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

S. KOVAL

São Luis

Seminario; Programa de Postgrado en Física de la Universidad Federal de Maranhao, São Luis, Brasil; 2013

Universidad Federal de Maranhao, São Luis, Brasil

The pressure dependences of the structural, thermoelastic and vibrational properties of SnO2in 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 theorycalculations using the localized basis code SIESTA. The results are employed to develop ashell model (SM) for application in future studies of nanostructured SnO2 . A good agreementof the SM results for the pressure dependences of the above properties with the ones obtainedfrom present and previous AI calculations as well as from experiments is achieved. Thetransition is characterized by a rotation of the Sn-centered oxygen octahedra around thetetragonal axis through the Sn. This rotation breaks the tetragonal symmetry of the lattice andan orthorhombic distortion appears above the critical pressure Pc . A zone-center phonon ofB1g symmetry in the rutile phase involves such rotation and softens on approaching Pc . Itbecomes an Ag mode which stabilizes with increasing pressure in the CaCl2 phase. Thisbehavior, together with the softening of the shear modulus (C11 − C12 )/2 related to theorthorhombic distortion, allows a precise determination of a value for Pc . An additionaldetermination is provided by the splitting of the basal plane lattice parameters. Both the AIand the experimentally observed softening of the B1g mode are incomplete, indicating a smalldiscontinuity at the transition. However, all results show continuous changes in volume andlattice parameters, indicating a second-order transition. All these results indicate that thereshould be sufficient confidence for the future employment of the shell model.