J. M. Recio

Thermodynamical properties of solids from microscopic theory: applications to MgF2 and Al2O3

We have recently developed a non-empirical Debye-like model for the inclusion of thermal effects in the equation of state (EOS) of solids. This model allows the calculation of many thermodynamical properties from the E-V relationship. We report the results of a theoretical investigation that explores the EOS of two ionic solids: MgF2 and Al2O3. The interionic interactions are modelized using either the ab initio Perturbed Ion (aiPI) method or the electron gas formalism along with aiPI electronic wavefunctions, which are allowed to relax with crystal strains. Our EOS results are in overall good agreement with experimental data. Other thermodynamic properties behave in the same way, although Gruneisen constant and related quantities have significant errors. This may be caused by numerical inaccuracies on the high order derivatives needed for its calculation.

Effects of a quantum‐mechanical lattice on the electronic structure and d–d spectrum of the (MnF6)4− cluster in Mn2+ :KZnF3

The electronic structure of the Mn2+ :KZnF3 impurity system has been computed by means of a Hartree–Fock–Roothaan cluster model. First, the Mn2+ center has been simulated by the (MnF6 )4− unit in vacuo. Then, the effects of the KZnF3 lattice have been included in the cluster calculation using three different lattice models. The well‐known point–charge approximation has been compared with two rigorous quantum–lattice models derived from the ideas of the theory of electronic separability. In these two models the lattice ions are represented by an effective lattice potential and a lattice projection operator that enforces the cluster–lattice orthogonality. In the Coulomb or Hartree model the cluster–lattice exchange interactions are neglected. The ab initio model potential (MP) lattice model makes use of model potentials for representing the lattice ions and includes an accurate nonlocal exchange operator. According to the present results, the point–charge lattice model destroys the acceptable picture of the...

A Quantum Chemical Interpretation of Compressibility in Solids.

The ability of the electron localization function to perform a partition of the unit cell volume of crystalline solids into well-defined, disjoint, and space-filling regions enables us to decompose the bulk compressibility into local contributions with a full chemical meaning. This partition has been applied to a set of prototype crystals of the chemical elements of the first three periods of the periodic table, and the equations of state for core, valence, bond, and lone electron pairs have been obtained. Solids are unequivocally classified into two groups according to their response to hydrostatic pressure. Those with sharing electrons (metals and covalent crystals) obey a simple relationship between the average valence electron density and the zero pressure bulk modulus. The stiffness of the closed-shell systems (molecular and ionic solids) is rationalized resorting to the Pauli principle. Overall, the results clearly correlate with chemical intuition: periodic trends are revealed, cores are almost incompressible and do not contribute appreciably to the macroscopic compressibility, and lone pair basins are rather easier to compress than bond basins.

How Electron Localization Function Quantifies and Pictures Chemical Changes in a Solid : The B3 -B1 Pressure Induced Phase Transition in BeO

Advantages of the analysis of the topology of the electron localization function (ELF) in the characterization of the chemical bonding in solids are illustrated in the study of the zinc blende --> rock salt transformation in BeO. The 4-fold to 6-fold coordination change is described as a two-step process: first, a catastrophic-like emergence of two new Be-O bonds reveals the onset of the rock salt structure, and second, the new interactions gradationally evolve to achieve the bonding network of the high-pressure phase. The increase in coordination, the volume collapse and the enhancement in the bulk modulus across the transition pathway are qualitatively and quantitatively traced back to the oxygens valence shell. Although several ELF indexes point toward the expected greater bond polarity in the B3 than in the B1 structure, it can be concluded that there is no substantial modification in the nature of the crystal interactions induced by the phase transformation.

High-pressure behaviour of selenium-based spinels and related structures?an experimental and theoretical study

The high-pressure structural behaviour of the cubic spinel CdCr2Se4 (space group ) and tetragonal CdGa2Se4 () has been investigated experimentally and theoretically in order to understand the large difference in compressibility between the two selenides. The experimental values of the bulk modulus for these compounds are 101(2) and 48(2)?GPa, respectively. These values compare well with 92 and 44?GPa obtained from first-principles calculations based on the density functional theory formalism. The observed difference in compressibility between the cubic and tetragonal structures can be understood in terms of polyhedral analysis. In a hypothetical cubic spinel structure (), the calculated bulk modulus for CdGa2Se4 is 85?GPa. This value together with the experimental and theoretical results for CdCr2Se4 suggest that the selenium-based cubic spinels should have a bulk modulus about 100?GPa, which is half the value found for the oxide spinels.

Computation of Local and Global Properties of the Electron Localization Function Topology in Crystals

We present a novel computational procedure, general, automated, and robust, for the analysis of local and global properties of the electron localization function (ELF) in crystalline solids. Our algorithm successfully faces the two main shortcomings of the ELF analysis in crystals: (i) the automated identification and characterization of the ELF induced topology in periodic systems, which is impeded by the great number and concentration of critical points in crystalline cells, and (ii) the localization of the zero flux surfaces and subsequent integration of basins, whose difficulty is due to the diverse (in many occasions very flat or very steep) ELF profiles connecting the set of critical points. Application of the new code to representative crystals exhibiting different bonding patterns is carried out in order to show the performance of the algorithm and the conceptual possibilities offered by the complete characterization of the ELF topology in solids.

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.

Anions in metallic matrices model: application to the aluminium crystal chemistry

We introduce and discuss an interpretative model of the structure and bonding of inorganic crystals containing metallic elements. The central idea is the conception of the crystal structure of such an inorganic compound as a metallic matrix whose geometric and electronic structures govern the formation and localization of the anions in the lattice. This is the reason for labelling the model anions in metallic matrices (AMM). Taking the AlX3 crystal family (X = F, Cl, OH) as a suitable test-bed class of compounds, we illustrate how this approach gives a direct interpretation of the crystalline structures and explains the variable coordination that Al exhibits in crystalline materials. An exhaustive analysis of the topology of the electron density allows us to provide a quantum-mechanical assessment of the main hypotheses of the AMM model and to uncover, using microscopic arguments, the behavior of anions as chemical pressure agents.

Bases for Understanding Polymerization under Pressure: The Practical Case of CO2.

We present a novel quantitative strategy for monitoring chemical bonding transformations in solids from the topology of their electronic structure. Developed in the context of the electron localization function formalism, it provides an unambiguous characterization of long-range interactions and bond formation. Charge flux between electron localization regions is found to hold the key for identifying the nature of the interaction between the chemically meaningful entities in the solid (valence shells, lone pairs, molecules, etc.). Because of the wide range of interesting properties that high pressure induces in molecular solids, we illustrate the potentialities of our strategy to unveil controversial questions involved in the bond reorganization along the polymerization of CO2. Our study confirms that the topology of the bonding network in the pseudopolymeric phases points toward the incipient formation of the new bonds in the higher pressure polymers. This transformation is identified as a synchronic weakening of the intramolecular (C==O) double bond and the birth of a new intermolecular C--O bond controlled by the oxygen lone pairs. Overall, the relationship that this type of analysis establishes between different polymorphs of the phase diagram could be further exploited for the prediction of the coordination of high pressure phases, opening new avenues for experimental synthesis and structure indexation.