A mechanism for aragonite to post-aragonite transition in MCO (M = Ca, Sr and Ba) carbonates: evidence of a hidden metastable polymorph.

To advance in the understanding of the Earth's carbon cycle, it is necessary to determine thermodynamic boundaries and kinetic barriers associated with the pressure-induced polymorphic sequence of alkaline-earth carbonates. Following a symmetry-based strategy within the martensitic approximation, we propose a two-step mechanism mediated by a hexagonal 6/ structure for the aragonite to post-aragonite transformation in the MCO (M = Ca, Sr, Ba) crystal family. The calculated transition pressures and activation energies, from ∼7 to 42 GPa and ∼0.

Pressure and temperature stability boundaries of cubic SiC polymorphs: a first-principles investigation.

A better understanding of the effects of temperature and pressure on the wide gap SiC semiconductor is necessary for both (i) an improvement of the performance of this compound in a variety of technological applications and (ii) a clarification of controversial issues related to the stability of its cubic polymorphs at high pressure and high temperature. Bearing in mind this double demand, we perform first-principles calculations of the phonon band structures, vibrational density of states, and thermal and mode Grüneisen parameters of the zinc blende (B3) and rock-salt (B1) cubic polymorphs of 3C-SiC covering pressures and temperatures up to 120 GPa and 3000 K, respectively. Under a martensitic description of the B3-B1 transformation, we found that the large hysteresis pressure range observed at room temperature (35-100 GPa) disappears at around 1100 K.

Pressure-Driven Metallization in Hafnium Diselenide.

The quest for new transition metal dichalcogenides (TMDs) with outstanding electronic properties operating under ambient conditions draws us to investigate the 1T-HfSe polytype under hydrostatic pressure. Diamond anvil cell (DAC) devices coupled to synchrotron X-ray, Raman, and optical (VIS-NIR) absorption experiments along with density functional theory (DFT)-based calculations prove that (i) bulk 1T-HfSe exhibits strong structural and vibrational anisotropies, being the interlayer direction especially sensitive to pressure changes, (ii) the indirect gap of 1T-HfSe tends to vanish by a -0.1 eV/GPa pressure rate, slightly faster than MoS or WS, (iii) the onset of the metallic behavior appears at ∼10 GPa, which is to date the lowest pressure among common TMDs, and finally, (iv) the electronic transition is explained by the bulk modulus - correlation, along with the pressure coefficient of the band gap, in terms of the electronic overlap between chalcogenide p-type and metal d-type orbitals.

Generalized Stress-Redox Equivalence: A Chemical Link between Pressure and Electronegativity in Inorganic Crystals.

The crystal structure of many inorganic compounds can be understood as a metallic matrix playing the role of a host lattice in which the nonmetallic atomic constituents are located, the Anions in Metallic Matrices (AMM) model stated. The power and utility of this model lie in its capacity to anticipate the actual positions of the guest atoms in inorganic crystals using only the information known from the metal lattice structure. As a pertinent test-bed for the AMM model, we choose a set of common metallic phases along with other nonconventional or more complex structures (face-centered cubic (fcc) and simple cubic Ca, CsCl-type BaSn, hP4-K, and fcc-Na) and perform density functional theory electronic structure calculations.

Chemical Pressure Maps of Molecules and Materials: Merging the Visual and Physical in Bonding Analysis.

The characterization of bonding interactions in molecules and materials is one of the major applications of quantum mechanical calculations. Numerous schemes have been devised to identify and visualize chemical bonds, including the electron localization function, quantum theory of atoms in molecules, and natural bond orbital analysis, whereas the energetics of bond formation are generally analyzed in qualitative terms through various forms of energy partitioning schemes. In this Article, we illustrate how the chemical pressure (CP) approach recently developed for analyzing atomic size effects in solid state compounds provides a basis for merging these two approaches, in which bonds are revealed through the forces of attraction and repulsion acting between the atoms.

Hysteresis and bonding reconstruction in the pressure-induced B3-B1 phase transition of 3C-SiC.

The determination of kinetic factors affecting phase metastability is crucial for the design of materials out of the ambient conditions. At a given temperature, the kinetic barrier associated with the reconstruction of the bonding network of a pressure-induced phase transition can be only overcome at pressures where the available vibrational energy of the system is equal or higher than the corresponding activation energy. Our work demonstrates that these pressures provide boundaries to hysteresis cycles that can be evaluated following a three-step computational strategy: (i) total energy electronic structure calculations, (ii) determination of vibrational contributions by means of a simple Debye model, and (iii) description of the energetic profile along the transition path in the framework of the martensitic approximation.

Structure and bonding in crystalline cesium uranyl tetrachloride under pressure.

A thorough investigation of pressure effects on the structural properties of crystalline cesium uranyl chloride was performed by means of first-principles calculations within the density functional theory framework. Total energies, equilibrium geometries and vibrational frequencies were computed at selected pressures up to 50 GPa. Zero pressure results present good agreement with available experimental and theoretical data.

Phosphorous acid and urea: valuable sources of phosphorus and nitrogen in the hydrothermal synthesis of ammonium-thorium phosphates.

Microcrystals of the first ammonium-thorium phosphates, (NH 4) 2Th(PO 4) 2.H 2O (tetragonal, I4 1/ amd, a = 7.0192(4) A, c = 17.

Theoretical study of P2O5 polymorphs at high pressure: hexacoordinated phosphorus.

Binary oxides of elements belonging to groups 13-15 are of special relevance from a fundamental point of view as well as because of their technological applications as a basis in zeotypes, glasses, or semiconductors. Aluminum oxide, Al2O3, crystallizes in the corundum structure, which is stable at low and high pressures, with the Al showing octahedral coordination. Silicon oxide, SiO2, crystallizes in phases with tetrahedral Si coordination at low pressures as alpha-quartz, but at high pressures, octahedral coordination is stable in the stishovite polymorph.

Synthesis and crystal structure of thorium bis(hydrogenphosphate) monohydrate.

Microcrystals of Th(HPO 4) 2.H 2O were hydrothermally obtained from a Th(NO 3) 4-CO(NH 2) 2-H 3PO 3-H 2O system ( T = 180 masculineC). The structure [orthorhombic, Pbca, a = 9.

New hydrothermal synthesis and structure of Th2(PO4)2(HPO4).H2O: the first structurally characterized thorium Hydrogenphosphate.

Th(2)(PO(4))(2)(HPO(4)).H(2)O was synthesized under wet hydrothermal conditions starting from a mixture of H(3)PO(3) and Th(NO(3))(4).5H(2)O.

Hydrothermal Synthesis and Structure of Fe(NH(3))(2)PO(4): A Novel Monophosphate.

Fe(NH(3))(2)PO(4) is synthesized under mild hydrothermal conditions in the presence of urea. The crystal structure is solved for powder X-ray diffraction data. The unit cell is orthorhombic, a = 10.