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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]

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. [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]

In this work, a search for stable structures of Rb 2 O and Cs 2 O was carried out using evolutionary algorithms within the density functional theory. In the studied pressure range of 0–100 GPa, for Rb 2 O, the transitions F m 3 ̄ m ↔ P n m a -I ↔ P n m a -II were detected, which are realized at pressures of 2.6 and 40 GPa, respectively. For Cs 2 O, the transitions R 3 ̄ m ↔ P n m a -I ↔ I 4 / m m m were revealed, which are realized at pressures of 6.3 and 63 GPa. As a result, previously unknown structures were found for these oxides Rb 2 O- P n m a -II and Cs 2 O- I 4 / m m m. According to the calculated phonon dispersion curves, the predicted structures are dynamically stable. Also, the P–T diagram for Rb 2 O and Cs 2 O was calculated for the first time. The electronic properties of the newly predicted high-pressure structures Rb 2 O- P n m a -II and Cs 2 O- I 4 / m m m were investigated, and the band structures and electron density of states (DOS) were calculated. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • For the first time the formation of Li-pyrocarbonate was demonstrated. • Li-pyrocarbonate is stable in the range of 4.5–27 GPa in the Mg 2 B 2 O 5 -type structure. • The calculated phonon spectrum at 0 GPa shows the dynamic stability of Li 2 C 2 O 5 -P-1. For the first time, the formation of compound Li 2 C 2 O 5 was demonstrated based on the first-principles calculations and crystal structure prediction algorithms. This compound is formed at a pressure of 4.5 GPa and a temperature of 0 K as a result of interaction Li 2 CO 3 with CO 2 and stable in the structure with triclinic P 1 ¯ symmetry. The thermodynamic stability field of Li 2 C 2 O 5 - P 1 ¯ is limited by a pressure of 27 GPa at 0 K and 40 GPa at high temperatures, above which it decomposes into Li 2 CO 3 +CO 2. The structure of Li 2 C 2 O 5 - P 1 ¯ is characterized by isolated [C 2 O 5 ]2− pyrogroups, which consist of two oxygen-sharing [CO 3 ]2− triangles. The structure of Li-pyrocarbonate is isostructural to Mg-pyroborate. [ABSTRACT FROM AUTHOR]

In this study we present the results of our density functional theory calculations within the quasi-harmonic approximation, which predict formation and properties of a new compound of the garnet family, La 3 Sc 2 Al 3 O 12 (LSAG). LSAG was shown to be formed from 3LaAlO 3 + Sc 2 O 3 at ambient pressure and room temperature. The thermodynamic stability region of LSAG is limited by a temperature of 685 K, above which its decomposition into LaAlO 3 (LAP) and Sc 2 O 3 occurs. With increasing pressure, LSAG retains its stability up to 0.23 GPa, where the decomposition reaction LSAG ↔ 3LAP＋ Sc 2 O 3 becomes thermodynamically favorable. In this work, not only the thermodynamic stability of LSAG was assessed, but also its thermal conductivity, elastic constants and band gap were calculated. At 300 K the thermal conductivity of LSAG is 13.204 Wm−1K−1. LSAG has the smallest elastic constants compared with YSAG and YAG, which indicates weaker atomic bonding in LSAG than in YSAG and YAG. The width of band gap of LSAG calculated using an hybrid functional is equal to 5.24 eV. In addition, the P–T phase diagram of the known Y 3 Sc 2 Al 3 O 12 (YSAG) was calculated for the first time. Unlike LSAG, YSAG is stable at fairly high temperatures, reaching values of at least 1500 K. With increasing temperature to 1500 K, the pressure of the phase transition increases slightly to 3.55 GPa, and the monovariant curve of the reaction YSAG ↔ 3YAP+Sc 2 O 3 is represented by an almost straight line. [Display omitted] [ABSTRACT FROM AUTHOR]

Based on the first-principles calculations within the density functional theory and crystal structure prediction algorithms iron phosphide phases stable under pressure of the Earth's core and temperatures up to 4000 K were determined. A new low-temperature modification FeP-P21/c stable above ∼75 GPa was predicted. Fe2P with the allabogdanite structure has been established to be stable in the low-temperature region at ambient conditions. At 750 K it transforms into the barringerite structure. The transition from Fe3P with schreibersite structure to Fe3P-Cmcm was observed at 27 GPa, and the phase transition boundary is nearly isobaric. Fe2P and FeP are thermodynamically stable at the Earth's inner core pressures and 0 K according to the obtained results, whereas Fe3P stabilizes with respect to decomposition to Fe + Fe2P at high temperatures above ∼3200 K. [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]

[Display omitted] First-principles calculations within the density functional theory of the stability of barium borates of the BaO-B 2 O 3 -BaF 2 ternary system have been performed at the pressure of up to 10 GPa. A brief summary on the known structures of ambient and high-pressure phases in the BaO–B 2 O 3 and BaF 2 –Ba 3 B 2 O 6 subsystems has been provided. In the BaO–B 2 O 3 subsystem the Ba 3 B 2 O 6 , BaB 2 O 4 , and BaB 4 O 7 phases tentatively are stable at up to 10 GPa, while the other known ambient-pressure borates Ba 5 B 4 O 11 , Ba 2 B 6 O 11 , and BaB 8 O 13 decompose under the pressure of above 7.1, 0.6, and 2 GPa, respectively. Two new high-pressure polymorphic modifications of BaB 2 O 4 compound, BaB 2 O 4 - Pna 2 1 and BaB 2 O 4 - Pa 3 - , stable above 1.0 and 6.1 GPa, respectively, have been predicted. In the BaF 2 –Ba 3 B 2 O 6 subsystem Ba 7 (BO 3) 4-x F 2+3x solid solution is suggested to be stable in the considered pressure range, and Ba 5 (BO 3) 3 F is suggested to decompose into Ba 3 B 2 O 6 and Ba 7 (BO 3) 4- x F 2+3 x at pressures above 3–5 GPa. It has been shown that the enthalpy of Ba 7 (BO 3) 4-x F 2+3x strongly depends on the distribution of the [(BO 3)F]4− and [F 4 ]4− groups in the structure. We consider the results obtained as a necessary basis for an experimental study aimed at obtaining barium borates under pressures of up to 10 GPa and studying their structure and properties. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 a ∗b ∗ 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. [ABSTRACT FROM AUTHOR]

• The extent of cationic substitution in antizeolite borates depends on their symmetry. • The symmetry is controlled by 'guest' anionic clusters in the channels. • This phenomenon can be used to control cation substitution in functional materials. 'Antizeolite' borates are a family of inorganic compounds with structure based on fixed (pseudo)tetragonal [Ba 12 (BO 3) 6 ]6+ sublattice, and various anionic clusters occupying 'channels' in the sublattice. An ability to host different anionic clusters provides a convenient way to tune such functional properties of materials based of 'antizeolite' borates as dichroism, luminescence etc. We demonstrated the dependence of extent of Sr2+→Ba2+ substitution in the cationic sublattice of antizeolite barium borates on the symmetry of initial compound. The latter is controlled by 'guest' anionic clusters in the channels of sublattice. In the case of initially orthorhombic [Ba 12-x Sr x (BO 3) 6 ](BO 3) 2 solid solution, the structure adapts to the increase in Sr2+ content (up to Sr/(Ba+Sr) ratio of at least 39 wt. %) by tetragonalization of the sublattice. In contrast, in tetragonal [(Ba 1-x Sr x) 12 (BO 3) 6 ](BO 3)[LiF 4 ] solid solution another structural mechanism of Sr2+ accommodation takes place, which results in the (2 a ,2 c) modulation and is limited by maximum Sr/(Ba+Sr) ratio of 10-20 wt. %. [Display omitted] [ABSTRACT FROM AUTHOR]

• Six systems for β-BaB 2 O 4 crystal growth have been analyzed. • Systems belong to the Na, Ba, B//O, F quaternary reciprocal system. • Concentration of sodium affects the quality of β-BaB 2 O 4 crystals. • PT-diagram and electronic properties of BaB 2 O 4 polymorphs have been studied. Nonlinear optical β-BaB 2 O 4 (BBO) crystals have been grown in six different systems belonging to the Na, Ba, B//O, F quaternary reciprocal system: I BaB 2 O 4 –BaF 2 , II BaB 2 O 4 –(NaF) 2 , III BaB 2 O 4 –Ba 2 Na 3 [B 3 O 6 ] 2 F, IV BaB 2 O 4 –NaBO 2 , V BaB 2 O 4 –NaBaBO3, and VI BaB 2 O 4 –NaBaBO 3 –Ba 2 Na 3 [B 3 O 6 ] 2 F. Comparative characteristics of the systems and grown crystals are given. The best results in terms of crystal quality have been achieved in systems V and VI. The observed occurrence of scattering centers in β-BaB 2 O 4 crystals might be associated with the sodium concentration in the initial high-temperature solution, and, as a consequence, with the sodium concentration in the β-BaB 2 O 4 crystal. The PT -diagram and electronic properties of BaB 2 O 4 polymorphs were studied using the first-principles calculations. [ABSTRACT FROM AUTHOR]