Carbon nitride compounds and methods

Abstract

A crystalline carbon nitride compound has the general formula C3N4, and has the Pearson symbol: oP28

Description

[0001]Carbon Nitride Compounds and Methods Field The present disclosure relates to a carbon nitride compound, and in particular toa novel crystalline nitride compound and to methods of preparation thereof. Diamond is the hardest substance known. It also has other appealing physicalproperties including optical transparency, high thermal conductivity, and high electricalresistivity. This makes diamond an industrially important material, used in varied applications such as metallurgy, mining, and electronics. The synthesis of diamond, or of alternative crystalline compounds exhibiting similar properties to those of diamond, has been the subject of research for several decades. For example, cubic boron nitride (c-BN) and diamond are similar: both are super- hard and are stable under high pressure and high temperature conditions. Boron nitridehas excellent thermal and chemical stability, but is not as hard as diamond.Binary carbon nitride compounds are inorganic compounds consisting of carbon and nitrogen atoms. Carbon nitrides featuring three-dimensional frameworks of CN4tetrahedra are one of the great aspirations of materials science, expected to have ahardness comparable to diamond. Various theoretical crystalline forms of carbon nitridehave been predicted, but synthesizing them has proved difficult.Laniel et al (Synthesis of Ultra-Incompressible and Recoverable Carbon NitridesFeaturing CN4 Tetrahedra, Advanced Materials, vol 36(3), 2024) discloses the synthesisof certain crystalline forms of carbon nitride, including tI14-C3N4, hP126-C3N4, and tI24-CN2. These compounds were produced above at least 72 GPa, and in some casesabove 100 GPa. These compounds were named using a conventional nomenclaturecommonly called “Pearson symbol” or “Pearson notation”, followed by their stoichiometry. In this nomenclature, a crystal is identified using a notation consisting oftwo letters (in italics) and a number, in which:- The first letter (in lower case) specifies the crystal family;- The second letter (in upper case) specifies the centring type;- The number specifies the number of atoms in each unit cell.55541016-1 It is an object of the present invention to obviate and / or mitigate the limitations and / or disadvantages associated with the prior art and / or with conventional methods and systems. It is an object of the present invention to provide novel and improved carbon nitride compounds. It is an object of the present invention to provide a method of manufacturing a carbon nitride compound, and in particular a compound having a three-dimensionalframework of CN4 tetrahedra, at lower pressures.Summary The present invention is based on the surprising findings that an alternative crystalline form of carbon nitride exhibiting one or more desirable physical properties, may be synthesised. The present invention is based on the further surprising findings that at least one alternative crystalline form of carbon nitride may be synthesised at a lower pressure than expected from the prior art. According to a first aspect there is provided a crystalline carbon nitride compound having the general formula C3N4, and having the Pearson symbol: oP28. It will be appreciated that the above general formula C3N4corresponds to the stoichiometry of the crystalline carbon nitride compound, as carbon nitride crystals aretypically made of repeating unit cells having a specific crystalline structure.The compound may define a three-dimensional framework of CN4 tetrahedra.The compound may have an orthorhombic crystal system.The compound may have a primitive centring type. The compounds may have an orthorhombic primitive Bravais lattice.The compound may have 28 atoms in a unit cell. The compound may have 4 formula units per unit cell. A / the unit cell may comprise or may consist of 12 carbon atoms and 16 nitrogen atoms. The compound may have a Pnnm space group and / or a space group #58.The compound may have one or more of the following lattice parameters: (i) a = 8.644(4), b = 7.3913(8) and c = 2.296(3) Å; and / or V = 146.7(3) Å3, ata pressure of 73 GPa and a wavelength of 0.2910 Å;(ii) a = 8.5741(8), b = 7.266(4) and c = 2.280(3) Å; and / or V = 142.1(2) Å3, ata pressure of 94 GPa and a wavelength of 0.29521Å;55541016-1(iii) a = 8.7516(8), b = 7.406(4) and c = 2.314(3) Å; and / or V = 150.0(2) Å3, ata pressure of 53 GPa and a wavelength of 0.3738 Å;(iv) a = 8.8035(8), b = 7.549(4) and c = 2.361(3) Å; and / or V = 156.9(2) Å3, ata pressure of 35 GPa and a wavelength of 0.3738 Å;(v) a = 8.9896(8), b = 7.674(4) and c = 2.373(3) Å; and / or V = 163.7(2) Å3, ata pressure of 17 GPa and a wavelength of 0.3738 Å;(vi) a = 9.1128(8), b = 7.819(4) and c = 2.396(3) Å; and / or V = 170.7(2) Å3, ata pressure of 4 GPa and a wavelength of 0.3738 Å;(vii) a = 9.1296(8), b = 7.851(4) and c = 2.400(3) Å; and / or V = 172.0(2) Å3, atambient pressure and / or at 1 atm, and a wavelength of 0.3738 Å,all measured at ambient temperature. In the above, the number in parentheses represents the uncertainty on the decimal it follows. For example, 8.644(4) means 8.644 + / - 0.004.The compound may have an X-ray powder diffraction pattern comprising one or more peaks at a 2^ ± 0.2 selected from: 4.69°, 5.43°, 5.46°, 5.94°, 7.20°, and 8.91°, or combinations thereof, as measured by X-ray powder diffraction at a wavelength of 0.3738 Å, whenmeasured at ambient pressure (e.g.1 atm) and ambient temperature (e.g. about 20°C).The compound may have an X-ray powder diffraction pattern comprising two or more peaks at a 2^ ± 0.2 selected from: 4.69°, 5.43°, 5.46°, 5.94°, 7.20°, and 8.91°, as measured by X-ray powder diffraction at a wavelength of 0.3738 Å, when measured at ambient pressure (e.g.1atm) and ambient temperature (e.g. about 20°C). The compound may have an X-ray powder diffraction pattern comprising two peaks at a 2^ ± 0.2 selected from: 4.69° and 7.20°; or4.69° and 8.91°; or7.20° and 8.91°,as measured by X-ray powder diffraction at a wavelength of 0.3738 Å, when measured at ambient pressure (e.g.1atm) and ambient temperature (e.g. about 20°C). The X-ray powder diffraction pattern may comprise further peaks at one or more, or all, of a 2^ ± 0.2 of: 55541016-15.43°, 5.46°, and 5.94° as measured by X-ray powder diffraction at a wavelengthof 0.3738 Å, when measured at ambient pressure (e.g.1atm) and ambient temperature (e.g. about 20°C). Advantageously, the compound was found to provide a superior alternative over cubic boron nitride (c-BN). Without wishing to be bound by theory, the compound isexpected to share with c-BN many properties that diamond lacks (i.e. inertness when incontact with Fe) but is calculated to be harder. The use of oP28-C3N4 as a superhard solid opens up a potentially more cost efficient and effective material for machining as well as protective coating. Its small band gap and its possible variability of chemical composition also offerspotential for advantageous optoelectronic properties. It also exhibits high energy density,i.e. above that of trinitrotoluene (TNT), and is environmentally-friendly upon detonation.Thus the compound may find use in one or more of the following applications: -Superhard and / or ultraincompressible materials, e.g. as protective coatings,machining tools, or the like; -High energy density material, e.g. environmentally-friendly explosives, forexample in mining. Due to its extreme hardness this compound has the potential to replace diamondin certain machining / cutting applications. Advantageously, (compared to diamond), it isbelieved to be non-reactive with iron, meaning it may be a better option when workingwith iron and iron containing metals such as steel. According to a second aspect there is provided a method of preparing a carbon nitride compound comprising: providing a crystal precursor in a diamond anvil cell (DAC); applying a target pressure in the DAC, wherein the target pressure is less than about 100 GPa; and heating the crystal precursor. The method may comprise preparing a carbon nitride compound having the general formula C3N4, and having the Pearson symbol: oP28. The method may comprise preparing the carbon nitride compound according to the first aspect. 55541016-1 The crystal precursor may be selected from the group consisting of cyanuric triazide (CTA), Pyrene + N2, Boron-doped diamond + N2, Boron-doped diamond + cyanuric triazide (CTA), and Anthracene + N2. In an embodiment, the crystal precursor may comprise or may consist of cyanuric triazide (CTA). The method may comprise laser-heating the crystal precursor. The method may comprise temperature quenching the crystal precursor, typically after laser-heating. The method may comprise heating the crystal precursor at a temperature in therange of about 1800 K to less than 2700 K, e.g. about 2000 K to about 2600 K. It will beappreciated that the target temperature may depend on the particular crystal precursor used, and / or the power of the laser being used. Without wishing to be bound by theory, it is believed that, when using a given crystal precursor, for example CTA, heating the crystal precursor at a relatively low temperature, e.g. between about 2000K and 2700K, may yield oP28-C3N4rather other crystalline forms such as hP126-C3N4. The method may comprise applying a target pressure in the DAC, wherein thetarget pressure is between about 50 GPa and less than about 100 GPa, e.g. betweenabout 50 GPa and about 99 GPa, e.g. between about 60 GPa and about 98 GPa, e.g.between about 70 GPa and about 95 GPa.In an embodiment, the method may comprise using a Nd:YAG laser with a wavelength of 1064 nm. The method may comprise analysing the compound. The method may comprise analysing the compound in a synchrotron. The method may comprise analysing the compound by single-crystal X-ray diffraction (SCXRD). The method may comprise solving the crystal structure of the compound, e.g. using data obtained from the SCRXD analysis. The method may comprise analysing the compound by SCXRD, at a plurality of pressures. The method may comprise analysing the compound by SCXRD, at a plurality of decreasing pressures. The method may comprise analysing the compound by SCXRD, at a plurality of pressures between about 100 GPa and ambient pressure or 1 atm, optionally between about 95 GPa and ambient pressure or 1 atm. The method may comprise determining pressure-unit cell volume data, e.g. a pressure-unit cell volume graph, from the SCXRD data. 55541016-1 For the avoidance of doubt, any feature described in respect of any aspect of the invention may be applied to any other aspect of the invention, in any appropriate combination. For example, method features may be applied to composition features and vice versa. Brief Description of the Drawings The present invention will now be further described in detail and with reference to the figures in which: Figure 1 is a schematic cross-sectional view of a conventional Diamond Anvil Cellused to prepare a compound according to the invention; Figure 2 is a graph representing Le Bail analysis for the powder XRD (pXRD)pattern containing oP28-C3N4and oP8-CN at ambient conditions; Figure 3 shows a crystal structure (unit cell) of oP28-C3N4at 73 GPa, where the C and N atoms are represented by grey and blue spheres, respectively; Figure 4 shows experimentally measured C-N bond lengths (4a), and N-C-N andC-N-C bond angles (4b), with pressure in oP28-C3N4;Figure 5 shows structural data collected on oP28-C3N4 upon its decompressionto ambient conditions. Unit cell volume (a), average volume of the CN4tetrahedra (b) and normalized tetrahedra volume (c) with respect to pressure. In all graphs, the black squares, the black line, the dashed blue line and the red full line represent the experimental data points, the second order Birch-Murnaghan equation of state fit to the experimental data, the data calculated from DFT and the experimental data on diamond, respectively; Figure 6 shows pXRD patterns for oP28-C3N4, compared to pXRD patterns for other carbon nitride crystals (oP8-CN,hP126-C3N4,tI14-C3N4,tI24-CN2); Figure 7 shows theoretical calculations on the thermodynamical stability of oP28-C3N4 vs hP126-C3N4, depending on temperature.Detailed of the In the present disclosure, reference is made to a number of terms, which have the meanings provided below, unless a context indicates to the contrary. The nomenclature used herein for defining compounds, in particular the compounds according to the invention, is in general based on the rules of the IUPAC organisation for chemical compounds, specifically the “IUPAC Compendium of Chemical Terminology 55541016-1 (Gold Book)”. For the avoidance of doubt, if a rule of the IUPAC organisation is in conflict with a definition provided herein, the definition herein is to prevail. Furthermore, if a compound structure is in conflict with the name provided for the structure, the structure is to prevail. The term “comprising” or variants thereof is to be understood herein to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The term “consisting” or variants thereof is to be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, and the exclusion of any other element, integer or step or group of elements, integers or steps. The term “about” herein, when qualifying a number or value, is used to refer to values that lie within ± 5% of the value specified. For example, if a temperature is specified to be about 5 to about 13 °C, temperatures of 4.75 to 13.65 °C are included. Reference to physical states of matter (such as liquid or solid) refer to the matter’s state at 25 °C and atmospheric pressure unless the context dictates otherwise. As explained above, the present inventors have discovered that a new crystallineform of carbon nitride exhibiting desirable physical properties, may be synthesised. The present invention is based on the further surprising findings that the alternative crystalline form of carbon nitride may be synthesised at a lower pressure than expected from the prior art. Figure 1 shows a conventional Diamond Anvil Cell (DAC) 100 used to prepare acompound according to the invention. The DAC 100 has an upper diamond anvil 111and a lower diamond anvil 112 on either side of a chamber 130. The chamber is sealedby a metal gasket 140. The shape of the diamond anvils 111,112 allows the application of large pressures to a sample 120 within the chamber 130 by application of a force F on the relatively large outer surfaces of the diamond anvils 111,112. The diamond anvils 111,112 are also transparent to radiations that can either heat the sample 120 during preparation of a crystal compound (e.g. laser radiation), or to radiations that may be used to solve or analyse the sample, typically X-ray radiation. 55541016-1 Examples Experimental Sample preparation Anumber of BX90 type diamond anvil cells (DACs) were equipped with 120 μmdiamond anvil culets. The DACs were loaded with a precursor. The following precursors were used, as follows: -cyanuric triazide (CTA, C3N12);- Pyrene + N2;- Boron-doped diamond + N2;- Boron-doped diamond + cyanuric triazide (CTA);- Anthracene + N2.Pressure and Temperature Measurements The pressure inside of the sample chamber was measured using the first order Raman mode of diamond and verified using the calibrated diffraction lines of the rhenium gasket. Laser heating was performed with double-sided Nd:YAG lasers (λ = 1064 nm) at a laboratory at the Bayerisches Geoinstitut (BGI), at the P02.2 beamline of the Deutsches Elektronen-Synchrotron (DESY) as well as at the GSECARS beamline of theAdvance Photon Source (APS) using CTA as a laser absorber. Temperatures weremeasured with an accuracy of ±200 K, using the thermoemission produced by the laser- heated samples. Synchrotron experiments The X-ray diffraction studies were done at the ID27 beamline (λ = 0.3738 Å) of the Extreme Brilliant Source European Synchrotron Radiation Facility (EBS-ESRF), at the P02.2 beamline (λ = 0.2910 Å) of DESY as well as at the GSECARS beamline (λ = 0.2952 Å) of the APS. To determine the position of the polycrystalline sample on which the single-crystal X-ray diffraction (SCXRD) acquisition is obtained, a full X-ray diffraction mapping of the pressure chamber was carried out. The sample position displaying the most and the strongest single-crystal reflections belonging to the phase of interest was chosen for the collection of single- crystal data, collected in step-scans of 0.5° from −38° to +38°. 55541016-1 The CrysAlisProsoftware was utilized for the single crystal data analysis. The analysis procedure includes the peak search, the removal of the diamond anvils’ and other ‘parasitic’ signal contributions, finding reflections belonging to a unique single crystal (achieved using DAFi), the unit cell determination, and the data integration. The crystal structures were then solved and refined using the OLEX2 andJANA2006 software. OLEX2 was employed to obtain a preliminary structural model andJANA2006 to cull parasitic reflections (e.g. diamonds, other single-crystals) and obtaina final structural model. The powder X-ray diffraction data was analysed using Dioptas, and Le Bail refinements performed with the FullProf software. Results Precursor # 1: cyanuric triazide (CTA, C3N12) A first sample (sample #1) of CTA was initially compressed to a pressure of 37 GPa and laser-heated. Due to the sample producing an intense flash of light, temperatures could not be accurately measured but are estimated to be above 3000 K. The heated portion of the sample visually changed, becoming transparent. An X-ray diffraction map revealed the decomposition of CTA into molecular nitrogen and diamond. Sample #1 was then further compressed to 59 GPa and laser-heated again, with the maximum measured temperature being of 2600(200) K. The heated areas again became optically transparent, and the pressure increased to 73 GPa. Sample mapping with X-ray diffraction unveiled the presence of known phases, i.e. ζ-N2, oP8-CN and CTA, alongwith sharp diffraction spots belonging to an unknown phase. To determine the crystal structure of the new compound, single-crystal X-ray diffraction (SCXRD) data were collected on the best sample positions. Given the high single-crystal data quality, the analysis of these SCXRD measurements amounted to a structure model and refinement at 73 GPa, as exemplified in Table 1, revealing the formation of a novel carbon nitride with a unique crystal structure. Table 1: Crystallographic data for oP28-C3N4 at 73 GPa. oP28-C3N4Exp. Pressure (GPa) 73Space group, # PnnmZ 4a (Å) 8.644(4)55541016-1b (Å) 7.3913(8)c (Å) 2.296(3)V (Å3) 146.7(2)Refinement details Wavelength (λ, Å) 0.2910μ (mm-1) 0.087# measured / independent 794 / 364 (245) reflections (I ≥ -13σ) (sin θ / λ)max (Å ) 1.034Rint (%) 8.46R1(%) 5.67 wR2 (%) 6.09R1 (all data, %) 8.04wR2 (all data, %) 6.53Goodness of fit 1.58No. of parameters 34Δρmin, Δρmax (eÅ-3) -0.63, 0.63Atomic positions Atom Wyckoff position Fractional atomic coordinates (x; y;Uiso(Å2) z) Exp. C1 4g 0.2791(4); 0.4655(2); 0 0.0055(4)C2 4g 0.1051(4); 0.2062(3); 0 0.0061(4)C3 4g 0.3824(4); 0.1604(2); 0 0.0054(4)N1 4g 0.5229(3); 0.2493(2); 0 0.0057(6)N2 4g 0.2550(4); 0.2820(2); 0 0.0088(7)N3 4g 0.1356(3); 0.0152(2); 0 0.0057(6)N4 4g 0.1289(3); 0.5476(2); 0 0.0053(6)This carbon nitride has an orthorhombic unit cell (Pnnm space group, #58) withlattice parameters a = 8.644(4), b = 7.3913(8) and c = 2.296(3) Å (V = 146.7(3) Å3). Theunit cell contains a total of 28 atoms, 12 carbon and 16 nitrogen atoms which are all onthe 4g Wyckoff position, resulting in three and four crystallographically unique atoms ofC and N, respectively. Given the atomic ratio, this solid has the C3N4 stoichiometry and, accordingly to the Pearson notation, is named oP28-C3N4. Remarkably, its structure (as illustrated in Figure 3) is composed exclusively of heteroatomic C-N bonds, with all carbon and nitrogen atoms being four-fold and three- fold coordinated, respectively, forming a 3D polymeric network of corner-sharing CN4tetrahedra. These CN4units assemble to produce, according to F. Liebau’snomenclature, sets of four dreier-rings (i.e. three-rings), together creating sechser-rings(i.e. six-rings). At 73 GPa, the C-N bond lengths range from 1.372(4) to 1.436(4) Å, foran average value of 1.409(3) Å, which can be attributed to a single bond. All carbon 55541016-1 atoms are sp3-hybridized, with the average N-C-N bond angle being of 109.4(2)°. Thenitrogen atoms, however, display a mixture of sp2- and sp3-hybridization. Indeed, the N1and N2 atoms form perfectly flat NC3 units with have an average C-N-C bond angle of 120.0(2)° while the N3 and N4 atoms have a corresponding value of 108.9(2)° with a bent NC3 unit. Moreover, the latter have slightly longer average C-N bond lengths of 1.423(3) Å (values between 1.413(2) and 1.436(4) Å) while the former have an average C-N bond length of 1.394(3) Å (values between 1.372(4) and 1.421(3) Å)—matching with the expectation of longer bonds for sp3-hybridized atoms compared to sp2-hybridized atoms. A second sample (sample #2) of CTA was prepared using a similar methodology, but pressurizing the CTA precursor to 94 GPa after laser-heating. The crystallographic data for oP28-C3N4 at 94 GPa are shown in Table 2.Table 2: Crystallographic data for oP28-C3N4at 94 GPa. oP28-C3N4 Exp. Pressure (GPa) 94Space group, # PnnmZ 4a (Å) 8.5741(8)b (Å) 7.266(4)c (Å) 2.280(3)V (Å3) 142.1(2)Refinement details Wavelength (λ, Å) 0.29521μ (mm-1) 0.09# measured / independent 409 / 195 (94) reflections (I ≥ -13σ) (sin θ / λ)max (Å ) 0.876Rint (%) 5.96R1 (I > 3σ, %) 4.42 wR2 (I > 3σ, %) 4.03R1 (all data, %) 15.72wR2 (all data, %) 4.62Goodness of fit 1.41No. of parametein max -rs 22Δρm , Δρ (eÅ3) -1.42, 1.42Atomic positions Atom Wyckoff position Fractional atomic coordinates (x; y; z) Uiso (Å2)Exp. C1 4g 0.2783(11); 0.4644(4); 0 0.0088(8)C2 4g 0.1036(10); 0.2012(4); 0 0.0104(9)55541016-1C3 4g 0.3828(9); 0.1558(4); 0 0.0079(9)N1 4g 0.5225(8); 0.2502(3); 0 0.0075(7)N2 4g 0.2550(8); 0.2820(3); 0 0.0082(8)N3 4g 0.1346(8); 0.0130(4); 0 0.0081(7)N4 4g 0.1285(7); 0.5455(3); 0 0.0084(8)The oP28-C3N4solid was progressively decompressed, with additional single- crystal data collected at 53 GPa, 35 GPa, 17 GPa, 4 GPa and 1 bar (as shown in Tables 3 to 7, respectively). Table 3: Crystallographic data for oP28-C3N4 at 53 GPa. oP28-C3N4Exp. Pressure (GPa) 53Space group, # PnnmZ 4a (Å) 8.7516(8)b (Å) 7.406(4)c (Å) 2.314(3)V (Å3) 150.0(2)Refinement details Wavelength (λ, Å) 0.3738μ (mm-1) 0.104# measured / independent 447 / 195 (135) reflections (I ≥ x-13σ) (sin θ / λ)ma (Å ) 0.891Rint (%) 2.36R1(%) 6.52 wR2 (%) 6.31R1 (all data, %) 9.02wR2 (all data, %) 6.42Goodness of fit 3.07No. of parameters 22Δρmin, Δρmax (eÅ-3) -041, 0.39Atomic positions Atom Wyckoff position Fractional atomic coordinates (x; y; z) Uiso (Å2)Exp. C1 4g 0.2784(3); 0.0343(10); 0 0.0109(7)C2 4g 0.1054(3); 0.2046(9); 0 0.0102(6)C3 4g 0.3818(3); 0.1600(10); 0 0.0118(6)N1 4g 0.5220(3); 0.2497(7); 0 0.0102(6)N2 4g 0.2529(4); 0.2870(10); 0 0.0158(7)N3 4g 0.1367(3); 0.0172(9); 0 0.0111(6)N4 4g 0.1306(3); 0.5514(8); 0 0.0099(6)55541016-1 Table 4: Crystallographic data for oP28-C3N4 at 35 GPa. oP28-C3N4Exp.Pressure (GPa) 35Space group, # PnnmZ 4a (Å) 8.8035(8)b (Å) 7.549(4)c (Å) 2.361(3)V (Å3) 156.9(2)Refinement detailsWavelength (λ, Å) 0.3738μ (mm-1) 0.099# measured / independent 390 / 208 (129) reflections (I ≥ -13σ)(sin θ / λ)max (Å ) 0.899Rint (%) 2.18R1(%) 7.78wR2 (%) 6.87R1 (all data, %) 11.64wR2 (all data, %) 7.14Goodness of fit 3.28No. of parameters 22Δρmin, Δρmax (eÅ-3) -0.43, 0.47Atomic positionsAtom Wyckoff position Fractional atomic coordinates (x; y; z) Uiso (Å2)Exp.C1 4g 0.2768(4); 0.4698(6); 0 0.0206(8)C2 4g 0.1077(4); 0.2079(6); 0 0.0198(8)C3 4g 0.3804(4); 0.1667(6); 0 0.0202(7)N1 4g 0.5222(3); 0.2527(5); 0 0.0196(8)N2 4g 0.2545(4); 0.2905(5); 0 0.0213(7)N3 4g 0.1346(8); 0.0229(5); 0 0.0216(7)N4 4g 0.1296(3); 0.5565(5); 0 0.0204(7)Table 5: Crystallographic data for oP28-C3N4 at 17 GPa. oP28-C3N4 Exp.Pressure (GPa) 17Space group, # PnnmZ 4a (Å) 8.9896(8)b (Å) 7.674(4)c (Å) 2.373(3)V (Å3) 163.7(2)Refinement details 55541016-1Wavelength (λ, Å) 0.3738μ (mm-1) 0.095# measured / independent 473 / 183 (119) reflections (I ≥ nθ / λ)max (Å-3σ)(si 1) 0.909Rint (%) 2.45R1 (%) 6.08wR2 (%) 5.14R1 (all data, %) 10.24wR2 (all data, %) 5.49Goodness of fit 2.74No. of parameters 22Δρmin, Δρmax (eÅ-3) -0.42, 0.40Atomic positionsAtom Wyckoff position Fractional atomic coordinates (x; y; z) Uiso (Å2)Exp.C1 4g 0.2750(3); 0.4689(9); 0 0.0128(6)C2 4g 0.1059(3); 0.2113(9); 0 0.0141(6)C3 4g 0.3809(3); 0.1707(9); 0 0.0140(6)N1 4g 0.5235(2); 0.2489(6); 0 0.0114(6)N2 4g 0.2526(3); 0.3017(11); 0 0.0177(6)N3 4g 0.1346(2); 0.0266(9); 0 0.0127(6)N4 4g 0.1317(3); 0.5575(8); 0 0.0128(6)Table 6: Crystallographic data for oP28-C3N4 at 4 GPa. oP28-C3N4Exp.Pressure (GPa) 4Space group, # PnnmZ 4a (Å) 9.1128(8)b (Å) 7.819(4)c (Å)3 2.396(3)V (Å ) 170.7(2)Refinement detailsWavelength (λ, Å) 0.3738μ (mm-1) 0.091# measured / independent 288 / 153 (101) reflections (I ≥ 3σ)(sin θ / λ)max (Å-1) 0.732Rint (%) 1.95R1(%) 5.74wR2 (%) 4.88R1 (all data, %) 9.59wR2 (all data, %) 5.10Goodness of fit 2.41No. of parameters 22Δρmin, Δρmax (eÅ-3) -0.28, 0.3155541016-1 Atomic positionsAtom Wyckoff position Fractional atomic coordinates (x; y; z) Uiso (Å2)Exp.C1 4g 0.2744(4); 0.4779(6); 0 0.0252(10)C2 4g 0.1077(4); 0.2122(7); 0 0.0286(10)C3 4g 0.3800(4); 0.1779(8); 0 0.0286(12)N1 4g 0.5250(3); 0.2494(5); 0 0.0275(10)N2 4g 0.2531(3); 0.2999(5); 0 0.0278(9)N3 4g 0.1414(3); 0.0274(6); 0 0.0283(9)N4 4g 0.1321(3); 0.5660(6); 0 0.0280(10)Table 7: Crystallographic data for oP28-C3N4at 1 bar. oP28-C3N4 Exp.Pressure (bar) 1Space group, # PnnmZ 4a (Å) 9.1296(8)b (Å) 7.851(4)c (Å) 2.400(3)V (Å3) 172.0(2)Refinement detailsWavele- ngth (λ, Å) 0.3738μ (mm1) 0.091# measured / independent 493 / 198 (135) reflections (I ≥ 3σ)(sin θ / λ)max (Å-1) 0.817Rint (%) 2.53R1 (%) 6.01wR2 (%) 5.70R1 (all data, %) 8.23wR2 (all data, %) 5.80Goodness of fit 2.59No. of parameters 22Δρmin, Δρmax (eÅ-3) -0.33, 0.32Atomic positionsAtom Wyckoff position Fractional atomic coordinates (x; y; z) Uiso (Å2)Exp.C1 4g 0.2742(10); 0.4783(4); 0 0.0157(7)C2 4g 0.1076(9); 0.2120(4); 0 0.0176(7)C3 4g 0.3807(9); 0.1776(4); 0 0.0163(7)N1 4g 0.5239(7); 0.2480(3); 0 0.0160(7)N2 4g 0.2532(10); 0.2994(4); 0 0.0244(8)N3 4g 0.1431(9); 0.0285(3); 0 0.0144(6)N4 4g 0.1336(8); 0.5680(3); 0 0.0152(7)55541016-1 Remarkably, oP28-C3N4 was found to be recoverable at ambient conditions and to be stable in air over prolonged a period of time; demonstrated by the analysis of the single-crystal and powder data obtained after the DAC’s opening, as demonstrated inTable 7 above, and in Figure 2 which shows Le Bail analysis for the XRD patterncontaining oP28-C3N4 and oP8-CN at ambient conditions. In particular, Figure 2 showsexcellent correlation between experimental and predicted (calculated) XRD patterns for oP28-C3N4. XRD patterns were further investigated, as shown in Figure 6, which showspXRD patterns for oP28-C3N4, compared to pXRD patterns for other carbon nitridecrystals (oP8-CN, hP126-C3N4, tI14-C3N4, tI24-CN2). This data shows distinctive peaksfor oP28-C3N4 at a 2^ ± 0.2 of, inter alia, 4.69°, 5.43°, 5.46°, 5.94°, 7.20°, and 8.91°, asmeasured by X-ray powder diffraction at a wavelength of 0.3738 Å, optionally whenmeasured at ambient pressure (e.g.1 atm) and ambient temperature (e.g. about 20°C).At ambient conditions, the lattice parameters were found to be a = 9.1296(8), b= 7.851(4) and c = 2.400(3) Å (V = 172.0(2) Å3). While the structural arrangement of theCN4 units constituting oP28-C3N4 remains the same, the interatomic distances increased compared to the values at 73 GPa. Indeed, the average C-N single bond length is of 1.468(6) Å, expanding compared to its 1.409(3) Å value at 73 GPa. Each of the atom’shybridization was maintained, i.e. with the C, N3 and N4 atoms being sp3-hybridized(average N-C-N and C-N-C angles of 109.4(4)° and of 110.5(4)°, respectively) and the N1 and N2 atoms being sp2-hybridized (average C-N-C angle of 120.0(4)°). The variation of the various bond length and bond angle are shown in Figure 4. Notably, the average C-N bond length decreases by only 5% between ambient pressure and 94 GPa. From the pressure-volume data collected upon the decompression of oP28-C3N4, its bulk modulus could be extracted using a second order Birch-Murnaghan equation ofstate (BM2), see Figure 5. A value of K0 = 337(3) GPa was obtained for the bulkmodulus, with the V0 parameter set to the experimentally measured value of 172.0 Å3.These values are quite close to those obtained from DFT calculations, i.e. V0 = 173.2(4)Å3, K0 = 323(13) GPa and K′ = 3.8(2) with a third order Birch-Murnaghan equation of state fit (BM3), and V0 = 173.4(3) Å3and K0 = 315(4) GPa with a BM2 fit. This makes oP28-C3N4the C3N4polymorph with the highest compressibility, and with a bulk modulus roughly 100 GPa lower than that of diamond. 55541016-1 The CN4 tetrahedra making up the oP28-C3N4 compound was found to be exceptionally incompressible. Indeed, its volume was extracted from the structure solutions obtained at all pressures and the data fit to a BM2, revealing a bulk modulus of 459(12) GPa. As shown in Table 8, the values obtained from fitting the CN4 volume calculated from DFT are in good agreement. Comparison with the CC4 and BN4tetrahedra constituting diamond and cubic boron nitride (c-BN) suggests CN4 to be moreincompressible. Table 8: Volume, bulk modulus and pressure derivative of the bulk modulus at ambient conditions for CN4, CC4 and BN4 tetrahedra obtained from fitting the pressure- volume data to second (BM2) or third (BM3) order Birch Murnaghan equation of state. V0 (Å3) K0 (GPa) K′CN4 units in oP28-C3N4, exp;1.615* 459(12) 4*BM2 CN4 units in oP28-C3N4, calc;1.616(2) 475(5) 4*BM2 CN4units in oP28-C3N4, calc;1.619(3) 444(18) 4.6(4)BM3 CC4 units in diamond, exp[; BM2 1.891* 402(1) 4*CC4 units in diamond, exp; BM3 1.891* 441(3) 3.13(6)BN4 units in c-BN, exp; BM2 1.969* 376.5(7) 4*BN4 units in c-BN, exp; BM3 1.969* 396(3) 3.56(6)* represents a value that was fixed. The direct comparison of the normalized tetrahedra volume for CN4versus the CC4and BN4units making up diamond and c-BN structure (see Figure 5c) further underlines this behaviour. Indeed, it can be seen that, at 100 GPa, while volume of the CC4and the BN4tetrahedra has shrunk by ~16% and ~17%, respectively, that of the CN4unit in oP28-C3N4 has decreased substantially less, i.e. of ~14.5%. It is believed thatthis provides evidence that the CN4tetrahedron may be the most incompressible knownpolyhedron. Keeping in mind C-N single bonds to be shorter than C-C and B-N singlebonds, this observation matches with the known relationship between a polyhedron’s bulk modulus and the average bond length between its center atom and its neighboringones, i.e. K0 ∝ <d3>, first proposed by Hazen and Prewitt in 1988. Keeping in mind thatthe bulk modulus of a material depends on both its constituting polyhedra as well as their 55541016-1linkage — a fact apparent when comparing the structure of the C3N4 polymorphs andtheir corresponding bulk modulus — this suggests that C3N4 polymorphs with an improvelinkage could be even more incompressible, and likely harder. Results Figure 7 shows theoretical calculations of the thermodynamical stability of oP28-C3N4 vs hP126-C3N4, depending on temperature.In these calculations, the free energy of both phases was calculated using Quantum Espresso in combination with the phonopy package. Phonon dispersion relations were calculated for oP28-C3N4 in 2x2x4 supercells with 4x4x8 k-points and for hP126-C3N41x1x3 supercells with 2x2x2 k-points using standard phonopy displacements. A 80 Ry cutoff for the plane wave expansion was used. Figure 7 shows the free energy difference at 73 GPa as a function of temperature.The difference is calculated as oP28-C3N4 - hP126-C3N4. As the energies are negative,a positive value of this difference indicates that oP28-C3N4 has a lower energy and a negative difference that hP126-C3N4 has a lower energy. A lower energy implies thermodynamical stability. A first observation is that the free energy difference between the two compounds is small, indicating that (i) the energies of both compounds are close, which is not surprising giving that both compounds are fully made up of CN4tetrahedra with the same type of chemical bonds, and (ii) small differences in parameters, e.g. temperature, may preferentially yield one species rather than the other. As shown in Figure 7, it can be seen that oP28-C3N4is expected to be the preferred phase at lower temperatures whereas hP126-C3N4becomes thermodynamically more stable at higher temperatures. 55541016-1

Claims

CLAIMS:

1. A crystalline carbon nitride compound having the general formula C3N4, andhaving the Pearson symbol: oP28.

2. A crystalline carbon nitride compound as claimed in claim 1, wherein thecompound defines a three-dimensional framework of CN4 tetrahedra.

3. A crystalline carbon nitride compound as claimed in claim 1 or claim 2, whereinthe compound has an orthorhombic crystal system and a primitive centring type.

4. A crystalline carbon nitride compound as claimed in claim 1 or claim 2, whereinthe compound has an orthorhombic primitive Bravais lattice.

5. A crystalline carbon nitride compound as claimed in any preceding claim, whereinthe compound has 28 atoms in a unit cell.

6. A crystalline carbon nitride compound as claimed in any preceding claim, whereinthe compound has 4 formula units per unit cell.

7. A crystalline carbon nitride compound as claimed in any preceding claim, whereineach unit cell comprises or consists of 12 carbon atoms and 16 nitrogen atoms.

8. A crystalline carbon nitride compound as claimed in any preceding claim, whereinthe compound has a Pnnm space group and / or a space group #58.

9. A crystalline carbon nitride compound as claimed in any preceding claim, whereinthe compound has one or more of the following lattice parameters: (viii) a = 8.644(4), b = 7.3913(8) and c = 2.296(3) Å; and / or V = 146.7(3) Å3, ata pressure of 73 GPa and a wavelength of 0.2910 Å;(ix) a = 8.5741(8), b = 7.266(4) and c = 2.280(3) Å; and / or V = 142.1(2) Å3, ata pressure of 94 GPa and a wavelength of 0.29521Å;(x) a = 8.7516(8), b = 7.406(4) and c = 2.314(3) Å; and / or V = 150.0(2) Å3, ata pressure of 53 GPa and a wavelength of 0.3738 Å;55541016-1(xi) a = 8.8035(8), b = 7.549(4) and c = 2.361(3) Å; and / or V = 156.9(2) Å3, ata pressure of 35 GPa and a wavelength of 0.3738 Å;(xii) a = 8.9896(8), b = 7.674(4) and c = 2.373(3) Å; and / or V = 163.7(2) Å3, ata pressure of 17 GPa and a wavelength of 0.3738 Å;(xiii) a = 9.1128(8), b = 7.819(4) and c = 2.396(3) Å; and / or V = 170.7(2) Å3, ata pressure of 4 GPa and a wavelength of 0.3738 Å;(xiv) a = 9.1296(8), b = 7.851(4) and c = 2.400(3) Å; and / or V = 172.0(2) Å3, ata ambient pressure and / or at 1 atm, and a wavelength of 0.3738 Å.

10. A crystalline carbon nitride compound as claimed in any preceding claim, whereinthe compound has an X-ray powder diffraction pattern comprising one or more peaks at a 2^ ± 0.2 selected from: 4.69°, 5.43°, 5.46°, 5.94°, 7.20°, and 8.91°, as measured by X-ray powder diffraction at a wavelength of 0.3738 Å, when measured at ambient pressure and ambient temperature.

11. A crystalline carbon nitride compound as claimed in any preceding claim, whereinthe compound has an X-ray powder diffraction pattern comprising two or more peaks at a 2^ ± 0.2 selected from: 4.69°, 5.43°, 5.46°, 5.94°, 7.20°, and 8.91°, as measured by X-ray powder diffraction at a wavelength of 0.3738 Å, when measured at ambient pressure and ambient temperature.

12. A crystalline carbon nitride compound as claimed in any preceding claim, whereinthe compound has an X-ray powder diffraction pattern comprising two peaks at a 2^ ± 0.2 selected from: 4.69° and 7.20°; or4.69° and 8.91°; or7.20° and 8.91°,as measured by X-ray powder diffraction at a wavelength of 0.3738 Å, when measured at ambient pressure and ambient temperature.

13. A crystalline carbon nitride compound as claimed in any of claims 10 to 12,wherein the X-ray powder diffraction pattern comprises further peaks at one or more, orall, of a 2^ ± 0.2 of: 55541016-15.43°, 5.46°, and 5.94° as measured by X-ray powder diffraction at a wavelengthof 0.3738 Å, when measured at ambient pressure and ambient temperature.

14. A method of preparing a carbon nitride compound comprising:providing a crystal precursor in a diamond anvil cell (DAC); applying a target pressure in the DAC, wherein the target pressure is less than about 100 GPa; and heating the crystal precursor at a temperature in the range of about 1800 K toless than 2700 K.

15. A method according to claim 14, wherein the method comprises laser-heating thecrystal precursor.

16. A method according to claim 14 or claim 15, wherein the method comprisesheating the crystal precursor at a temperature in the range of about 2000 K to about 2600 K.

17. A method according to any one of claims 14 to 16, wherein the method comprisesapplying a target pressure in the DAC, wherein the target pressure is between about 50GPa and less than about 100 GPa, optionally between about 50 GPa and about 99 GPa,optionally between about 60 GPa and about 98 GPa, optionally between about 70 GPaand about 95 GPa.

18. A method according to any one of claims 14 to 17, wherein the method comprisesanalysing the compound by single-crystal X-ray diffraction (SCXRD).

19. A method according to any one of claims 14 to 18, wherein the method comprisesanalysing the compound at a plurality of pressures between about 95 GPa and ambient pressure or 1 atm.

20. A method according to any one of claims 14 to 19, wherein the crystal precursoris selected from the group consisting of cyanuric triazide (CTA), pyrene + N2, boron- doped diamond + N2, boron-doped diamond + cyanuric triazide (CTA), and anthracene 55541016-1

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