First-principles characterisation of the pressure-dependent elastic anisotropy of SnO2 polymorphs. Issue 18 (22nd June 2016) - Das, Pratik Kumar
Abstract: Using DFT calculations, this study investigates the pressure-dependent variations of elastic anisotropy in the following SnO2 phases: rutile-type (tetragonal; P4 2 /mnm ), CaCl2 -type (orthorhombic; Pnnm )-, α-PbO2 -type (orthorhombic; Pbcn )- and fluorite - type (cubic; Fm - 3m ). Experimentally, these polymorphs undergo sequential structural transitions from rutile-type → CaCl2 - type → α-PbO2 -type → fluorite-type with increasing pressure at 11.35, 14.69 and 58.22 GPa, respectively. We estimate the shear anisotropy ( A 1 and A 3 ) on {1 0 0} and {0 0 1} crystallographic planes of the tetragonal phase and ( A 1, A 2 and A 3 ) on {1 0 0}, {0 1 0} and {0 0 1} crystallographic planes of the orthorhombic phases. The rutile-type phase shows strongest shear anisotropy on the {0 0 1} planes ( A 2 > 4.8), and the degree of anisotropy increases nonlinearly with pressure. In contrast, the anisotropy is almost absent on the {1 0 0} planes (ie A 1 ~ 1) irrespective of the pressure. The CaCl2 -type phase exhibits similar shear anisotropy behaviour preferentially on {0 0 1} ( A 3 > 5), while A 1 and A 2 remain close to 1. The α-PbO2 -type phase shows strikingly different elastic anisotropy characterised by a reversal in anisotropy ( A 3 > 1 to < 1) with increasing pressure at a threshold value of 38 GPa. We provide electronic density of states and atomic configuration to account for this pressure-dependent reversal in shear anisotropy. Our study also analyses the directional Young's moduli for the tetragonal and orthorhombic phases as a function of pressure. Finally, we estimate the band gaps of these four SnO2 phases as a function of pressure which are in agreement with the previous results.