Ricardo A. Casali

Ab initio study of mechanical and thermo-acoustic properties of tough ceramics: applications to HfO2 in its cubic and orthorhombic phase

By means of the ab initio all-electron new full-potential linear-muffin-tin orbitals method, calculations were made for elastic constants C11, C12 and C44 for Si, ZrO2 and HfO2 in their cubic phase, and constants C11, C22, C33, C12, C13, C23, C44, C55 and C66 for HfO2 in its orthorhombic phase. Using the Voigt and Reuss theory, estimations were made for polycrystals of their bulk, shear and Young moduli, and Poisson coefficients. The speed of elastic wave propagations and Debye temperatures were estimated for polycrystals built from Si and the above mentioned compounds. The semicore 4f 14 electrons should be included in the valence set of Hf atom in this all-electron approach if accurate results for elastic properties under pressures are looked for.

Ab?initio and shell model studies of structural, thermoelastic and vibrational properties of SnO2 under pressure

The pressure dependences of the structural, thermoelastic and vibrational properties of SnO2 in its rutile phase are studied, as well as the pressure-induced transition to a CaCl2-type phase. These studies have been performed by means of ab initio (AI) density functional theory calculations using the localized basis code SIESTA. The results are employed to develop a shell model (SM) for application in future studies of nanostructured SnO2. A good agreement of the SM results for the pressure dependences of the above properties with the ones obtained from present and previous AI calculations as well as from experiments is achieved. The transition is characterized by a rotation of the Sn-centered oxygen octahedra around the tetragonal axis through the Sn. This rotation breaks the tetragonal symmetry of the lattice and an orthorhombic distortion appears above the critical pressure P(c). A zone-center phonon of B1g symmetry in the rutile phase involves such rotation and softens on approaching Pc. It becomes an Ag mode which stabilizes with increasing pressure in the CaCl2 phase. This behavior, together with the softening of the shear modulus (C11-C12)/2 related to the orthorhombic distortion, allows a precise determination of a value for Pc. An additional determination is provided by the splitting of the basal plane lattice parameters. Both the AI and the experimentally observed softening of the B(1g) mode are incomplete, indicating a small discontinuity at the transition. However, all results show continuous changes in volume and lattice parameters, indicating a second-order transition. All these results indicate that there should be sufficient confidence for the future employment of the shell model.

Mechanical anisotropy and thermoacoustic properties of SnO2 under high pressures: an ab initio approach

The effects of hydrostatic pressures on the electronic, thermoacoustic and elastic anisotropies of SnO2 in the rutile structure is analyzed up to 18 GPa. It is found that the polycrystalline bulk modulus B increases from 227 to 312 GPa between 0 and 18 GPa while the Young and shear moduli slightly decrease with pressures. The resulting polycrystalline ductility increases with pressures. The speed of the sound for longitudinal waves increases with pressure, while the transverse polarizations and the Debye temperature decrease. Large crystal anisotropy for the shear planes {001} between ⟨ 110⟩ and ⟨ 010⟩ directions under pressures, associated with the phase transition to the Cl2Ca, is found.

HYDROSTATIC DEFORMATION POTENTIAL IN THE FRAMEWORK OF PSEUDOPOTENTIAL: COULOMBIC CORRECTIONS

Abstract A model based on overlapped neutral atomic spheres is developed to correct the hydrostatic deformation potential calculated with a normconserving pseudopotential, density functional theory and self-consistent scheme. Applied to several IV diamond structures and III–V zinc-blenda semiconductors, the results show overall good agreement with the approach of Cardona-Christensen, based on Dielectric Mid Point Energy correction and in the case of GaAs, with direct experimental data using uniaxial stress spectroscopy.