Pb2[C2O6]-P3̄m1: new insights into the high-pressure behavior of carbonates.

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]-P3̄m1. This structure differs significantly from previously known modifications of lead carbonate. The Pb2[C2O6]-P3̄m1 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 → P3̄m1 phase transition increases to 100 GPa. The high-pressure modification Pb2[C2O6]-P3̄m1 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.

Growth, crystal structure and IR luminescence of KSrY1-xErx(BO3)2.

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.

The intrinsic twinning and enigmatic twisting of aragonite crystals.

It is generally accepted that aragonite crystals of biogenic origin are characterized by significantly higher twin densities compared to samples formed during geological processes. Based on our single crystal X-ray diffraction (SCXRD) and transmission electron microscopy (TEM) study of aragonite crystals from various localities, we show that in geological aragonites, the twin densities are comparable to those of the samples from crossed lamellar zones of molluscs shells. The high twin density is consistent with performed calculations, according to which the Gibbs free energy of twin-free aragonite is close to that of periodically twinned aragonite structure. In some cases, high twin densities result in the appearance of diffuse scattering in SCXRD patterns. The obtained TEM and optical micrographs show that besides the twin boundaries (TBs) of growth origin, there are also TBs and especially stacking faults that were likely formed as the result of local strain compensation. SCXRD patterns of the samples from Tazouta, in addition to diffuse scattering lines, show Debye arcs in the [Formula: see text] plane. These Debye arcs are present only on one side of the Bragg reflections and have an azimuthal extent of nearly 30°, making the whole symmetry of the diffraction pattern distinctly chiral, which has not yet been reported for aragonite. By analogy with biogenic calcite crystals, we associate these arcs with the presence of misoriented subgrains formed as a result of crystal twisting during growth.

Phase relations, thermal conductivity and elastic properties of ZrO2 and HfO2 polymorphs at high pressures and temperatures.

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 → P6̄2m, 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 → P6̄2m), while for HfO2 similar polymorphic transitions are obtained at 9 GPa (P21/c → Pbca), 16 GPa (Pbca → Pnma), and 126 GPa (Pnma → P6̄2m), 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.

Crystal structures and P-T phase diagrams of SrC 2 O 5 and BaC 2 O 5 .

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.

Halogen-Doped Chevrel Phase Janus Monolayers for Photocatalytic Water Splitting.

Chevrel non-van der Waals crystals are promising candidates for the fabrication of novel 2D materials due to their versatile crystal structure formed by covalently bonded (Mo6X8) clusters (X-chalcogen atom). Here, we present a comprehensive theoretical study of the stability and properties of Mo-based Janus 2D structures with Chevrel structures consisting of chalcogen and halogen atoms via density functional theory calculations. Based on the analysis performed, we determined that the S2Mo3I2 monolayer is the most promising structure for overall photocatalytic water-splitting application due to its appropriate band alignment and its ability to absorb visible light. The modulated Raman spectra for the representative structures can serve as a blueprint for future experimental verification of the proposed structures.

High-pressure transformations of CaC2O5 - a full structural trend from double [CO3] triangles through the isolated group of [CO4] tetrahedra to framework and layered structures.

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 I4̄2d, 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-I4̄2d 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.

Ba3(BO3)2: the first example of dynamic disorder in a borate crystal.

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.

Synthesis and Growth of Rare Earth Borates NaSrR(BO3)2 (R = Ho-Lu, Y, Sc).

NaSrR(BO3)2 (R = Ho-Lu, Y, Sc) compounds were obtained for the first time. Their structures exhibit disordered positions of Sr2+ and Na+ atoms while RO6 polyhedra are connected through the BO3 groups. Large distances between R atoms and high transparency in the range of 250-900 nm make them promising for phosphor applications. A pathway to obtain single crystals was shown by growing NaSrY(BO3)2 and NaSrYb(BO3)2 by the top seeded solution growth method with Na2O-B2O3-NaF flux.

γ-BaB2O4: High-Pressure High-Temperature Polymorph of Barium Borate with Edge-Sharing BO4 Tetrahedra.

The α- and ß-modifications of barium metaborate are important functional materials used in optoelectronic devices. A new theoretically predicted modification of BaB2O4 has been synthesized under conditions of 3 GPa and 900 °C, using the DIA-type apparatus. The new high-pressure modification, γ-BaB2O4, crystallizes in a centrosymmetrical group of monoclinic syngony (P21/n (#14), a = 4.6392(4) Å, b = 10.2532(14) Å, c = 7.066(1) Å, ß = 91.363(10)°, Z = 4). A distinctive feature of the γ-BaB2O4 structure is the presence of edge-sharing tetrahedra [B2O6] which form infinite double chains ∞[B4O4O8/2] stretching along the a axis. The number of known structural types with the [B2O6] group is limited. Phase γ-BaB2O4 has the shortest distance between boron atoms of shared tetrahedra among all currently known compounds. The [B2O6] group angles are 95.5° and 105.5°. Thermodynamic stability and electronic properties of the γ-BaB2O4 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-BaB2O4 and ß-BaB2O4 modifications. In the Raman spectra of γ-BaB2O4, 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.

Phase relations, and mechanical and electronic properties of nickel borides, carbides, and nitrides from ab initio calculations.

Based on density functional theory and the crystal structure prediction methods, USPEX and AIRSS, stable intermediate compounds in the Ni-X (X = B, C, and N) systems and their structures were determined in the pressure range of 0-400 GPa. It was found that in the Ni-B system, in addition to the known ambient-pressure phases, the new nickel boride, Ni2B3-Immm, stabilizes above 202 GPa. In the Ni-C system, Ni3C-Pnma was shown to be the only stable nickel carbide which stabilizes above 53 GPa. In the Ni-N system, four new phases, Ni6N-R3̄, Ni3N-Cmcm, Ni7N3-Pbca, and NiN2-Pa3̄, were predicted. For the new predicted phases enriched by a light-element, Ni2B3-Immm and NiN2-Pa3̄, mechanical and electronic properties have been studied.

(Fe,Ni)2P allabogdanite can be an ambient pressure phase in iron meteorites.

An orthorhombic modification of (Fe,Ni)2P, allabogdanite, found in iron meteorites was considered to be thermodynamically stable at pressures above 8 GPa and temperatures of 1673 K according to the results of recent static high-pressure and high-temperature experiments. A hexagonal polymorphic modification of (Fe,Ni)2P, barringerite, was considered to be stable at ambient conditions. Experimental investigation through the solid-state synthesis supported by ab initio calculations was carried out to clarify the stability fields of (Fe,Ni)2P polymorphs. Both experimental and theoretical studies show that Fe2P-allabogdanite is a low-temperature phase stable at ambient conditions up to a temperature of at least 773 K and, therefore, is not necessarily associated with high pressures. This is consistent with the textural relationships of allabogdanite in iron meteorites.

New high-pressure phases of Fe7N3 and Fe7C3 stable at Earth's core conditions: evidences for carbon-nitrogen isomorphism in Fe-compounds.

We carried out ab initio calculations on the crystal structure prediction and determination of P-T diagrams within the quasi-harmonic approximation for Fe7N3 and Fe7C3. Two new isostructural phases Fe7N3-C2/m and Fe7C3-C2/m which are dynamically and thermodynamically stable under the Earth's core conditions were predicted. The Fe7C3-C2/m phase stabilizes preferentially to the known h-Fe7C3 at 253-344 GPa in the temperature range of 0-5000 K, and the Fe7N3-C2/m stabilizes preferentially relative to the ß-Fe7N3 - at ∼305 GPa over the entire temperature range. This indicate that carbon and nitrogen can mutually coexist and replace each other in the Earth's and other planetary cores similarly to low pressure phases of the same compounds.