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Silver carbonate (Ag 2 CO 3) is a material currently used for artificial carbon storage. In this work, we report synchrotron X-ray powder diffraction (XRD) experiments under high pressure and high temperature in combination with density-functional theory (DFT) calculations on silver carbonate up to 13.3 GPa. Two pressure-induced phase transitions were observed at room temperature: at 2.9 GPa to a high-pressure (HP1) phase and at 10.5 GPa to a second high-pressure phase (HP2). The facts that a) the HP2 phase can be indexed with the initial P 2 1 / m structure, b) our DFT calculations predict the initial structure is stable in the entire pressure range, and c) the HP2 phase is stable under decompression suggest that the intermediate HP1 phase is a product of the appearance of non-hydrostatic stresses in the sample. The observed structural transformations are associated to a high sensitivity of this compound to non-hydrostatic conditions. The compressibility of Ag 2 CO 3 has also been determined, showing the c axis is the most compressible and that the bulk modulus increases quickly with applied pressure. We attribute both observations to the weak nature of the closed-shell Ag–Ag interactions in this material. The behavior of Ag 2 CO 3 under heating at approximately 3 GPa was also studied. No temperature-induced phase transitions were found at this pressure, and the thermal expansion was determined to be relatively high for a carbonate. [Display omitted] • Two pressure-induced phase transitions have been found in Ag 2 CO 3 silver carbonate. • The structural transformations are associated to a high sensitivity to non-hydrostatic stresses. • Ag 2 CO 3 presents a strong anisotropic compressibility and a quickly increase of the bulk modulus with applied pressure. • Closed-shell d10-d10 interactions between Ag atoms play an important role in this silver-rich compound. [ABSTRACT FROM AUTHOR]

Highlights: [•] Single crystals of HgGa2S4 with defect-chalcopyrite (DC) structure were synthesized. [•] High-pressure X-ray diffraction in DC-HgGa2S4 was performed. [•] Equation of state of DC-HgGa2S4 determined (bulk modulus of 48.4GPa). [•] Calculated elastic constants of DC-HgGa2S4 reported at different pressures. [•] DC-HgGa2S4 becomes mechanically unstable above 13.8GPa. [ABSTRACT FROM AUTHOR]

The structures of the silicides #x03B5;-FeSi, Fe2Si and Fe5Si3 are analysed in connection with the cation arrays of the corresponding oxides FeSiO3, Fe2SiO4 and Fe5Si3O12. In FeSiO3 and in Fe2SiO4, the Fe&z.sbnd;Si subarray reproduces the structures of the corresponding FeSi and Fe2Si alloys, providing new examples of structural identity between the cations in oxides and the corresponding alloys. In contrast, Fe5Si3, when oxidised, produces the Fe5Si3O12 garnet whose cation array differs from the structure of the Fe5Si3 alloy. As oxidation stabilises high-pressure phases of the corresponding alloy for several oxides, the possibility of a phase transition Mn5Si3 → garnet-cation-type was assumed for Fe5Si3. X-ray diffraction experiments were carried out on Fe5Si3 up to 30 GPa using a diamond anvil cell (DAC). The contraction of the unit cell parameters with pressure is rather isotropic. In this pressure range no phase transition was observed. The equation of state was obtained from a Murnaghan fit, leading to a bulk modulus of B0=243±9 GPa with a pressure derivative B0′=3.4±0.9 at room temperature. [Copyright &y& Elsevier]

We have theoretically investigated the elastic properties of calcite-type orthoborates A BO 3 ( A =Sc and In) at high pressure by means of ab initio total-energy calculations. From the elastic stiffness coefficients, we have obtained the elastic moduli ( B , G and E ), Poisson's ratio ( ν ), B / G ratio, universal elastic anisotropy index ( A U ), Vickers hardness, and sound wave velocities for both orthoborates. Our simulations show that both borates are more resistive to volume compression than to shear deformation ( B > G ). Both compounds are ductile and become more ductile, with an increasing elastic anisotropy, as pressure increases. We have also calculated some thermodynamic properties, like Debye temperature and minimum thermal conductivity. Finally, we have evaluated the theoretical mechanical stability of both borates at high hydrostatic pressures. It has been found that the calcite-type structure of InBO 3 and ScBO 3 becomes mechanically unstable at pressures beyond 56.2 and 57.7 GPa, respectively. [ABSTRACT FROM AUTHOR]

The stabilities of the body-centered-tetragonal and distorted-diamond phases of titanium are investigated by first-principles methods. Our results, together with previous experimental and theoretical work, confirm two interesting observations. First, that the metal arrays in oxides correspond to stable or metastable phases of the parent metal; and second, that oxygen provides the pressure medium that stabilizes these phases. In addition, we have confirmed that the bulk modulus of oxygen matrices follows a nearly universal behavior with pressure, and that pressure-induced phase transitions tend to occur when the compressibility of the oxygen matrix reaches the compressibility of the high-pressure phase of the oxide. [Copyright &y& Elsevier]