Miriam Marqués

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.

The bulk modulus of cubic spinel selenides: an experimental and theoretical study

It is argued that mainly the selenium sublattice determines the overall compressibility of the cubic spinel selenides, AB2Se4, and that the bulk modulus for these compounds is about 100 GPa. The hypothesis is supported by experiments using high-pressure X-ray diffraction and synchrotron radiation, and by first-principles calculations using density functional theory.

Nature and stability of ice X

A study of pressure effects on the vibrational frequencies of ice X is performed to shed light on the existence and stability of this phase. The analysis reveals: (i) its stability range, (ii) the soft phonon nature of the transition at high pressures to the Pbcm structure proposed by Benoit et al. (M. Benoit, M. Bernasconi, P. Focher, and M. Parrinello, New high-pressure phase of ice, Phys. Rev. Lett. 76 (1996), pp. 2934–2936.) and (iii) the phonon collapse associated with the dynamical disordered structure at low pressures. Additionally, a topological analysis of the electron localization function and the electron density through the atoms-in-molecules formalism clarifies the chemical nature of ice X. †This paper was presented at the XLVIth European High Pressure Research Group (EHPRG 46) Meeting, Valencia (Spain), 7–12 September, 2008.

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.

Bonding Changes Along Solid-Solid Phase Transitions Using the Electron Localization Function Approach

Recent computational developments on the application of the Electron Localization Function in the solid state allow to perform a rich characterization of chemical changes along phase transitions induced by thermodynamic variables in crystals. Chemical entities, in the sense of the Lewis theory, can be idengified and classified according to the role they play in these processes. Covalent (SiO2), ionic (BeO), molecular (CO2, O2), and metallic (Na, K) systems have been selected to illustrate the ability of ELF to gain insight into the global understanding of the transformations. Detailed topological analysis of the bonding reconstruction process clearly distinguishes transitions where the bonding nature of the solid is not altered, and just a reorganization takes place, to those where the chemical pattern suffers a dramatic change. We have highlighted the close relationship between energy, structure and bonding across several transition pathways and how ELF can be of help to anticipate pressure induced emerging structures and to discard among competitive transition mechanism

Simple thermodynamic model for the hydrogen phase diagram

We describe a classical thermodynamic model that reproduces the main features of the solid hydrogen phase diagram. In particular, we show how the general structure types that are found by electronic structure calculations and the quantum nature of the protons can also be understood from a classical viewpoint. The model provides a picture not only of crystal structure, but also for the anomalous melting curve and insights into isotope effects, liquid metallisation and InfraRed activity. The existence of a classical picture for this most quantum of condensed matter systems provides a surprising extension of the correspondence principle of quantum mechanics, in particular the equivalent effects of classical and quantum uncertainty.