.0 g of rose-colored trigonal rhombohedral manganese carbonate crystals and add them to a notched beaker containing 10% dilute nitric acid solution, and place a magnetic stirring rotor in it; Step 3: Cover the notched beaker with a glass watch glass, place it on a ventilated high-temperature magnetic stirring hot plate, and react at room temperature and 700 rpm for 72 hours; Step 4: Weigh out 17.7677 g of solid aluminum isopropoxide powder and 180 mg of solid chromium (III) acetylacetonate crystalline powder, and add them to the dilute nitric acid solution containing manganese carbonate; Step 5: Cover the notched beaker with a glass watch glass. Step 6: Place the notched beaker on a ventilated high-temperature magnetic stirring hot plate and stir at room temperature and 800 rpm for 48 hours; Step 7: Weigh out 2 grams of solid oxalic acid powder and add it to the notched beaker; Step 8. Place the notched beaker of the mixture back on a ventilated high-temperature magnetic stirring hot plate, cover with a glass watch glass, and stir at 80 °C and 1000 rpm for 36 hours. Step 9: Remove the glass watch glass from the beaker and increase the temperature of the high-temperature magnetic stirring hot plate to 110°C until the mixed solution in the notched beaker is completely evaporated. Step 10: Take out the magnetic stirring rotor in the notched beaker, take out all the sample powder and place it in a graphite crucible; Step 11: Place the graphite crucible in a muffle furnace, increase the temperature to 1100 °C at a heating rate of 300 °C / hour, and keep the temperature constant for 5 hours; Step 12: Cool the sample powder in the muffle furnace to room temperature at a cooling rate of 200 °C / hour, and take out the sample powder; Step 13: Grind the sample powder in a corundum mortar for 1 hour to obtain a fine-grained and homogenized manganese spinel powder sample.

4. The method for preparing a chromium-doped anhydrous manganese spinel single crystal under high temperature and high pressure according to claim 1, characterized in that: The method for obtaining a chromium-doped anhydrous manganese spinel single crystal by subjecting a cylindrical manganese spinel sample to a high-temperature and high-pressure reaction comprises: Step 24: seal the cylindrical manganese spinel sample in a graphite tube with an inner diameter of Φ 4.0 mm and a wall thickness of 4.4 mm and a wall thickness of 0.2 mm, and encapsulate the upper and lower ends of the sample tube with graphite sheets with a diameter of Φ 4.0 mm and a height of 0.2 mm; Step 25. Place the graphite tube containing the sample on a typical Kawai‒1000t laboratory 6-8 type multi-faceted large cavity high temperature and high pressure equipment. Set the pressure and temperature increase rates to 0.5 GPa / hour and 10 °C / minute, respectively. Raise the pressure and temperature to 3.0 GPa and 1050 °C, respectively, and perform hot pressing sintering. The reaction time is 72 hours at constant temperature and pressure. Step 26: After maintaining constant temperature and pressure at 3.0 GPa and 1050°C for 72 hours, the temperature in the sample chamber was reduced from 1050°C to 800°C at a cooling rate of 3°C / min and maintained at this temperature for 1 hour; then, the temperature in the sample chamber was reduced from 800°C to room temperature at a cooling rate of 5°C / min. Step 27: After the temperature in the sample chamber drops to room temperature, the pressure in the sample chamber is reduced from 3.0 GPa to atmospheric pressure at a pressure reduction rate of 0.5 GPa / hour; Step 28. Remove the sample from the Kawai‒1000t typical 6-8 type multi-faceted large cavity high temperature and high pressure equipment, remove the graphite tube wrapping the sample, and select the chromium-doped anhydrous manganese spinel single crystal.

5. The method for preparing a chromium-doped anhydrous manganese spinel single crystal under high temperature and high pressure according to claim 4, characterized in that: During high temperature and high pressure reactions, the temperature is calibrated using two sets of high temperature resistant tungsten-rhenium thermocouples. Each set of tungsten-rhenium thermocouples is made of two different tungsten-rhenium alloys, with a chemical composition of W. 95% Re 5% and W 74% Re 26% Each set of tungsten-rhenium thermocouples is symmetrically placed at the upper and lower ends of the graphite tube sample cavity.

6. The method for preparing a chromium-doped anhydrous manganese spinel single crystal under high temperature and high pressure according to claim 3, characterized in that: By changing the amount of chemical reagent added with chromium (III) acetylacetonate crystalline powder from 151.9617 mg to 212.7464 mg, the corresponding chromium content in the chromium-doped anhydrous manganese spinel single crystal sample was obtained from 5000 ppm wt% to 7000 ppm wt%.