T. Ouahrani

Effect of pressure on the global and local properties of cubic perovskite crystals

The influence of pressure on the structural, elastic, thermal and bonding properties of four perovskite-type oxides AMO3 is studied from the point of view of the quantum theory of atoms in molecules. Ab initio investigations are performed by means of the full-potential linear augmented plane-wave method as implemented in the wien2k code. The integrated basin charges resulting from the topological analysis of electronic density provide a partition of the bulk modulus and compressibility into atomic contributions. Special attention is paid to the nonlinear behaviour of the local bonding properties.

Density functional theory calculation of the optical properties and topological analysis of the electron density of MBi2B2O7 (M = Ca,Zn) compounds

The topology of the electron density for congruent melting oxyborate Bi2ZnOB2O6 and CaBi2B2O7 is studied in light of the theory of atoms in molecules. All the electron density critical points in the unit cell are systematically calculated. What makes these compounds most interesting is a rich collection of B–O long-distance bond paths. We focus on the study of the asymmetric bonds and basins forming the anisotropic B2O5 groups in these compounds. B2O5 shows transferable contributions to the crystal, with long bond paths. We relate these observations to the strong behavior, which favorites its application to the second harmonic generation field. Wherefore, the analyses of bonding and related optical properties as well as the multipole moments of the CaBi2B2O7 compounds are predicted for the first time. CaBi2B2O7 exhibits some uniaxial dielectric anisotropy resulting in a strong birefringence. We also report calculations of the complex second-order optical susceptibility dispersion for the principal tensor ...

A new method of calculation in the Fractional Quantum Hall Effect regime

The electron-electron and electron-background interaction energies are calculated analytically for systems with up to N = 6 electrons. The method consists of describing the position vectors of electrons using complex coordinates and all the interaction energies with complex notation, whereby simplifications become possible. As is known, in this type of calculation, complicated expressions involving integrals over many variables are encountered and the trick of using complex coordinates greatly facilitates the exact calculation of various quantities. Contrary to previous analytical calculations, using complex coordinates avoids complicated trigonometric functions from appearing in the integrand, simplifying the exact evaluation of the integrals. The method we have used can be straightforwardly extended to larger systems with N > 6 electrons.

Thermo-elastic and bonding features of the CdMgS2 crystal: an ab initio study

An investigation of the chalcopyrite and rock-salt phase of the CdMgS2 compound was carried out in terms of FP-LAPW calculations. We found that the electron density of crystals contains important information required for studying the phase transition in this compound. By means of the atoms-in-molecules (AIM) and electron localization function (ELF) formalisms, we found that the chalcopyrite semiconductor phase of the investigated compound undergoes a phase transition to a metallic rock salt. The closure of the band gap is reflected in an increasing electronic delocalization across a change in the nature of the bonding. Thermal effects were included via a quasi-harmonic non-empirical Debye model in order to study their linear expansion parameters. Calculations of the elastic constants of this crystal are possible in both phases, but, obviously, this does not imply that the rock-salt phase is the thermodynamically stable one for this crystal.

Quantum-mechanical simulations of pressure effects on MgIn2S4 polymorphs

ABSTRACT We have carried out a computational study of the behavior under pressure of three polymorphs of MgInS. Ab initio calculations are performed to determine the equation of state parameters of direct (DS) and inverse (IS) spinel phases, and a defect-LiTiO-type (LTO) structure observed at high pressure. It has been shown that the DS phase has higher enthalpy at zero pressure than the IS one, and the latter undergoes a pressure-induced transition to the LTO structure at 8 GPa, very close to the experimental value. Additionally, the topological analysis of the electron density of this IS phase indicates that the size and atomic-like bulk modulus associated with the sulfur atom are the key contributions to of the IS phase of MgInS. This result is in concordance with the anomalous trend found for at spinels (, S, Se) as X increases its atomic number.

Structural, Elastic, Electronic and Optical Properties of SrTMO3 (TM = Rh, Zr) Compounds: Insights from FP-LAPW Study

The structural, mechanical, electronic and optical properties of SrTMO3 (TM = Rh, Zr) compounds are investigated by using first principle calculations based on density functional theory (DFT). The exchange-correlation potential was treated with the generalized gradient approximation (GGA) for the structural properties. Moreover, the modified Becke-Johnson (mBJ) approximation was also employed for the electronic properties. The calculated lattice constants are in good agreement with the available experimental and theoretical results. The elastic constants and their derived moduli reveal that SrRhO3 is ductile and SrZrO3 is brittle in nature. The band structure and the density of states calculations with mBJ-GGA predict a metallic nature for SrRhO3 and an insulating behavior for SrZrO3. The optical properties reveal that both SrRhO3 and SrZrO3 are suitable as wave reflectance compounds in the whole spectrum for SrRhO3 and in the far ultraviolet region (FUV) for SrZrO3.

Local and global properties of the ordered double perovskite Sr2YTaO6: an ab initio study

A density functional-based method has been used to investigate the systematic trends for the structural parameters and electronic structure of four phases of the ordered double-perovskite Sr2YTaO6 compound. The chemical bonding has been investigated by making use of topological analyses grounded on the theory of atoms in molecules (AIM). Local charge and local charge transfer results lead to a prediction of strong ionic behavior. Using a quasiharmonic Debye model, in which the phononic effects are taken into account, studies of the effects of temperature on the heat capacity and entropy are performed. We have found that octahedral coordination of each of the structures has a negligible impact on the electronic and bonding properties. However, distortions from cubic structure lead to increases in the band gap. Optical as well as thermal and transport properties are also analyzed.