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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]

A series of novel KSrY1–xErx(BO3)2 (x = 0–1) phosphors that emit near‐infrared radiation was synthesized using solid‐state methods. Pure Y and Er crystals were grown using a KF flux via the top‐seeded solution growth technique. In situ high‐temperature single crystal X‐ray diffraction, Raman spectroscopy and DFT calculations were used for characterization. Within the series, a polymorphic phase transition from space group P21/m to R3m was discovered between 550 and 600°C. The concentration dependence of the luminescence intensity was measured for the samples. A strong emission of Er3+ electron transition 4I13/2 → 4I15/2 was detected within the 1529–1549 nm range, with the maximum observed for the KSrY0.4Er0.6(BO3)2 composition. [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]

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]