Method for producing a tellurate

Abstract

The invention relates to a method for producing a crystalline tellurate comprising at least zinc; the method comprising the following steps: providing a cationic aqueous solution comprising Zn2+ and Te6+ cations; co-precipitating the crystalline tellurate in liquid medium by applying a pH to the cationic aqueous solution, the pH being maintained at a value ranging from 5.00 to 10.00; and optionally separating the crystalline tellurate from the liquid medium.

Description

[0001] PROCESS FOR MANUFACTURING A TELLURATE

[0002] Field of invention

[0003] The invention relates to a method for manufacturing a crystalline zinc tellurate compound, in particular of garnet structure, by ionic precipitation and condensation.

[0004] Prior art

[0005] Tellurates are chemical compounds containing a tellurium oxyanion in which this element is in the oxidation state (+VI). Tellurates can adopt different crystalline phases such as a lamellar structure, a dagenaisite-type structure or a yafsoanite-type structure, which is similar to the garnet structure. These tellurates can be used in many fields, particularly in the field of:

[0006] - transparent ceramics, and, in particular, for laser applications (when the compound contains a rare earth ion, which will constitute the optically active element);

[0007] - inorganic light-emitting diodes and in particular white diodes

[0008] - scintillators (comprising a rare earth ion, often Ce 3+ ) ;

[0009] - jewelry; and

[0010] - the manufacture of colored pigments.

[0011] These uses require a high degree of purity. Tellurates can also be considered in certain areas of thermomechanical protection.

[0012] Currently, crystalline compounds, and in particular those of the garnet type, are prepared using various processes, including solid-state processes, including high-temperature thermal crystallization steps, generally called calcination, and possibly the use of high pressures. Such steps are considered necessary to obtain both the particular garnet structure and the required high degree of purity. Such energy-intensive processes are therefore expensive and not very environmentally friendly.

[0013] Thus, EP0030170A1 relates to the manufacture of polycrystalline garnet-type compounds and teaches dissolving the starting compounds in acid to allow the formation of co-hydroxides by coprecipitation. These are subsequently calcined at temperatures ranging from 800 °C to 1400 °C.

[0014] For garnet-type tellurates, the methods described also include energy-intensive thermal steps. For example, Deng et al. in the article "Photoluminescence properties of a Dy 3 *, N / A + -activated yafsoanite Ca3ZnsTe20i2 phosphor with excellent thermal stability » Journal of Materials Science: Materials in Electronics, https: / / doi.ora / 10.1007 / s10854-019-02012-6; describes a process where a powder of starting compounds present under stoichiometric conditions is heated to 1050 °C for 12 h in air.

[0015] In addition, the various processes commonly used to prepare crystalline tellurates, particularly those of the garnet type, generally require a high-energy grinding step to reduce the particle size of the resulting tellurates, for example using a planetary mill. However, such grinding is time-consuming and energy-intensive. They can also introduce impurities.

[0016] There is therefore a need to improve the known processes for synthesizing crystalline compounds with a garnet structure in order to simplify them, increase their yield, and / or make them less energy-intensive while making it possible to obtain compounds with a sufficient degree of purity for the intended uses and / or controlling production costs.

[0017] Description of the invention

[0018] An object of the invention relates to a process for the manufacture of a crystalline tellurate comprising at least zinc; said process comprising the following steps: providing a cationic aqueous solution comprising Zn cations 2+ and you 6+ ; coprecipitating said crystalline compound in a liquid medium by applying a pH to said cationic aqueous solution, said pH being maintained at a value ranging from 5.00 to 10.00; and, optionally, separating said crystalline tellurate from said liquid medium.

[0019] The process according to the invention makes it possible to obtain a crystalline tellurate in the form of a microcrystalline powder, preferably nanocrystalline and / or of homogeneous density. Advantageously, the crystalline tellurate can be obtained according to the process of the invention at room temperature, without a high-energy grinding step and / or without a secondary phase. The process of the invention has the advantages of being simple to implement and inexpensive. In the present invention, the term "without secondary phase" means that at least 99%, preferably 100%, of the tellurates obtained have the same structure, for example a garnet-type structure or a dagenaisite-type structure. A single crystalline phase is then obtained.

[0020] The process of the invention makes it possible, surprisingly, to control the tellurate obtained, in particular its structure, as a function of the pH of the coprecipitation step. The process of the invention advantageously makes it possible to prepare a crystalline tellurate of complex composition, in particular comprising, in addition to the element tellurium and zinc, at least one, preferably at least two, other elements, zinc and this other element, or these other elements, preferably belonging to different chemical families. The process of the invention makes it possible to obtain, for example, a pure crystalline tellurate powder of intense blue color with low cobalt contents. In the present invention, the term "chemical family" means that the elements belonging to the same chemical family are grouped in the same column of the periodic table.

[0021] The crystalline tellurate comprising zinc which can be obtained according to the process of the invention may be a tellurate comprising only zinc, such as ZnsTeOe. However, the crystalline zinc tellurate may advantageously comprise at least one other element.

[0022] This other element may be an alkaline earth metal, such as beryllium, magnesium, calcium, strontium and / or barium. Advantageously, the alkaline earth metal is calcium and / or barium, preferably calcium. Alternatively or additionally, this other element may be an alkali metal such as lithium, sodium, potassium, rubidium, cesium or a mixture thereof. Sodium, lithium and / or cesium are particularly preferred. The alkali metal is very preferably sodium. Alternatively or additionally, this other element may be a transition metal such as silver, chromium, cobalt, nickel, copper, iron, iridium, manganese, mercury or a mixture thereof. Silver, cobalt, nickel, copper, iron, manganese, or a mixture of these metals is preferred. Cobalt is particularly preferred.Alternatively or additionally, this other element may be a rare earth such as scandium, yttrium or a lanthanide, notably europium or cerium.

[0023] According to a particular aspect of the invention, the crystalline zinc tellurate which can be obtained according to the process of the invention preferably comprises cobalt or calcium, very preferably calcium.

[0024] According to a particular aspect of the invention, the crystalline zinc tellurate which can be obtained according to the process of the invention can advantageously comprise at least two other elements defined above. By way of example, the crystalline zinc tellurate which can be obtained according to the process of the invention comprises three other elements defined above.

[0025] According to a preferred embodiment of the invention, zinc and the other element defined above, preferably zinc and the at least two other elements defined above, belong to different chemical families. For example, the crystalline tellurate obtainable by the method of the invention comprises zinc, an alkaline earth metal and / or an alkali metal and / or a transition metal and / or a rare earth. Advantageously, the crystalline tellurate preferably comprises at least zinc, an alkaline earth metal and a transition metal, very preferably at least zinc, calcium and cobalt. For example, the zinc tellurate may be of composition Ca3Zn2,sCoo,5Te20i2 or of composition Ca3Zn2.7Co0.3T e20i2.

[0026] The crystalline tellurate comprising zinc obtainable by the process according to the invention may have a dagenaisite-type structure or a yafsoanite-type structure (also called "garnet structure", or "garnet-type structure"), preferably a yafsoanite-type structure.

[0027] When zinc tellurate has a dagenaisite-type structure, the tellurium oxyanion is generally of the type (TeOe) 6 '. This tellurate advantageously comprises an alkaline earth metal, such as barium, and / or a transition metal, such as cobalt. It may in particular be ZnsTeOe, Ba2ZnTeOe and / or Zn2CoTeOe.

[0028] When zinc tellurate has a garnet-like structure, the tellurium oxyanion is usually of the type (TeOe) 6'. This tellurate advantageously comprises an alkaline earth metal and in particular calcium, for example this compound can be of formula Ca3Zn3Te20i2. This compound, yafsoanite, adopts a structure similar to garnet and consists of a three-dimensional network of octahedra and tetrahedra which share vertices and which are constituted by oxygen atoms. It can also be described by the formula Ca3Te2(ZnC>4)3. According to a particular aspect of the invention, part of the zinc in this structure can be replaced by another element. This other element is advantageously chosen for a property associated with its presence. For example cobalt, nickel and / or copper can be present and replace part of the zinc in the yafsoanite type structure. Cobalt can thus replace part of the zinc and make it possible to obtain in particular the tellurate Ca3Zn2CoTeOi2 or Ca3Zn2,7Coo,3TeOi2.In this case, the presence of cobalt gives the crystalline compound obtained a particular blue color.

[0029] According to a particular aspect of the invention, the crystalline tellurate obtained according to the process of the invention is obtained in the form of a mixture of crystalline tellurates of different structures, for example of the yafsoanite or dagenaisite type; the proportion of tellurate of one structure being preferably greater than or equal to 90%, very preferably greater than or equal to 95%, more preferably greater than or equal to 99%. In particular, the crystalline tellurate obtained according to the process of the invention is obtained without a secondary phase.

[0030] According to a particular aspect of the invention, the crystalline tellurate obtained according to the method of the invention may be of composition Ca3Zn3Te20i2 and has a garnet-type structure in which the calcium cations are in a distorted cubic site, the Te ions 6+are on the octahedral sites and the Zn ions 2+ are on the tetrahedral sites, as shown in Figure 1.

[0031] The zinc tellurates which can be obtained according to the process of the invention, and in particular the compounds having a garnet-type structure described above, can advantageously be used in luminescent devices. Also, the tellurates, and particularly the yafsoanite-type tellurates, can advantageously be doped with at least one element such as an alkali, and in particular Li + and Na + , and / or a rare earth, such as Ce 3+. Doping a compound consists of introducing elements in very low concentrations into its matrix that can replace other atoms. The presence of these dopants causes the appearance of new electron acceptor and donor levels in the band structure of the doped material. In such applications, a mixture of at least two doping elements, such as Li + and This 3+ , or Na + and This 3+ , is particularly used. The dopant(s) may replace a very small part (for example less than 5%, preferably less than 2%) of the sites occupied by other elements of the compound, in particular by the alkaline earth(s) (for example Ca 2+ ). Such doped tellurates are, for example, Ca2,94(Nao,5,Ceo,5)o,o6Zn3TeOi2, Ca2,94(Lio,5,Ce0,5)o,o6Zn3TeOi2 or Ca2,94(Nao,5,Ceo,5)o,o6(Zn(i- y )Co y)3TeOi2. In the present invention, the tellurate is said to be “doped” when the other element(s) included in the crystalline tellurate have the effect of modifying the conductivity properties of the crystalline tellurate.

[0032] Doping can also be performed on compounds with different structures.

[0033] The method according to the invention comprises providing an aqueous solution comprising Zn cations 2+ and you 6+ (cationic aqueous solution). These cations are preferably added in a stoichiometric amount corresponding to the proportions present in the desired tellurate. This cationic aqueous solution is generally an acidic solution with a pH of less than 5.

[0034] This cationic aqueous solution can be obtained by using a salt of these cations, such as zinc nitrate hexahydrate and / or orthotelluric acid (Te(OH)e). Other types of zinc salts, such as a chloride, can also be used.

[0035] The method according to the invention may comprise providing a cationic aqueous solution comprising Zn cations 2+ and you 6+ and at least one other cation corresponding to another element as defined above. The cationic aqueous solution can be obtained by using a salt of this other cation.

[0036] When a compound includes other cations, the chosen salt may advantageously be a nitrate of the chosen metal, e.g., calcium nitrate tetrahydrate. Other types of salts, such as a chloride, may also be considered. Thus, the following salts may be used: a sodium salt (e.g., NaCl, NaNO3), a barium salt (e.g., Ba(NO3)2, BaCh), a cobalt salt (e.g., Co(NC>3)2, C0Cl2), a cesium salt (e.g., CsNCh, CsCl), a lithium salt (e.g., LiNOs, LiCl); etc. In the case where the salt contains a nitrate, the latter will very often be hydrated.

[0037] According to a particular aspect of the invention, the cationic aqueous solution comprises at least two other cations corresponding to the other elements defined above. The cationic aqueous solution, for example, comprises, in addition to Te 6+ , Zn 2+ , three other cations corresponding to the other elements defined above.

[0038] According to a particular aspect of the invention, the cationic aqueous solution comprises, in addition to Te 6+ , Zn 2+ and at least 1, preferably 2, other cations corresponding to the other elements as defined above, zinc and the other element, preferably the other elements, belonging to different chemical families.

[0039] For example, the method may comprise providing a cationic aqueous solution comprising Zn cations 2+ , You 6+ and the respective cations of cobalt and / or calcium.

[0040] To obtain a doped tellurate according to the invention, the other doping element(s) may be introduced into the cationic aqueous solution. This may, for example, comprise the cations Li + and This 3+ or Na + and This 3+ .

[0041] The cationic aqueous solution is preferably a concentrated solution of cationic species. By "concentrated" it is understood that the concentration of Te 6+ is at least 0.05 mol / L, preferably at least 0.1 mol / L and preferably at least 0.4 mol / L, for example 0.5 mol / L. This cationic aqueous solution is preferably a homogeneous solution which can be obtained by simple stirring. Preferably, the solvent of the cationic aqueous solution comprises or consists of water, preferably pure and / or osmosed water. Distilled water may also be used, in particular in the case where the cationic aqueous solution contains Ce cations 3+ .

[0042] According to the method of the invention, crystalline zinc tellurate is obtained by coprecipitation in a liquid medium by applying a pH to said cationic aqueous solution. This application is advantageously carried out by adding to the cationic aqueous solution an alkaline and / or neutral or basic buffer solution. This addition is preferably controlled, for example by adding a quantity of the cationic aqueous solution proportionally small compared to said buffer solution, so as to minimize the variation in pH of the solution. On a laboratory scale, this addition can therefore be made by the drop-by-drop method. In particular, crystalline zinc tellurate can be obtained by reverse coprecipitation.

[0043] It has been found, surprisingly, that the pH value of the liquid medium during the coprecipitation step influences the structure of the crystalline tellurate obtained. Advantageously, when the pH is maintained at a value ranging from 8 to 10, preferably 9, the crystalline tellurate obtained can have a crystalline structure typical of yafsoanite and can be obtained without a secondary phase. For comparison, it has been found, when the pH is for example maintained at pH 4, that the crystalline tellurate obtained has a crystalline structure typical of dagenaisite and is obtained without a secondary phase. It has therefore been found surprisingly that a crystalline tellurate comprising zinc and calcium with a yafsoanite-type structure cannot be obtained by coprecipitation in a liquid medium at pH=4.

[0044] When the pH is maintained at a value below pH 8 and above pH 5, the crystalline tellurate is obtained in the form of a mixture of crystalline tellurates which may, for example, include crystalline tellurate having a yafsoanite-type structure and / or a dagenaisite-type structure. The proportion of crystalline tellurate having a yafsoanite-type structure or a dagenaisite-type structure depends on the pH during co-precipitation. The higher the pH value, the more predominantly the typical yafsoanite structure will be obtained.

[0045] It is therefore necessary to maintain the pH at a given value during the coprecipitation step, this value being chosen in particular according to the desired structure of the crystalline tellurate.

[0046] It is thus very advantageous to control and / or minimize the variation of said pH during the coprecipitation. When this takes place by adding the cationic aqueous solution to a buffer solution, this can be done by carrying out a time- and volume-controlled addition of the cationic aqueous solution to the buffer solution. According to a particularly preferred aspect of the invention, this can also be done by measuring the pH of the reaction medium thus created, this measurement preferably being able to be carried out continuously (or at regular and frequent intervals). The pH control can be done semi-automatically by using an ion metering pump. Any variation in pH can thus be compensated for quickly, for example by adding a basic compound to the reaction medium if the pH decreases. Similarly, continuous stirring can be applied to accelerate the homogenization of the mixture.

[0047] Advantageously the reaction is carried out at room temperature and / or atmospheric pressure.

[0048] The buffer solution may be an ammonia buffer solution, i.e. include ammonia and / or a NH4NO3 / NH4OH mixture.

[0049] The alkaline solution can be sodium hydroxide NaOH or ammonia NH4OH solution.

[0050] Advantageously, the pH is maintained at a value ranging from 8 to 10, preferably at a pH of 9 + / - 0.3 (more particularly 9 + / - 0.2). More specifically, the pH may be maintained at a value greater than or equal to 8.0; preferably 8.5; very preferably 8.6; more preferably 8.7, even more preferably 8.8 or even preferably 8.9; and is maintained at a value less than or equal to 10.0; preferably 9.5; very preferably 9.4, more preferably 9.3; even more preferably 9.2; or even preferably 9.1. For example, the pH is maintained at pH 9.0 or at a value greater than or equal to 8.7; preferably 8.8; and less than or equal to 9.3; preferably 9.2. The choice of such a pH is particularly preferred for obtaining a compound having a crystal structure typical of yafsoanite.In particular, this pH makes it possible to obtain, according to the process of the invention, a crystalline tellurate having a garnet-type structure with a high yield, in particular greater than 85% calculated on the basis of the theoretical total mass expected at the end of the precipitation, washing, filtration and drying operations, as well as to obtain crystals (crystalline tellurates) of high purity and is particularly suitable for the synthesis of yafsoanite and its derivatives of formula (I) defined below.

[0051] This operation, namely the co-precipitation of crystalline tellurate in a liquid medium by applying a pH to the cationic aqueous solution, can be complete or almost complete. Also, by using limiting or stoichiometric conditions for certain metal cations (especially toxic ones) these can be consumed in their entirety during the formation of crystals (crystalline tellurates) and the resulting filtrate (the liquid medium) does not contain or contains little of such elements (the cations of tellurium, zinc and / or other elements). In addition to the advantageous economic aspect, the ecological aspect of a method producing little or no liquid products (filtrate) containing (toxic) metals is a particularly appreciable technical effect.

[0052] According to a variant of the process according to the invention, the coprecipitation step comprises a maturation step allowing the formation of the compounds (zinc tellurates). This step is subsequent, preferably immediately subsequent, to the step of applying said pH. It can last from a few minutes (e.g. 5 min.) to a few hours (e.g. 2 to 6 hours). It can be carried out at rest, but is preferably carried out with continuous stirring.

[0053] The separation of said crystalline tellurate from said liquid medium can be carried out by simple or vacuum filtration, repeated or not. Alternatively, this separation can be carried out by centrifugation or any other usual technique known in this field. It can optionally be followed by one or more rinsing steps, for example with water (for example osmosis). Once obtained, the crystalline zinc tellurate can be dried and optionally ground. In the present invention, unless otherwise indicated, the term "grinding" refers to the manual deagglomeration of the crystalline zinc tellurate obtained. This manual grinding, unlike high-energy grinding, does not modify the diameter of the crystalline zinc tellurates obtained.

[0054] The process may include an additional heat treatment step. During this step, the crystalline tellurate obtained is subjected for a short time to a temperature below 1000°C, preferably below or equal to 800°C, very preferably below or equal to 725°C, even more preferably below or equal to 700°C. This step is optionally carried out at low pressure, in particular under primary vacuum corresponding to a few 10' 2 mbar. The short duration of the heat treatment is variable. For example, it can be from 5 to 30 minutes, especially 15 min. This step allows in particular to increase the purity of the crystalline tellurates obtained. For example, this step allows to eliminate impurities such as NO groups Xand / or ammonium chemisorbed on the crystalline tellurate. This step facilitates the shaping of the crystalline tellurates obtained, in particular when these crystalline tellurates are used to produce ceramics. Unlike the calcination step commonly carried out in the synthesis of crystalline tellurates, the heat treatment useful in the process which is the subject of the invention is carried out on the crystalline tellurate formed. This heat treatment step is therefore subsequent to the crystallization of the tellurate.

[0055] Crystalline zinc tellurate is advantageously obtained in the form of microcrystalline powder, and preferably nanocrystalline powder. Optionally, this powder is obtained without a grinding step, in particular without high-energy grinding. In the present invention, a microcrystalline powder defines a powder whose individual grains have a diameter of the order of a micron, or more. Similarly, a nanocrystalline powder designates a powder whose individual grains have a diameter ranging from a few nanometers to a few hundred nanometers. Crystalline zinc tellurate can for example be obtained in the form of nanocrystalline powder whose individual grains have a diameter less than or equal to 500 nm, in particular less than or equal to 200 nm. Preferably, the individual grains have a diameter less than or equal to 100 nm, in particular between 50 and 100 nm.According to a variant of the process of the invention, the crystalline zinc powder obtained can have a specific surface area greater than 60 m. 2 / g measured by the BET N2 method. The micrometric, preferably nanometric, size of the grains makes it possible to obtain a zinc tellurate powder having a homogeneous density. Advantageously, the degree of purity of the crystalline zinc tellurate obtained and / or the homogeneity of the powder obtained are sufficient for the process according to the invention to be preferably carried out without the application of the heat treatment step.

[0056] Crystalline zinc tellurate is advantageously obtained pure or with a degree of purity greater than or equal to 99%.

[0057] The method according to the invention is advantageously implemented without the application of a calcination step. The calcination step, otherwise called high temperature thermal crystallization, is commonly defined in the prior art as being carried out optionally under pressure, at a temperature ranging from 150°C to 1600°C. In the prior art, the calcination step is generally carried out prior to, or even simultaneously with, the crystallization of the tellurate. The method according to the invention, unlike the methods of the prior art, therefore does not require the implementation of a calcination step to obtain the crystallization of the tellurate.

[0058] According to an advantageous variant of the process according to the invention, the coprecipitation step is carried out at a temperature ranging from 5°C to 60°C and, preferably, at room temperature, for example around 20°C + / - 5°C. The process can be carried out under pressure as intended by a person skilled in the art. According to an advantageous variant of the process according to the invention, the coprecipitation step is carried out under a pressure ranging from 90 to 120 kPa (absolute pressure), and preferably at atmospheric pressure.

[0059] According to a variant of the method according to the invention, the latter can be automated to allow control of at least one parameter such as the concentration of chemical species, pressure, temperature, agitation, and, in particular, pH.

[0060] Another subject of the invention relates to a crystalline compound, in particular of the garnet type, of formula (I) (Cai- x HAS x )3(Zn(iy)By)3Te2Oi2, where

[0061] - A is, or includes, an element of the group HA and / or an “alkali M, rare earth RE” couple of form (M + , D 3+ ) where 0 < x < 1,

[0062] B is, or comprises, an element of group HB and / or a transition metal (in particular Cu 2+ , Co 2+ , Neither 2+ ) with 0 < y < 1, and excluding yafsoanite. The expression "0 < x < 1" means that x is a number ranging from 0 to 1, the limits being inclusive. The expression "0 < y < 1" means that y is a number ranging from 0 to 1, the lower limit "0" being inclusive and the upper limit "1" being excluded. This compound can be obtained according to the process of the invention. Preferably, B is an element of group HB and / or a transition metal. When x is equal to 0, a cobalt-containing compound of formula Ca3Zn3- a Co aTe2Oi2 where 0 < a < 1, is a particularly preferred object of the invention. Similarly, the expression “0 < a < 1” means that a is a number varying from 0 to 1, the lower limit “0” being excluded and the upper limit “1” being included.

[0063] When A is, or includes, cerium, the resulting compound is a phosphor emitting at visible wavelengths (especially yellow-green). It can be used in the development of new white inorganic light-emitting diodes by combination with other compounds. It can also be used as a new scintillator material (converter of ionizing radiation (X-rays or gamma rays) into visible light). The use of one or more specific rare earth ions, especially an ion other than cerium, in particular Nd 3+ , Yb 3+ , Er 3+ , Tm 3+, can also be used for stimulated emission of coherent light (laser emission), with the possibility of partially substituting the Ca2+ with an alkaline couple M+ / Rare Earth +3. A compound of formula (I) and its use as described above is therefore an object of the invention.

[0064] The crystalline compound of formula (I) or obtained according to the process of the invention can advantageously be used in the field of colored inorganic pigments and / or jewelry. In this case, element B comprises, or is made up of cobalt, which makes it possible to obtain a crystalline compound of intense blue color, and / or turquoise blue.

[0065] The crystalline compound of formula (I) or obtained according to the process of the invention can advantageously be used in the field of optics, for the conversion and / or emission of light. In this case, element A preferably comprises, or consists of, a rare earth.

[0066] Brief description of the figure

[0067] The invention will be better understood by reading the following examples.

[0068] Figure 1 is one of the schematic representations of the crystal structure of the compound Ca3ZnaTe20i2 and a possible alternation of oxygenated polyhedra.

[0069] Figure 2 represents the diagram of the X-ray diffraction analysis of the compound obtained in Example 1.

[0070] Figure 3 represents the particle size distribution of the grains of the powder obtained in Example 1.

[0071] Figure 4 shows the X-ray diffraction pattern of the compound obtained in Comparative Example 1.

[0072] Example of implementation of the invention

[0073] Example 1: Preparation of a crystalline tellurate from a cationic aqueous solution comprising tellurium, zinc and calcium at pH 9.0 Preparation of the cationic aqueous solutions and ammoniacal pH buffer

[0074] First, a concentrated aqueous cationic solution with a volume of 20 mL is prepared. The starting precursors are: calcium nitrate tetrahydrate Ca(NO3)2.4H2O (Sigma Aldrich, C1396-500G, 99.0%), zinc nitrate hexahydrate Zn(NC>3)2.6H2O (Sigma Aldrich, 96482-500G, 99.0%) and orthotelluric acid Te(OH)e (Sigma Aldrich, 86375-100G, 99.0%). The concentrations chosen for the Ca cations 2+ , Zn 2+ and you 6+ are respectively equal to 0.75 mol / L, 0.75 mol / L and 0.5 mol / L and the mass of each reagent to be introduced is determined. A precision balance (U(m)=0.0001g) is used to weigh the masses of precursors:

[0075] - m(Ca(NO3)2.4H2O) = 3.5421 g

[0076] - m(Zn(NO3)2.6H2O) = 4.4621 g

[0077] - m(Te(OH)6) = 2.2964 g

[0078] The precursors are introduced into a 50 mL beaker with 20 mL of reverse osmosis water. The mixture is then stirred and homogenized at room temperature and atmospheric pressure. The stirring time depends on the dissolution of the solids (the precursors) in the water but leads after a few minutes to a clear solution. This cationic aqueous solution is then introduced into the graduated burette (or a dropping funnel) in order to carry out an acid-base neutralization. The acid-base neutralization carried out consists of adjusting the pH of the cationic aqueous solution to pH 9.00. The cations in the neutralized cationic aqueous solution co-precipitate.

[0079] In a second step, a 20 mL ammonia buffer solution with a total concentration of C = 2 mol / L is prepared from a mixture of ammonium nitrate NH4NO3 (Sigma Aldrich, A9642-500G, 99.0%) and 35% ammonia NH4OH. A mass of ammonium nitrate (n NhLNOs) = 2.1586 g weighed using the precision balance is introduced into a 50 mL tall beaker, also called a reaction beaker, with a volume of osmosis water of 18.5 mL. A volume V = 1.5 mL of ammonia (previously prepared solution with a mass content t = 35% and density d = 0.88) is taken and added to the reaction beaker with a graduated glass pipette (0.01 mL graduation). The buffer solution is stirred on a magnetic stirrer at room temperature until a homogeneous solution is obtained. As with the cationic aqueous solution, the stirring time depends on the dissolution of the solids (ammonium and ammonia nitrates) in the water.

[0080] pH conditions to be respected

[0081] To obtain a crystalline tellurate with a yafsoanite-type structure without a secondary phase, at the end of the neutralization, and more precisely at the end of the coprecipitation step including the neutralization and the maturation step, the pH in the reaction beaker is kept constant at a value of 9.00. A pH meter (Eutech Instruments pH 700 model) allows the pH of the solution contained in the beaker to be monitored in situ during the neutralization. The measuring electrode needs to be calibrated regularly using commercial buffer solutions (OAKTON) whose pH standards are equal to 4.01, 7.00 and 10.01 respectively. The pH of the cationic aqueous solution and the ammonia buffer are also measured before the start of the synthesis.

[0082] Start of the reverse coprecipitation reaction

[0083] The cationic aqueous solution is introduced into a 25 mL graduated burette. The reaction beaker containing the ammonia buffer solution is placed on a magnetic stirrer (the stirring speed is set at 500 rpm) and positioned below the graduated burette containing the cationic aqueous solution. The measuring electrode was immersed in the reaction beaker (or reaction beaker) for the entire duration of the acid-base neutralization in order to monitor the evolution of the pH.

[0084] The stopcock of the graduated burette containing the cationic aqueous solution is opened so as to have a flow rate of one drop per second. The more cationic aqueous solution is introduced into the buffer solution, the more the pH decreases since an acid-base neutralization is carried out by adding an acidic cationic aqueous solution. Concentrated ammonia at 35% (v / v) was also added dropwise during the neutralization in order to limit the variation in pH partly absorbed by the buffering capacity of the mixture and to readjust the pH to pH 9.00. At the beginning of the neutralization, one or two drops of NH4OH are sufficient to maintain the pH of the solution at 9.00. However, as the neutralization progresses, it is necessary to add more and more drops of ammonia in order to maintain the pH at 9.00. The total volume of ammonia added to the reaction beaker during neutralization is measured: it can vary between 8.0 and 11.0 mL.At the end of neutralization and before the 6-hour maturation step at room temperature and atmospheric pressure, the pH value in the reaction beaker is checked. At the end of neutralization, crystalline tellurate suspended in the liquid medium in the reaction beaker is obtained. The measuring electrode is then removed and a parafilm is placed on the reaction beaker for the duration of maturation.

[0085] At the end of the maturation stage (6 hours), the pH in the reaction beaker is measured again.

[0086] Separation of crystalline tellurate from the liquid medium The suspension of crystalline tellurate in the liquid medium is then filtered by the vacuum filtration method (or Büchner filtration). A sintered glass filter crucible with porosity 4 is used. This is rinsed 3 times with distilled water before the start of filtration.

[0087] A first portion of the suspension is poured into the crucible and filtered. Once no more liquid remains, the second remaining portion is then poured into the funnel and the beaker is rinsed with fresh osmosis water to recover all of the solid formed. When the suspension is completely filtered, the solid (the crystalline tellurate) is rinsed twice with 20 mL of osmosis water and twice with 20 mL of ethanol (faster drying thereafter). Throughout the filtration, the filtrate (the liquid medium) remained clear (no cloudiness) and transparent.

[0088] The solid (crystalline tellurate) is then recovered in the form of a powder and placed in a crystallizer. It is dried for 24 hours in an oven set at 90 °C. The individual grains of the powder are of the order of 50 to 100 nm. These agglomerate a little to give agglomerates whose median size (d50) is very close to the micron. Thus, after drying and grinding to obtain a fine powder, the mass of solid obtained is approximately equal to 3.7 g. The solid and dry compound (crystalline tellurate) will finally be stored in a dry box.

[0089] Specifically, the powder was analyzed by scanning electron microscopy and X-ray diffraction (Bruker D8 Advance equipped with a LynxEyeXE detector). The resulting diagram, shown in Figure 2, shows the production of a pure product of composition Ca3Zn3Te20i2 with a structure typical of yafsoanite. No secondary phase was synthesized. The analysis of the particle size distribution of the grains of the obtained powder was then carried out by laser diffraction, also known as static light scattering, with a Horiba Partica La-950. Approximately 250 mg of powder was suspended in 40 mL of water. The suspension was sonicated for one minute before being measured with a Horiba Partica La-950. The ultrasonic treatment prevents powder sedimentation and ensures good reproducibility of the results.This result, represented in Figure 3, shows that the powder obtained is monodisperse with a dso of 1.3 pm.

[0090] These results show that the process according to the invention implemented at room temperature (and therefore without calcination step) makes it possible to obtain a pure crystalline tellurate, without secondary phase, comprising in addition to tellurium, two cations of different chemical families, in the form of a powder of homogeneous density having individual grains of nanometric diameter and whose agglomerates have a size of the order of a micron. of a crystalline tellurate from a solution ellure, zinc and calcium at pH 4.0

[0091] The process implemented in Example 1 was carried out while maintaining the pH at 4.00. As in Example 1, the powder obtained was analyzed by X-ray diffraction, the resulting diagram of which is shown in Figure 4.

[0092] The results show the production of a pure powder of a crystalline tellurate of composition ZnsTeOe with a dagenaisite structure. No secondary phase was synthesized. The crystalline tellurate obtained does not contain calcium although this element was present in the cationic aqueous solution.

[0093] These results (Example 1 and Comparative Example 1) show that the pH value during coprecipitation influences the structure and composition of the product obtained. The pH value influences the retention of metals in crystalline tellurates, the crystalline compound obtained not containing calcium.

[0094] Comparative Example 2: Solid-state preparation of a crystalline tellurate comprising tellurium, zinc and calcium

[0095] A crystalline tellurate comprising zinc and calcium of composition Ca3ZnaTe20i2 was prepared according to a commonly used method, by solid route.

[0096] The starting precursors are: calcium carbonate CaCOs (CAS 471-34-1), zinc oxide ZnO (CAS: 1314-13-2) and tellurium dioxide TeC>2 (CAS 7446-07-3). These powders were weighed and mixed in stoichiometric proportions and then ground at high energy with a Pulverisette 7 planetary mill (Fritsch). The resulting mixture was calcined at 750°C for 2 hours, then ground manually and then calcined again at 950°C for 2 hours. The powder obtained after these two calcination steps was ground at high energy by the planetary mill.

[0097] The powder was analyzed as in Example 1 and the results obtained show the production of a pure product of composition Ca3Zn3Te20i2 having a structure typical of yafsoanite.

[0098] The particle size distribution of individual grains of the powder obtained before and after the high-energy grinding step was analyzed by laser diffraction as in Example 1. The median size (dso) of the agglomerates of the powder before high-energy grinding is 135.90 pm while the dso of the powder after high-energy grinding is 2.95 pm.

[0099] This result shows that the process according to the invention (Example 1, dso=1.3 pm) makes it possible to obtain a finer powder than the solid-state process (Comparative Example 2) while being more easily implemented, particularly at room temperature and without the use of a high-energy grinder.

[0100] Example 2: Preparation of a crystalline tellurate from a cationic aqueous solution comprising tellurium, zinc, calcium and cobalt of composition Ca3Zn27Coo.3Te20i2

[0101] The cationic aqueous solution and buffer solution are prepared as in Example 1, maintaining the pH at pH 9.00, using as starting precursors calcium nitrate tetrahydrate, zinc nitrate hexahydrate, orthotelluric acid and cobalt nitrate hexahydrate. The concentrations of the Ca cations 2+ , Zn 2+ , Co 2+ and you 6+ are respectively equal to 0.75 mol / l, 0.675 mol / L, 0.075 mol / L and 0.5 mol / L. The powder obtained was analyzed by X-ray diffraction. The results show that a pure tellurate of yafsoanite structure is obtained in the form of a sky blue powder, without any calcination step, the tellurate having crystallized at room temperature.

[0102] This result shows that the process according to the invention makes it possible to prepare a pure crystalline tellurate, without secondary phase comprising, in addition to tellurium, 3 cations from different chemical families.

[0103] The powder obtained was then subjected to a heat treatment step at 725°C for 15 min in air or at 725°C under primary vacuum corresponding to some 10' 2 mbar. At the end of the heat treatment in air, the powder obtained is intense blue while at the end of the heat treatment in primary vacuum, the powder obtained is intense turquoise blue.

[0104] These results also show that the process according to the invention makes it possible to obtain a pure crystalline tellurate powder with low cobalt contents having an intense blue coloration. These results also show that the operating conditions of the heat treatment make it possible to modulate the coloration of the powder obtained.

Claims

Claims 1. A process for the manufacture of a crystalline tellurate comprising at least zinc; said process comprising the following steps: providing a cationic aqueous solution comprising Zn cations 2+ and you 6+ ; coprecipitating said crystalline tellurate in a liquid medium by applying a pH to said cationic aqueous solution, said pH being maintained at a value ranging from 5.00 to 10.00; and, optionally, separating said crystalline tellurate from said liquid medium.

2. The method of claim 1, wherein the cationic aqueous solution comprises Zn cations 2+ and you 6+ and at least one other cation.

3. Method according to claim 1 or 2 in which the cationic aqueous solution comprises Zn cations 2+ and you 6+ and at least two other cations.

4. A method according to claim 2 or 3, wherein the other cation corresponds to another element, this other element being an alkaline earth metal, an alkali, a transition metal or a rare earth.

5. A method according to any one of claims 2 to 4, wherein the zinc and the other cation, or the zinc and the two other cations, belong to different chemical families.

6. A method according to any one of claims 1 to 5, wherein the cationic aqueous solution comprises Zn cations 2+ and you 6+ and the respective cations of cobalt and / or calcium.

7. A method according to any one of claims 1 to 6, wherein the cationic aqueous solution is obtained using a nitrate salt or a chloride salt.

8. Method according to any one of claims 1 to 7, wherein said crystalline tellurate is obtained in the form of a microcrystalline powder, and preferably a nanocrystalline powder.

9. Method according to any one of claims 1 to 8, in which the crystalline tellurate is obtained in the form of a mixture of crystalline tellurates of different structures, for example of the yafsoanite or dagenaisite type; the proportion of tellurate of one structure being preferably greater than or equal to 90%, very preferably greater than or equal to 95%, more preferably greater than or equal to 99%.

10. Method according to any one of claims 1 to 9, in which the crystalline tellurate is obtained without secondary phase and has a yafsoanite type structure, in other words at least 99%, preferably 100%, of the tellurates obtained have the same structure, this structure being a yafsoanite type structure.

11. Method according to any one of claims 1 to 10, comprising a step of heat treatment of the tellurate obtained at a temperature below 1000°C for a short time, preferably between 5 and 30 minutes.

12. A method according to any one of claims 1 to 11, wherein said crystalline tellurate is obtained without the application of a calcination step.

13. Process according to any one of claims 1 to 12, wherein the coprecipitation step is carried out at a temperature ranging from 5°C to 60°C and preferably at room temperature.

14. Process according to any one of claims 1 to 13, wherein the coprecipitation step is carried out under a pressure ranging from 90 to 120 kPa, and preferably at atmospheric pressure.

15. A method according to any one of claims 1 to 14, wherein said pH is maintained at a pH ranging from 8 to 10 and preferably is maintained at a pH of 9.0 + / - 0.

3.

16. The method according to any one of claims 1 to 15, wherein the coprecipitation comprises a step of bringing the cationic aqueous solution into contact with an alkaline solution and / or buffer solution, preferably by adding the cationic aqueous solution to the buffer solution, said buffer solution advantageously comprising ammonia and / or an NH4NO3 / NH4OH mixture.

17. A method according to any one of claims 1 to 16, wherein a variation of said pH during the coprecipitation is minimized, in particular by carrying out a time- and volume-controlled addition of the cationic aqueous solution to the buffer solution.

18. Method according to any one of claims 1 to 17, wherein the coprecipitation step comprises a maturation step subsequent to the step of applying said pH and / or comprises a step of washing and / or drying said crystalline compound.

19. A crystalline compound, of formula (I) (Cai- xAx)3(Zn(iy)By)3Te2Oi2 , where A is an element of group 11 A and / or an alkali couple M + / rare earth RE 3+ , where 0 < x < 1 ; B is an element of group HB and / or a transition metal, in particular Cu 2+ , Co 2+ , Neither 2+ , with 0 < y < 1, and excluding yafsoanite. SUBSTITUTION SHEET (RULE 26)

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