J.M. Menéndez

Compression of Silver Sulfide: X-ray Diffraction Measurements and Total-Energy Calculations

Angle-dispersive X-ray diffraction measurements have been performed in acanthite, Ag(2)S, up to 18 GPa in order to investigate its high-pressure structural behavior. They have been complemented by ab initio electronic structure calculations. From our experimental data, we have determined that two different high-pressure phase transitions take place at 5 and 10.5 GPa. The first pressure-induced transition is from the initial anti-PbCl(2)-like monoclinic structure (space group P2(1)/n) to an orthorhombic Ag(2)Se-type structure (space group P2(1)2(1)2(1)). The compressibility of the lattice parameters and the equation of state of both phases have been determined. A second phase transition to a P2(1)/n phase has been found, which is a slight modification of the low-pressure structure (Co(2)Si-related structure). The initial monoclinic phase was fully recovered after decompression. Density functional and, in particular, GGA+U calculations present an overall good agreement with the experimental results in terms of the high-pressure sequence, cell parameters, and their evolution with pressure.

Local pressures in Zn chalcogenide polymorphs

Using the rich polymorphism of ZnX (X: S, Se, Te) compounds, we show how local pressures can be unequivocally determined from i) first-principles total energy calculations, and ii) atomic volumes derived by means of topological analysis of crystalline electron densities. An analogy between atoms and mechanical resistors is put forward since these local pressures lead to the inverse of the thermodynamic pressure once their respective inverses are added up. Accordingly, we define the atomic-like mechanical conductance as a measure of the atomic volume reduction for energy unit under pressure, and prove that, in agreement with chemical hardness expectations, Zn has lower values than S, Se, and Te in all the polymorphs of the chalcogenide crystal family.

From ELF to Compressibility in Solids

Understanding the electronic nature of materials’ compressibility has always been a major issue behind tabulation and rationalization of bulk moduli. This is especially because this understanding is one of the main approaches to the design and proposal of new materials with a desired (e.g., ultralow) compressibility. It is well recognized that the softest part of the solid will be the one responsible for its compression at the first place. In chemical terms, this means that the valence will suffer the main consequences of pressurization. It is desirable to understand this response to pressure in terms of the valence properties (charge, volume, etc.). One of the possible approaches is to consider models of electronic separability, such as the bond charge model (BCM), which provides insight into the cohesion of covalent crystals in analogy with the classical ionic model. However, this model relies on empirical parametrization of bond and lone pair properties. In this contribution, we have coupled electron localization function (ELF) ab initio data with the bond charge model developed by Parr in order to analyze solid state compressibility from first principles and moreover, to derive general trends and shed light upon superhard behavior.

Guest–host interactions in gas clathrate hydrates under pressure

First-principles calculations were performed to determine equilibrium geometries, static equation of state parameters, the energetics and orientation of the guest molecule inside the 5 and 56 cages, and vibrational frequencies of methane clathrate hydrate. According to our results, the progressive inclusion of one molecule in each clathrate cavity is always a stabilizing process up to saturation. The released energy is very similar for both types of cages. In agreement with the experimental observation of roto-vibrational spectra in this hydrate, we calculate an energy barrier of less than 0.5 kcal/mol, indicating free rotation of methane inside the cages. The stabilizing effect of applied pressure leads to a red shift of the O–H stretching frequencies of the water molecules of around 80 cm in average at 1 GPa.

Theoretical analysis of vibrational modes in uranyl aquo chloro complexes

AbstractThe electronic structure of uranyl-based complexes of water and chlorine, with the general formula

First-principles study of structure and stability in Si–C–O-based materials

[\hbox {UO}_2(\hbox {H}_2\hbox {O})_{x}\hbox {Cl}_{y}]^{2-y} \,(y = 1,\, 2,\, 3,\, 4;\, x + y = 4, 5)

Microscopic partition of pressure and elastic constants in CdTe polymorphs

[UO2(H2O)xCly]2-y(y=1,2,3,4;x+y=4,5), have been computed both in vacuo and in aqueous solution. Within the density functional theory framework, total and relative energies (including basis set superposition error corrections), equilibrium geometries, and vibrational frequencies were determined and briefly compared with available experimental and previous theoretical data. New results on vibrational modes of these complexes are emphasized. Our focus is on the trend exhibited by the frequency of the stretching mode (