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Based on density functional theory and evolutionary algorithms the thermodynamically stable structures of MnN and their stability P–T fields were revealed in the pressure range of 0–50 GPa. The MnN-F4¯3m polymorph (zinc blende-type) was shown to be the ambient pressure phase, while experimentally known MnN-I4/mmm (distorted NaCl-type) is a high-temperature phase quenchable to the ambient condition. At 5.0 GPa and below 500 K the MnN-F4¯3m transforms into a NiAs-type structure, MnN-P63/mmc. With temperature increase, MnN-P63/mmc undergoes the phase transition into MnN-I4/mmm. Above 41.2 GPa the polymorph with a distorted NiAs-type structure, MnN-Pnma, was shown to be the thermodynamically stable phase. Thus, we have shown that the recently synthesized above ∼55 GPa new high-pressure phase of MnN should have the distorted NiAs-type structure. In addition, for all considered MnN polymorphs the mechanical properties were estimated. It was shown that all MnN polymorphs fall below the minimum threshold for hard materials (HV > 20 GPa) and therefore they could not be classified as hard materials. [ABSTRACT FROM AUTHOR]

In the present study, based on density functional theory and crystal structure prediction approaches, we found a new high-pressure structure of lead carbonate, named Pb2[C2O6]-P%3m1. This structure differs significantly from previously known modifications of lead carbonate. The Pb2[C2O6]-P%3m1 structure is characterized by the presence of ethane-like [C2O6] groups, which can also be classified as orthooxalate groups. This structure is most energetically favorable at pressures above 92 GPa at low temperatures, while Pmmn (post-aragonite structure) is most favorable below this pressure. As temperature increases to 2000 K, the pressure required for the Pmmn - P%3m1 phase transition increases to 100 GPa. The high-pressure modification Pb2[C2O6]-P%3m1 retains its stability at least up to 200 GPa. In addition, the Raman spectrum of the newly discovered modification was calculated, which may be useful for subsequent identification of this phase in high-pressure experiments. At 100 GPa, the most intense band located at 1148 cm-1 corresponds to the symmetric stretching mode of the C-C bond in the [C2O6] orthooxalate groups. The second and third most intense modes appear at 1021 and 726 cm-1, correspondingly. [ABSTRACT FROM AUTHOR]

In the present study, P–T phase diagrams of ZrO2 and HfO2 for a wide pressure range of 0–150 GPa at 0–2500 K were calculated for the first time using density functional theory with the method of lattice dynamics within the quasi-harmonic approximation. We calculated P–T conditions for a full sequence of high-pressure transformations, P21/c → Pbca → Pnma → P6¯2m, for both compounds. At low temperatures, these transformations for ZrO2 are obtained at 7.6 GPa (P21/c → Pbca), 13.4 GPa (Pbca → Pnma), and 143 GPa (Pnma → P6¯2m), while for HfO2 similar polymorphic transitions are obtained at 9 GPa (P21/c → Pbca), 16 GPa (Pbca → Pnma), and 126 GPa (Pnma → P6¯2m), correspondingly. At high temperatures, for both ZrO2 and HfO2 the P21/c and Pbca structures transform into the P42/nmc modification. In addition, the thermal conductivity and elastic properties of the ZrO2 and HfO2 polymorphs were calculated and compared with the available experimental and theoretical data. [ABSTRACT FROM AUTHOR]

In this study, we present the results of a search for new stable structures of SrC2O5 and BaC2O5 in the pressure range of 0–100 GPa based on the density functional theory and crystal structure prediction approaches. We have shown that the recently synthesized pyrocarbonate structure SrC2O5‐P21/c is thermodynamically stable for both SrC2O5 and BaC2O5. Thus, SrC2O5‐P21/c is stable relative to decomposition reaction above 10 GPa, while the lower‐pressure stability limit for BaC2O5‐P21/c is 5 GPa, which is the lowest value for the formation of pyrocarbonates. For SrC2O5, the following polymorphic transitions were found with increasing pressure: P21/c→Fdd2 at 40 GPa and 1000 K, Fdd2→C2 at 90 GPa and 1000 K. SrC2O5‐Fdd2 and SrC2O5‐C2 are characterized by the framework and layered structures of [CO4]4− tetrahedra, respectively. For BaC2O5, with increasing pressure, decomposition of BaC2O5‐P21/c into BaCO3 and CO2 is observed at 34 GPa without any polymorphic transitions. [ABSTRACT FROM AUTHOR]

Both numerical and experimental studies of the stability and electronic properties of barium–sodium metaborate Ba2Na3(B3O6)2F (P63/m) at pressures up to 10 GPa have been carried out. Electronic-structure calculations with HSE06 hybrid functional showed that Ba2Na3(B3O6)2F has an indirect band gap of 6.289 eV. A numerical study revealed the decomposition of Ba2Na3(B3O6)2F into the BaB2O4, NaBO2, and NaF phases above 3.4 GPa at 300 K. Subsequent high-pressure high-temperature experiments performed using 'Discoverer-1500' DIA-type apparatus at pressures of 3 and 6 GPa and temperature of 1173 K confirmed the stability of Ba2Na3(B3O6)2F at 3 GPa and its decomposition into BaB2O4, NaBO2, and NaF at 6 GPa, which was verified by energy-dispersive X-ray analysis and Raman spectroscopy. The observed Raman bands of the Ba2Na3(B3O6)2F phase were assigned by comparing the experimental and calculated spectra. The experimental Raman spectra of decomposition reaction products obtained at 6 GPa suggest the origin of a new high-pressure modification of barium metaborate BaB2O4. [ABSTRACT FROM AUTHOR]

Based on the density functional theory and crystal structure prediction approaches, we found a novel high-pressure structure of Fe 2 CO 4 - P 1 ¯ . It is characterized by the presence of ethane-like O 3 C–CO 3 groups or so-called orthooxalate groups. The formation of such O 3 C–CO 3 groups has been proposed earlier in melts and aqueous carbonate solutions, but no such examples were known in inorganic crystalline materials. We found that this structure is dynamically and thermally stable at pressures of 50 GPa. Similar structures were also predicted to be dynamically stable for Mn 2 CO 4 , Ni 2 CO 4 , and Co 2 CO 4 . In addition, FeCO 3 was found to transform into a similar structure with O 3 C–CO 3 orthooxalate groups at a pressure above 275 GPa. Additionally, for the first time, we describe the self-diffusion of metal atoms in carbonates at high pressure and at high temperatures. The prediction of novel carbonate structures extends the crystal chemistry of inorganic carbonates beyond the established ones with [CO 3 ] triangles, [C 2 O 5 ] pyro-groups, and [CO 4 ] tetrahedra. [ABSTRACT FROM AUTHOR]

Over the past few years, the concept of carbonates, as the salts of MCO3 or composition with [CO3] triangles in the crystal structures, was sufficiently extended. In addition to carbonates, crystal structures with stoichiometry M3CO5, M2CO4 and MC2O5 were predicted and successfully synthesized. In the present study, based on density functional theory and crystal structure prediction algorithms, we found a novel structure of CaC2O5, namely Ca-pyrocarbonate with monoclinic symmetry Cc, which is one of the possible agents of the global carbon cycle. This structure is characterized by the isolated [C2O5] groups consisting of two [CO3] triangles connected through a common oxygen atom. The thermodynamic stability field of Ca-pyrocarbonate with respect to the decomposition reaction into calcium carbonate and carbon dioxide begins at a pressure of 10 GPa. As the pressure increases to 21 GPa, the structure of Ca-pyrocarbonate transforms into the recently synthesized tetragonal modification I4¯2d, in the structure of which carbon is in the sp3-hybridized state and [CO4] tetrahedra form isolated pyramidal [C4O10] anionic groups. At 59 GPa in the temperature range of 0–2500 K, CaC2O5-I4¯2d undergoes a phase transition to CaC2O5-Fdd2, with the framework structure of [CO4] tetrahedra. On further compression to about 80 GPa, the framework structure transforms into layered ones, C2 and Pc. In addition, we estimated the thermodynamic stability of CaC2O5 with respect to the minerals of the Earth's mantle. We found that CaC2O5 can coexist with bridgmanite up to pressures of 54 GPa at 300 K, where it reacts with the formation of a Ca-perovskite, magnesite, and solid CO2-V. [ABSTRACT FROM AUTHOR]

The search of the stable intermediate compounds in the system PbO−CO2 have been performed using crystal structure prediction technique in the pressure range of 0–50 GPa. Stable structures were found for the compositions Pb2CO4 and PbC2O5. The earlier known oxy‐carbonate Pb2CO4‐P212121 was shown to transform at 10 GPa into another oxy‐carbonate Pb2CO4‐Pnma and than at 22 GPa – into the sp3‐hybridized orthocarbonate with the same symmetry. PbC2O5 became thermodynamically stable at almost 10 GPa in the form of the pyrocarbonate structure P21/c. At 35–45 GPa, PbC2O5‐P21/c transforms into PbC2O5‐Fdd2 with the framework of [CO4] tetrahedra. Another energetically favorable, although dynamically unstable, pyrocarbonate structure PbC2O5‐P1‾ was found in the narrow pressure interval around 10 GPa. We suggest that some analogue of this structure can appear in experiment. One structure, sp3‐hybridized Pb2CO4‐Pnma, is dynamically and thermally stable at ambient pressure, and therefore can be decompressed and extracted from the high‐pressure environment. [ABSTRACT FROM AUTHOR]

Based on ab initio molecular dynamic simulations, dynamic disorder of [BO3] groups in the Ba3(BO3)2 compound has been established. This is the first example of dynamic disorder in borates. It has been shown that static disorder of BO3 groups in the Ba3(BO3)2 crystals [Bekker et al., J. Am. Ceram. Soc., 2018, 101, 450] can be the result of quenching of dynamically disordered high-temperature modifications. [ABSTRACT FROM AUTHOR]

In the present study, based on density functional theory and crystal structure prediction approaches, we found a new high-pressure structure of lead carbonate, named Pb 2 [C 2 O 6 ]- P 3̄ m 1. This structure differs significantly from previously known modifications of lead carbonate. The Pb 2 [C 2 O 6 ]- P 3̄ m 1 structure is characterized by the presence of ethane-like [C 2 O 6 ] groups, which can also be classified as orthooxalate groups. This structure is most energetically favorable at pressures above 92 GPa at low temperatures, while Pmmn (post-aragonite structure) is most favorable below this pressure. As temperature increases to 2000 K, the pressure required for the Pmmn → P 3̄ m 1 phase transition increases to 100 GPa. The high-pressure modification Pb 2 [C 2 O 6 ]- P 3̄ m 1 retains its stability at least up to 200 GPa. In addition, the Raman spectrum of the newly discovered modification was calculated, which may be useful for subsequent identification of this phase in high-pressure experiments. At 100 GPa, the most intense band located at 1148 cm -1 corresponds to the symmetric stretching mode of the C-C bond in the [C 2 O 6 ] orthooxalate groups. The second and third most intense modes appear at 1021 and 726 cm -1 , correspondingly.

In this study, we present the results of a search for new stable structures of SrC 2 O 5 and BaC 2 O 5 in the pressure range of 0-100 GPa based on the density functional theory and crystal structure prediction approaches. We have shown that the recently synthesized pyrocarbonate structure SrC 2 O 5 - P 2 1 / c is thermodynamically stable for both SrC 2 O 5 and BaC 2 O 5 . Thus, SrC 2 O 5 - P 2 1 / c is stable relative to decomposition reaction above 10 GPa, while the lower-pressure stability limit for BaC 2 O 5 - P 2 1 / c is 5 GPa, which is the lowest value for the formation of pyrocarbonates. For SrC 2 O 5 , the following polymorphic transitions were found with increasing pressure: P 2 1 / c → F d d 2 at 40 GPa and 1000 K, F d d 2 → C 2 at 90 GPa and 1000 K. SrC 2 O 5 - F d d 2 and SrC 2 O 5 - C 2 are characterized by the framework and layered structures of [CO 4 ] 4 - tetrahedra, respectively. For BaC 2 O 5 , with increasing pressure, decomposition of BaC 2 O 5 - P 2 1 / c into BaCO 3 and CO 2 is observed at 34 GPa without any polymorphic transitions., (© 2023 Wiley Periodicals LLC.)

In the present study, P - T phase diagrams of ZrO 2 and HfO 2 for a wide pressure range of 0-150 GPa at 0-2500 K were calculated for the first time using density functional theory with the method of lattice dynamics within the quasi-harmonic approximation. We calculated P - T conditions for a full sequence of high-pressure transformations, P 2 1 / c → Pbca → Pnma → P 6̄2 m , for both compounds. At low temperatures, these transformations for ZrO 2 are obtained at 7.6 GPa ( P 2 1 / c → Pbca ), 13.4 GPa ( Pbca → Pnma ), and 143 GPa ( Pnma → P 6̄2 m ), while for HfO 2 similar polymorphic transitions are obtained at 9 GPa ( P 2 1 / c → Pbca ), 16 GPa ( Pbca → Pnma ), and 126 GPa ( Pnma → P 6̄2 m ), correspondingly. At high temperatures, for both ZrO 2 and HfO 2 the P 2 1 / c and Pbca structures transform into the P 4 2 / nmc modification. In addition, the thermal conductivity and elastic properties of the ZrO 2 and HfO 2 polymorphs were calculated and compared with the available experimental and theoretical data.

Over the past few years, the concept of carbonates, as the salts of MCO 3 or composition with [CO 3 ] triangles in the crystal structures, was sufficiently extended. In addition to carbonates, crystal structures with stoichiometry M 3 CO 5 , M 2 CO 4 and MC 2 O 5 were predicted and successfully synthesized. In the present study, based on density functional theory and crystal structure prediction algorithms, we found a novel structure of CaC 2 O 5 , namely Ca-pyrocarbonate with monoclinic symmetry Cc , which is one of the possible agents of the global carbon cycle. This structure is characterized by the isolated [C 2 O 5 ] groups consisting of two [CO 3 ] triangles connected through a common oxygen atom. The thermodynamic stability field of Ca-pyrocarbonate with respect to the decomposition reaction into calcium carbonate and carbon dioxide begins at a pressure of 10 GPa. As the pressure increases to 21 GPa, the structure of Ca-pyrocarbonate transforms into the recently synthesized tetragonal modification I 4̄2 d , in the structure of which carbon is in the sp 3 -hybridized state and [CO 4 ] tetrahedra form isolated pyramidal [C 4 O 10 ] anionic groups. At 59 GPa in the temperature range of 0-2500 K, CaC 2 O 5 - I 4̄2 d undergoes a phase transition to CaC 2 O 5 - Fdd 2, with the framework structure of [CO 4 ] tetrahedra. On further compression to about 80 GPa, the framework structure transforms into layered ones, C 2 and Pc . In addition, we estimated the thermodynamic stability of CaC 2 O 5 with respect to the minerals of the Earth's mantle. We found that CaC 2 O 5 can coexist with bridgmanite up to pressures of 54 GPa at 300 K, where it reacts with the formation of a Ca-perovskite, magnesite, and solid CO 2 -V.

Based on ab initio molecular dynamic simulations, dynamic disorder of [BO 3 ] groups in the Ba 3 (BO 3 ) 2 compound has been established. This is the first example of dynamic disorder in borates. It has been shown that static disorder of BO 3 groups in the Ba 3 (BO 3 ) 2 crystals [Bekker et al., J. Am. Ceram. Soc. , 2018, 101, 450] can be the result of quenching of dynamically disordered high-temperature modifications.

The α- and β-modifications of barium metaborate are important functional materials used in optoelectronic devices. A new theoretically predicted modification of BaB 2 O 4 has been synthesized under conditions of 3 GPa and 900 °C, using the DIA-type apparatus. The new high-pressure modification, γ-BaB 2 O 4 , crystallizes in a centrosymmetrical group of monoclinic syngony ( P 2 1 / n (#14), a = 4.6392(4) Å, b = 10.2532(14) Å, c = 7.066(1) Å, β = 91.363(10)°, Z = 4). A distinctive feature of the γ-BaB 2 O 4 structure is the presence of edge-sharing tetrahedra [B 2 O 6 ] which form infinite double chains ∞ [B 4 O 4 O 8/2 ] stretching along the a axis. The number of known structural types with the [B 2 O 6 ] group is limited. Phase γ-BaB 2 O 4 has the shortest distance between boron atoms of shared tetrahedra among all currently known compounds. The [B 2 O 6 ] group angles are 95.5° and 105.5°. Thermodynamic stability and electronic properties of the γ-BaB 2 O 4 modification were studied. The width of the band gap, calculated using the HSE06 functional, is 7.045 eV which implies transparency in the deep-UV region. Experimental and numerical methods which demonstrate a good match were used to the study the Raman spectra of γ-BaB 2 O 4 and β-BaB 2 O 4 modifications. In the Raman spectra of γ-BaB 2 O 4 , the most intense band at a frequency of 853 cm -1 was found to correspond to the symmetric bending mode of the B-O-B-O ring in edge-sharing tetrahedra.