M. Flórez

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...

Structural and chemical stability of halide perovskites

Abstract The ab initio Perturbed Ion ( ai PI) quantum mechanical method is used to study the solid state reaction: AX + MX 2 → AMX 3 from a thermodynamical point of view. The reaction energy is first determined by means of static calculations (i.e. at null absolute temperature) on the ideal cubic structures of the components. The very difficult problem of determining the most stable crystal structure of a compound is then undertaken by examining the differences in energy among many structures reported for AX , MX 2 and AMX 3 compounds. Finally, the reaction energy is again examined in the light of those corrections, and the results are used to analyze the experimental data available on the synthesis of perovskites.

Ab initio calculation of the local geometry of Cu+:NaF and Cu+:NaCl

A new description of the nature and scope of the impurity centers Cu:NaF and Cu:NaCl emerges when the local equilibrium geometry and wave function are obtained from cluster‐in‐the‐lattice calculations involving clusters from 7 to 33 ions. The numerical results reveal the importance of simulating the defects by means of clusters having, at least, a boundary shell of fixed ions whose wave functions can follow the geometrical changes of the cluster inner shells. Inward relaxations of 0.089 A for Cu:NaF and 0.085 A for Cu:NaCl are deduced from the best calculations, in agreement with recent measurements on the last system.

Local geometry and resonant vibrations of Cu+:NaF. Results of abinitio perturbed ion, cluster‐in‐the‐lattice calculations involving clusters of 179 ions

The ground state electronic structure and energy of a CuF92Na5−86 cluster (Cu+ plus 12 shells of neighbors) embedded into a quantum lattice representing the NaF crystal are determined by using the ab initio perturbed ion (aiPI) method, with unrelaxed Coulomb–Hartree–Fock (uCHF) correlation energy corrections. Parallel calculations are performed on the NaF92Na5−86 cluster of the pure crystal in order to identify the changes induced by the impurity and to estimate the systematic errors in our calculations. The geometry of the first four shells (32 ions) is allowed to relax by following symmetric breathing modes. An inwards relaxation of −0.12 A is predicted for the nearest neighbors (nn) shell, but negligible relaxations are found for the outer shells. The substitution of the Na+ ion by the Cu+ impurity is favored by −1.03 eV. The Cu+ ion is found to occupy an on‐center octahedral position. The 138 independent Oh force constants corresponding to the vibration of the Cu+ and its first four shells of neighbor...

Ab initio pair potentials from quantum-mechanical atoms-in-crystals calculations

A new technique for deriving pairwise potentials from ab initio quantum-mechanical calculations of atoms in crystals is presented. The total energy of the crystal referred to the infinitely separated atoms is partitioned into two components: (i) a monocentric deformation energy arising from the changes of atomic electron density in passing from the free atom to the crystal state, and (ii) a bicentric energy due to the atomic interactions in the crystal. The authors show that the first component can be meaningfully separated into pairwise contributions. The new technique is used to derive Buckingham-type potentials for the alkali chloride crystals that (a) reproduce the ab initio crystal energy, (b) predict good static equations of state and defect properties, and (c) give a realistic description of the crystal binding.

Universal compressibility behaviour of ions in ionic crystals

The theory of atoms in molecules leads to a convenient partition of the crystalline space into atomic regions that are space filling and allow a decomposition of the bulk compressibility of a crystal as a volume-weighted sum of local compressibilities. Using available ab initio calculations for the complete alkali halides (AX) family in the rock salt phase and some selected spinels, we find that the pressure at the limit of stability of the crystal matches exactly those of the individual ions (or group of ions), pointing to the conclusion that the ionic volumes at the equilibrium define the relative compressibilities of the ions (or group of ions) in the crystal at all pressures. We also analyse the functional dependence between the ionic compressibilities and volumes for the AX family. We show that these ions exhibit universal behaviour when the local bulk moduli are correlated with the pressure referred to the spinodal pressure value, instead of volume. This fact allows us to define a generalised equation of state for the individual ions based on the spinodal instability hypothesis.

Static simulations of Cu+ centers in alkali halides

Abstract Quantum-mechanical calculations and atomistic simulations have been used to characterize the local geometry, stability and resonant vibrations of CuA centers in alkali halides

Quantum Mechanical Cluster Calculations of Solids: The ab initio Perturbed Ion Method

There has been, in the last few years, a significant interest in the application of molecular quantum mechanical methods to describe the bulk and surface electronic structure of solids [1–5]. We are justified to take this route due to the existence of clusters, i.e. electronic groups in the solid that correlate only slightly with its environs. Even so, the interaction energy between the cluster and the rest of the crystal is decisive for many cluster properties and cannot be ignored in the calculation. The accurate representation of the embedding of the cluster is precisely the largest difference between methods derived to treat isolated molecules and those needed for solid state systems.

Microscopic Study of the Rock Salt-Caesium Chloride Phase Stability in Alkali Halides

In this work we present a microscopic study of observable magnitudes linked to the relative stability of the rock salt (B1) and caesium chloride (B2) phases in the AX (A: Li, Na, K, Rb, Cs; X: F, Cl, Br, I) crystal family. Transition pressures and j Y = Y ( B 2) m Y ( B 1) differences in total energies, volumes, and bulk moduli at zero and transition pressures are computed following a localized Hartree-Fock scheme. The arrangement of the data in clear trends is shown to be dominated by the cation atomic number, being weaker the dependence of the data on the anion. These systematics are well interpreted in terms of a variety of microscopic arguments that emerge from the decomposition of the energy, pressure, and bulk modulus in anionic and cationic contributions.

Energetics of the RbF + CaF2 → RbCaF3 solid state reaction : a first-principles study

In this contribution, we report the preliminary results of a theoretical calculation of relevant thermodynamic magnitudes involved in the RbF+CaF 2 → RbCaF 3 solid state reaction. We combine pairwise and quantum-mechanical simulations to determine the static equations of state for the three crystals involved in this heterogeneous reaction. Then, we compute the standard enthalpy and volume of the reaction (ΔH°, ΔV°) and the dependence of ΔH and ΔV with pressure. Finally, the influence of crystal polymorphism in these magnitudes is examined.