First-principles characterisation of the pressure-dependent elastic anisotropy of SnO sub(2) polymorphs

• Published in: Philosophical magazine (Abingdon, England) England), 2016-06, Vol.96 (18), p.1861-1882
• Main Authors: Das, Pratik Kumar, Chowdhury, Anjan, Mandal, Nibir, Arya, A
• Format: Article
• Language: English
• Subjects: Anisotropy; Elastic anisotropy; Orthorhombic phase; Phases; Planes; Shear; Tin dioxide; Tin oxides
• ISSN: 1478-6435; 1478-6443
• DOI: 10.1080/14786435.2016.1177228

Using DFT calculations, this study investigates the pressure-dependent variations of elastic anisotropy in the following SnO sub(2) phases: rutile-type (tetragonal; P4 sub()2/mnm, CaCl sub(2)-type (orthorhombic; Pnnm)-, alpha -PbO sub(2)-type (orthorhombic; Pbcn)- and fluorite-type (cubic; Fm-3m). Experimentally, these polymorphs undergo sequential structural transitions from rutile-type arrow right CaCl sub(2)-type arrow right alpha -PbO sub(2)-type arrow right fluorite-type with increasing pressure at 11.35, 14.69 and 58.22 GPa, respectively. We estimate the shear anisotropy (A sub(1) and A sub(3)) on {1 0 0} and {0 0 1} crystallographic planes of the tetragonal phase and (A sub(1), A sub(2) and A sub(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 sub(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 sub(1) ~ 1) irrespective of the pressure. The CaCl sub(2)-type phase exhibits similar shear anisotropy behaviour preferentially on {0 0 1} (A sub(3) > 5), while A sub(1) and A sub(2) remain close to 1. The alpha -PbO sub(2)-type phase shows strikingly different elastic anisotropy characterised by a reversal in anisotropy (A sub(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 SnO sub(2) phases as a function of pressure which are in agreement with the previous results.