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Carbon Nitride Frameworks and Dense Crystalline Polymorphs

We used ab initio random structure searching (AIRSS) to investigate polymorphism in C3N4 carbon nitride as a function of pressure. Our calculations reveal new framework structures, including a particularly stable chiral polymorph of space group P43212 containing mixed sp2 and sp3-bonding, that we have produced experimentally and recovered to ambient conditions. As pressure is increased a sequence of structures with fully sp3-bonded C atoms and three-fold coordinated N atoms is predicted, culminating in a dense Pnma phase above 250 GPa. Beyond 650 GPa we find that C3N4 becomes unstable to decomposition into diamond and pyrite-structured CN2.


I. INTRODUCTION
Carbon nitrides with N:C ratios >1 form a class of solid state compounds with properties that are being investigated for applications ranging from gas sorption and catalysis to energy conversion and storage 1-6 . Experimentally known structures are based on sp 2 -bonded carbon atoms and include polymeric to graphitic layered compounds. Within the C-N-H system, materials related to Liebig's melon ((C 6 N 9 H 3 ) n ) contain tris-triazine (heptazine: C 6 N 7 ) structural units linked via -NH-groups 1,2,7-9 . Additional graphitic materials containing intercalated Li + , Cl − and Br − ions are formed by polytriazine imide (PTI) layers consisting of triazine (C 3 N 3 ) rings linked by -N= or -NH-species [10][11][12][13] . Synthesis of macroscopic flakes of triazine-based graphitic carbon nitride (TGCN) composed of nitrogen-linked triazine (C 3 N 3 ) units was reported recently 14 . This structure was first proposed in the mid-1990's as "graphitic carbon nitride" ("g-C 3 N 4 ") by analogy with the structurally related graphite 15,16 . A nanocrystalline material with C 3 N 4 stoichiometry had been reported from chemical vapor deposition 17,18 and related models were studied theoretically 16,19,20 . Triazine-based carbon nitrides can also be exfoliated to produce layered materials analogous to few-layered graphene 21,22 . Theory shows that the optical bandgap (1.6-2.7 eV) important in photocatalysis can be tuned by controlling the layer buckling at high pressure 23,24 .
Interest in dense carbon nitride polymorphs based on sp 3 bonded C atoms began with predictions from density functional theory (DFT) calculations of superhard properties 16,[25][26][27][28] . First investigations began by considering known structures such as those of Si 3 N 4 ceramic phases, and more recently using structure searching methods 29 . However none of the predicted high density C 3 N 4 polymorphs has been demonstrated experimentally to date 1, 30 . One carbon nitride solid containing sp 3 bonded C atoms was produced by laser heating dicyandiamide (DCDA: C 2 N 4 H 4 ) at P > 27 GPa in a diamond anvil cell 31 . This material with a defective wurtzite structure could be recovered to ambient conditions 32 . A recent study has identified formation of a CN phase that also contains sp 3 bonded carbon 33 .
Experimental compression studies and DFT calculations have also revealed the existence of carbon nitride structures containing both sp 2 and sp 3 bonded C atoms obtained from the graphitic layered PTI com-pound C 6 N 9 H 3 ·HCl. Layer buckling causes C-N distances in adjacent layers to approach a covalently bonded value and an interlayer bonded (ILB) form becomes stabilized above 47 GPa 34 . This result shows that carbon nitride framework structures containing mixed sp 2 and sp 3 bonding can exist over a range of densities. Microporous carbon nitride solids have also been produced at ambient pressure by linking C 3 N 3 triazine units into three-dimensional covalent networks [35][36][37][38][39] .
The structural polymorphism of sp 3 -bonded carbon nitride is usefully compared with that of elemental carbon. Here high-pressure synthesis is well known to produce dense structures 40 . Low pressure allotropes dominated by sp 2 bonding consist of planar graphite and graphene structures formed from fused six-membered rings. Introducing curvature into the layers leads to fullerenes and C-nanotubes, while hierarchical low density mesoporous solids assembled at different length scales are important for nanotechnology and planetary science studies [40][41][42] . Theoretical modelling now also points to the potential existence of metastable zeolitic sp 3 -bonded carbon framework structures in the 0-20 GPa range 43 . In equilibrium at low temperatures and pressures above 0.76 GPa 44,45 the dense sp 3 -bonded structure of diamond becomes the most stable form and no further structural transformations are expected until a body-centered (BC8) polymorph is predicted above 1000 GPa (1 TPa) 40 . Other new dense allotropes relevant to understanding C-rich exoplanets are predicted to exist at extreme pressures extending into the TPa range [46][47][48] .
Here we used ab initio random structure searching (AIRSS) 49,50 to explore carbon nitride polymorphism as a function of density throughout a wide pressure range extending between 0-1000 GPa (0-1 TPa), focusing on behavior at the C 3 N 4 composition. At low density the results reveal new crystalline architectures including previously unsuspected framework structures based on mixed sp 2 -sp 3 bonding. One of these structures (P 4 3 2 1 2 or its enantiomorph P 4 1 2 1 2) has been verified experimentally in our study. The AIRSS results allow a ranking of stability among predicted C 3 N 4 polymorphs up to 650 GPa, above which C 3 N 4 becomes unstable relative to diamond + pyrite-structured CN 2 .

II. STRUCTURE SEARCHING
AIRSS is a powerful theoretical prediction technique that has led to the discovery of new structures subsequently verified by experiment, in materials ranging from hydrogen 51,52 , ammonia 53 , ammonia hydrates 54,55 , aluminium hydride 56 , silane 49 to xenon oxides 57 . Within the AIRSS method a cell volume and shape are selected at random, atoms are added at random positions to provide the desired composition, and the system is relaxed until the forces on the atoms are negligible and the pressure takes the required value. This procedure is repeated many times, giving a relatively sparse sampling of the "structure space". We typically constrain the minimum separations between atom pairs (C-C, C-N, N-N) in the initial structures, which helps to space out the atoms appropriately, while retaining a high degree of randomness. These minimum distances are obtained from preliminary short AIRSS runs. In almost all of the subsequent searches we impose symmetry constraints on the structures, for example, we might demand that all structures belong to randomly selected space groups with 8 or more symmetries. In the next stage of searching we might choose to allow structures with 4 or more symmetries, etc. With this approach we obtain a very good sampling of the high-symmetry structures, which can be extended towards lower symmetries as the available computing resources allow. The use of symmetry constraints greatly reduces the size of the "structure space", although they also break it up into disconnected regions which can prevent some structures from relaxing fully. However, there is a strong tendency for low energy structures to possess symmetry 58 , and we find that the application of symmetry constraints throughout the searching is a very useful tool. Searches for low pressure structures are performed using heptazine-and triazine-based units and we consider structures containing up to sixteen formula units (f.u.).
The structure searches were performed using AIRSS and first-principles DFT methods, version 8.0 of the castep plane-wave basis set pseudopotential code 59 , default castep ultrasoft pseudopotentials 60 , a plane-wave basis set energy cutoff of 280 eV, and an initial Brillouin zone sampling grid of spacing 2π×0.1Å −1 . The PBE generalized gradient approximation density functional 61 was used for the structure searches. The final numerical results reported in this paper were obtained using the PBEsol functional with a basis set energy cutoff of 700 eV, castep "on-the-fly" ultrasoft pseudopotentials, as described in the Supplemental Material 62 , and a k-point spacing of 2π×0.05Å −1 , except for the data in Table  I, which were calculated with a finer k-point spacing of 2π×0.03Å −1 .

III. CHOICE OF DENSITY FUNCTIONAL
Estimating the pressure of polymorphic transitions among sp 2 and sp 3 bonded phases for carbon nitride materials is difficult because no reliable data for the structures and energetics of sp 3 bonded C 3 N 4 polymorphs are available. In order to choose a density functional that would provide reasonable transition pressures for comparison with experiment we considered the well-known transition from sp 2 -bonded graphite to sp 3 -bonded diamond in elemental carbon, which has distinct similarities to C 3 N 4 . The structures and lattice constants of diamond and graphite are known, which allows accurate comparisons to be made with the results of DFT calculations 44,45 .
In Fig. 1 we plot the differences in enthalpy between the graphite and reference diamond structures calcu- lated using five density functionals: the local density approximation (LDA) 63 , the semi-local PBE 61 , WC 64 and PBEsol generalized gradient approximations 65 , and the PBE+G06 generalized gradient approximation with empirical van der Waals corrections 61,66 . The calculated graphite/diamond transition pressures ranged from -1 GPa (LDA) to +6 GPa (PBE), which provides an estimate of the range of errors in the pressure arising from the approximate density functionals of about ±4 GPa, see Fig. 1.
The most important factor for obtaining an accurate graphite/diamond transition pressure is to use a functional that provides a good account of the volume of diamond and the in-plane lattice constant of graphite. Here the effects of zero-point vibrational motion are estimated to be small and have been neglected. The LDA normally tends to underestimate volumes, while PBE tends to overestimate them. PBEsol, WC, and PBE+G06 give volumes for diamond intermediate between those of LDA and PBE and are in reasonable agreement with experiment. The description of graphite is more variable. The graphite/diamond transition pressure is calculated to be 0.7 GPa using PBEsol, in close agreement with experiment. WC and PBE-G06 both give 2 GPa, while PBE predicts the transition at 6 GPa. We therefore elected to use PBEsol for our studies of C 3 N 4 polymorphs. The PBE and PBEsol functionals gave the same sequence of polymorphic phase transitions although the PBEsol transition pressures are somewhat larger.

IV. STRUCTURES FOUND IN THE HIGH PRESSURE SEARCHES
Enthalpy-pressure relations for C 3 N 4 polymorphs found by AIRSS using the PBEsol functional are shown in Fig. 2. Below approximately 5 GPa we find that carbon nitride compounds are metastable with respect to elemental carbon (diamond or graphitic polymorphs) and N 2 phases. At low pressure a wide variety of corrugated layers or open framework structures based on linked heptazine or triazine motifs are predicted to occur and the number of potential polymorphs and structure types increases dramatically with system size 67 . These low-density materials typically have very large unit cell volumes and they rapidly become destabilized with increasing pressure 29 . The importance of such mesoporous phases and their relationship to layered and frameworkstructured carbon nitrides is discussed below. Heptazinebased structures of Cc, F dd2 and P bca symmetries are the most stable below 2 GPa in PBEsol, and the P 4 3 2 1 2 structure with mixed sp 2 -sp 3 bonding is stable in the range 2-11 GPa. At higher pressures we find structures with sp 3 bonding around the carbon atoms to be the most stable. We found the following sequence of transitions among the most stable phases as a function of pressure: The numbers above the arrows indicate the PBEsol transition pressures in GPa. (The Cm structure found in DFT searches by Dong et al. 29 is not predicted to be stable at any pressure because our study has discovered new structures that are more stable.) Structures of the high-density phases predicted to become stable at high pressure are illustrated in Fig. 3. The low-enthalpy structures contain only C-N bonds, no C-C or N-N bonding was found to occur in energetically competitive structures over the pressure range studied of 0-1 TPa. Between 11-47 GPa (using PBEsol) the α-C 3 N 4 structure with P 31c symmetry analogous to the ceramic phase α-Si 3 N 4 is predicted to be stable. This is one of the polymorphs considered by Cohen, Sung and others to constitute a likely ultra-low compressibility, superhard material 16,[25][26][27][28] . The α-C 3 N 4 structure contains sp 3 bonded 4-coordinated C atoms and sp 2 bonded three-fold coordinated N atoms characteristic of other low-enthalpy structures at high pressures. In addition to its low compressibility and potentially high hardness it has been predicted to have the interesting materials property of a negative Poisson's ratio 68 . The β-C 3 N 4 phase considered in early discussions is not stable at any pressure within the range studied. Above 47 GPa we find that α-C 3 N 4 is overtaken in stability by a P nnm structured phase that remains the most stable polymorph until 100 GPa (Fig.  3). At this pressure a structure with P mc2 1 symmetry makes a brief appearance, before the introduction of a cubic (I42m) structure 29 containing nearly tetrahedrally bonded N atoms along with the sp 3 -bonded C atom centers. Neither the P nnm nor P mc2 1 polymorphs were identified as stable phases in structure searches reported by Dong et al. 29 . At 195 GPa another cubic (I43d) phase makes a brief appearance, followed by a Cmc2 1 structured polymorph that is stable until 238 GPa. This phase was described by Dong et al. as occurring between 224-300 GPa. It appears to resolve the competing structural situation between C-and N-based bonding with the emergence of clearly separated C-and N-atomic layers within a structure that could be described as "defective" diamond or c-BN. Above 238 GPa the most stable predicted polymorph has orthorhombic P nma symmetry. This remains the most stable phase until 650 GPa where it decomposes into diamond and a pyrite-structured CN 2 polymorph according to

V. DIAMOND ANVIL CELL EXPERIMENTS
To begin to test our theoretical predictions we carried out an exploratory synthesis experiment in a laser-heated diamond anvil cell (DAC) using a sample of crystalline TGCN g-C 3 N 4 as a starting material 14 . This compound has been shown to have P 6 3 cm or P6m2 symmetry corresponding to ABC or AB stacking of the buckled graphitic layers. A thin sample flake of approximately 60 × 60 micrometers in lateral dimensions was loaded into the DAC chamber and isolated from the diamond window using three small ruby spheres. The pressure was determined by ruby fluorescence from a different ruby sphere placed away from the sample. The sample was compressed using Ne as the pressure-transmitting medium (PTM). In situ X-ray diffraction experiments were carried out at beamline ID27 of the European Synchrotron Radiation Facility using monochromatic radiation (λ = 0.3738Å). The starting material exhibits an intense peak at 2θ = 6.6 • due to the 002 reflection of the g-C 3 N 4 TGCN phase (Fig. 4(A)). During initial compression to 53 GPa this peak first shifts to larger 2θ values then broadens and finally disappears above approximately 40 GPa (Fig.  4(A)). This most likely occurs due to structural disordering associated with layer buckling and possible interlayer C-N bonding at high pressure as previously observed for structurally related C 6 N 9 H 3 ·HCl 34 . By 53 GPa the only peaks remaining in the pattern correspond to the face centered cubic lattice of solid Ne used as the PTM (Fig.  4(A)).
Following initial pressurization the material was heated at 1545±80 K using a CO 2 laser beam focused on the sample while monitoring the X-ray diffraction pattern 69 . After approximately 5 minutes new single crystal spots appeared in the 2D diffraction pattern that had been previously dominated by broad diffraction rings from the compressed TGCN phase and Ne PTM material (Fig.   4(A) and (B)). Interestingly, the appearance of the crystalline peaks was accompanied by a pressure drop inside the DAC chamber to 44 GPa indicated by ruby fluorescence, as well as by an 18 % lattice expansion of the solid Ne used as PTM (Fig. 4(A)). This possibly indicates a corresponding densification within the C 3 N 4 sample. Analysis of our X-ray data showed no evidence for diamond or any elemental N 2 phases formed during the experiment. There was likewise no evidence for any of the other predicted carbon nitride phases with different N/C ratios such as CN or CN 2 . We conclude that the sample retained its initial composition close to C 3 N 4 throughout the DAC laser heating experiment. The crystalline material produced in the experiment corresponded to only a few crystallites distributed throughout the sample. While the sample remained at high pressure in the DAC we carried out a mapping study using a focused (∼2 micrometers) X-ray beam that revealed crystals present in only 3 out of 36 patterns obtained. A typical 2D diffraction image is shown in Fig. 4(B) showing the crystalline spots against the sharper rings from the Ne PTM and diffuse scattering from the densified disordered C 3 N 4 material.
During recovery to ambient conditions the diffraction pattern emerged more clearly. Once removed from the DAC the sample which remained mounted inside its Re gasket was examined by X-ray diffraction. Although there were more crystalline features than at high pressure the powder averaging remained poor and it was not possible to carry out ab initio or Rietveld structure refinements (( Fig. 4(C)). However we tested the range of diffraction peaks against calculated patterns for the various polymorphs produced in our AIRSS study as well as other theoretically predicted structures. Only one phase contained peaks within the range of d spacings observed experimentally. This corresponded to a new C 3 N 4 polymorph with P 4 3 2 1 2 symmetry as predicted by the AIRSS calculations. This phase is predicted to have a stability range between 2 to 11 GPa using PBEsol. At 50 GPa it is highly metastable relative to the predicted α-C 3 N 4 (P 31c) or P nnm structures, but it lies lower in energy than the metastably compressed graphitic forms (Fig. 2). We found an excellent match between all the observed d spacings for the new crystalline phase and predicted reflections for the P 4 3 2 1 2 polymorph. We then used Le Bail analysis to obtain refined unit cell parameters a = b = 4.021Å, c = 13.922Å, c/a = 3.46, and V = 56.26Å 3 per f.u., with structural residual or reliability factors R wp = 6.49 and R p = 4.23 ( Fig. 4(C)). These cell parameters are in reasonable agreement with the predicted values for the new structure type at 0 GPa (Table I). The observed pattern of X-ray intensities differs substantially from those expected for a fully averaged powder diffraction pattern because only a few crystallites were present within the laser heated sample. In addition the crystallites might exhibit preferred orientation further complicating any more detailed analysis of the experimental data. We were able to generate patterns that approximately modelled the observed intensity distribution by constructing composites containing a few oriented crystallites with fixed atomic positions from the P 4 3 2 1 2 theory predictions, applying a March-Dollase function to correct for preferred orientation. Using this approach we obtained solutions to a constrained Rietveld refinement of the structure with large but not unreasonable residual values, e.g., with R wp and R p on the order of ∼20 (Fig.  4(D)).
To test the possibility that other atomic arrangements could give rise to the same unit cell we then used AIRSS to examine alternative structure solutions within the experimentally determined cell shape and volume under ambient conditions. Using the PBEsol functional for these runs gave a very small residual pressure of about 0.4 GPa in the experimental cell while PBE indicated a pressure of 3 GPa. Our searches consistently returned P 4 3 2 1 2 as the most stable phase, with the next possibility lying at much higher energy (0.75 eV/f.u.). We conclude from this analysis that the newly predicted P 4 3 2 1 2 polymorph was produced in our high pressure-high temperature experiment. This is the first recorded experimental synthesis of a crystalline C 3 N 4 framework structure predicted by ab initio theory. Along with the defective-wurtzite structure C 2 N 3 H 31,32 and the recent report of a new CN phase 33 it constitutes a new example of a crystalline carbon nitride containing sp 3 -bonded carbon atoms. Various schematic drawings of the new P 4 3 2 1 2 polymorph are shown in Fig. 5 to highlight different important features of the structure. It corresponds to a new framework type containing fused triazine rings linked via sp 3 -bonded C atoms. This is important because mixed sp 2 /sp 3 bonding around the C atoms does not occur for pure carbon polymorphs in equilibrium at any pressure. In addition the geometrical arrangement containing sp 3 -bonded carbon linked to four nitrogen atoms is highly unusual and perhaps absent in all organic chemistry except perhaps among transition states. Our experimental findings indicate that the probability of forming the new carbon nitride phase is low. This can be ascribed to high kinetic barriers as well as the difficulty of forming the unusual C-N linked tetrahedral geometry associated with the formation reaction.

VI. STRUCTURAL COMPLEXITY AMONG CARBON NITRIDE POLYMORPHS AT LOW PRESSURE
In the low pressure range where C x N y compounds are predicted to be metastable with respect to elemental carbon and nitrogen 29 , it has typically been assumed by analogy with elemental carbon that layered "graphitic" motifs should constitute the ground state for carbon nitride materials at low pressures, taking into account the existence of "voids" within the layers due to the different valencies of C and N atoms 1,2, 16,[18][19][20]29,67 . One basic unit of connectivity is the planar triazine (C 3 N 3 ) ring that may be interconnected via linking -N= bonds to form polytriazine imide (PTI) layers with C 3 N 4 stoichiometry 14,16,[18][19][20] . The polymeric TGCN material synthesized by Algara-Siller et al. 14 and used as the precursor for our high pressure experiments was based on such PTI layers with either AB (space group P6m2) or ABC (P 6 3 cm) stacking. The most stable TGCN polymorph found in our AIRSS study had P 6 3 cm symmetry (Fig. 6). It is predicted to lie at 0.329 eV/f.u. above the ground state at ambient pressure (see Supplemental  Table I). It has been shown that layers based on linked tri-s-triazine (polyheptazine: PH) units are more stable than the PTI motifs 1,67 . Various levels of complexity can appear among both PTI and PH connectivity patterns within a single layer, and the layers are predicted to be far from planar. Substantial buckling around the bridging -N= units that act as mechanical hinges lead to structural models with "corrugated" layers 67 .
Dong et al. 29 found a Cc structure to be the ground state at 0GPa, and we have found a very similar structure of F dd2 symmetry that is only a fraction of an meV per f.u. higher in energy than Cc (see Fig. 6 and Supplemental Material 62 ). We have also found a new corrugated PH structure of P bca symmetry that is predicted to be about 7 meV per f.u. higher in energy than Cc andon analysis is the same as a structure found by Gracia and Kroll but in a larger unit cell with P 1 symmetry. We predict that P bca is the most stable phase in the range 0.05-2 GPa. The Cc, F dd2, and P bca structures are based on the ThSi 2 motif 67 . The P bca phase, which has a starting volume near 73Å 3 /f.u. is expected to cross the P 6 3 cm enthalpy-pressure curve at approximately 25 GPa and so would not be accessible to the experimental synthesis procedure (Fig. 2). Gracia and Kroll 67 noted tubular PH structures that could lead to macroscopic forms analogous to carbon nanotubes.
Many of these structure types have very large unit cell volumes (>100Å 3 /f.u.) that are rapidly destabilized at high pressure, see Fig. 2. Others with energies up to 1 eV/f.u. above the ground state have volumes in the 65-90Å 3 /f.u. range and they appear as metastable structure solutions in Fig. 3. One structure stands out. This is the new P 4 3 2 1 2 phase described above that we have synthesized experimentally (Fig. 5). The experimental volume of P 4 3 2 1 2 of 56.26Å 3 /f.u. is a little smaller than the PBEsol volume of 59.97Å 3 /f.u.
A further new phase of F dd2 symmetry consisting of triazine rings linked into a three-dimensional framework via sp 2 -bonded atoms is found to occur at 0.152 eV/f.u. above the ground state at ambient pressure (Fig. 6). The identification of such a new family of framework structures built from thermally stable C-N bonds opens up new directions for carbon nitride research. The challenge for carbon nitride synthesis is now to produce some of these predicted phases and their unusual bonded architectures in a controlled way, and verify their structures and determine their properties experimentally.
Of particular interest is the behavior of the P 6 3 cm  Fig. 2. During initial compression the TGCN phase loses its initial crystallinity but it can be expected to continue along a similar enthalpy-pressure path to reach approximately 0.5 eV/f.u. above the P 4 3 2 1 2 energy by 50 GPa. By this pressure P 4 3 2 1 2 has become substantially metastable (by ∼3 eV/f.u.) relative to α-C 3 N 4 and other dense phases but it is the next lowest in enthalpy that we have found following destabilization of the disordered and metastably compressed P 6 3 cm material. These predicted results can allow us to better understand the nucleation of only a few crystals of the P 4 3 2 1 2 phase following laser heating within the metastably compressed P 6 3 cm TGCN compound. Interestingly, several other C 3 N 4 polymorphs are also predicted to occur within the 20-80 GPa pressure range, with enthalpies extending to ∼0.5 eV/f.u. above the ground state. We might expect that further high-P ,T experiments carried out on metastably compressed starting materials could lead to identification of these new C 3 N 4 polymorphs.

VII. CONCLUSIONS
Extended sp 2 -bonded framework and layered carbon nitride structures are found to be thermodynamically stable at pressures below about 2 GPa. The lowest energy structures are built from linked polyheptazine (PH) units: at higher energies layered and 3D microporous frameworks based on triazine rings linked by imide groups (PTI structures) appear. The enthalpies and volumes per f.u. are reported in the Supplemental Material 62 .
We find that the microporous PH Cc structure is the most stable at zero pressure, although the similar F dd2 structure is only a fraction of an meV per f.u. higher in energy. A layered P bca PH structure is predicted to be stable in the range 0.05-2 GPa.
The P 4 3 2 1 2 structure is a new structure prototype not found in databases that we have discovered using AIRSS. It has mixed sp 2 and sp 3 bonding and is predicted to be stable in the range 2-11 GPa. C 3 N 4 therefore differs from pure carbon in that there is a thermodynamically stable phase between the sp 2 and sp 3 bonded forms. This structure could be accessible through suitable precursors and synthetic routes at low pressure.
All of the predicted thermodynamically stable phases at pressures above 11 GPa consist entirely of four-fold coordinated C atoms and three-fold coordinated N atoms.
The relative stabilities of the most favorable C 3 N 4 have uncertainties due to the use of approximate density functionals which amount to a few GPa at low pressures. We have, however, sought to assess the sizes of these errors by comparing with results for the related graphite/carbon diamond system. AIRSS has predicted new P bca, P 4 3 2 1 2, P nnm, P mc2 1 , and P nma C 3 N 4 structures to have regions of thermodynamic stability and, in addition, the new F dd2 structure is almost stable. Our calculations predict decomposition of C 3 N 4 at pressures above 650 GPa into diamond and pyrite-structured CN 2 .
Our experimental study has provided strong evidence for the synthesis in laser-heated diamond anvil cell experiments of the newly-predicted P 4 3 2 1 2 structure. The identification of a new family of structures with mixed sp 2 and sp 3 bonding opens up a new area of exploration for these important photo-and catalytically active solid materials.   in the C3N4 system. N atoms are depicted as blue spheres and C atoms are white/grey. The α-C3N4 (P 31c) phase is predicted to be stable between 11-47 GPa using PBEsol, followed by P nnm, P mc21, I42m, I43d, Cmc21 and P nma polymorphs at the transition pressures given in the text.  (Table I).
(iii) The plot shows the calculated powder pattern expected from complete sampling of randomly oriented crystallites.