A computational study into the (tetrahedral) distortion of TX2α-quartz materials: The effect of changing the chemical composition away from SiO2

We report a periodic density functional study into the tetrahedral distortion of a range of quartz-like TX₂ materials. The total tetrahedral distortion and its most sizable contribution, the angular part (angular tetrahedral distortion), are found to be strongly dependent on the chemical composition and to increase in the order of BeF₂<SiO₂<CO₂<GeO₂<SiS₂<GeS₂. The bond stretching contribution to the total tetrahedral distortion appears to be significantly smaller and to vary more erratically with changes in the chemical composition. The sulphide materials in the study were found to have tetrahedral distortion values commonly associated in silica with experimentally unrealizable hypothetical frameworks, suggesting that such frameworks might become realizable when stepping away from silica. None of the tetrahedra were found to be strictly regular, in line with the analysis of Smith Acta. Cryst. 16 (1963) 542-545], demonstrating that regular tetrahedra are in principle possible in quartz but that distortions from ideality are energetically advantageous. The energetic reason for this distortion is still an open question; we propose a simple electrostatic model that explains the ease with which tetrahedra can be distorted in terms of charge transfer and the relative charge on the X (O,S,F) atom.