Modelling the structural variation of quartz and germanium dioxide with temperature by means of transformed...

The pseudocubic (PC) parameterization of O₄ tetrahedra [Reifenberg & Thomas (2018). Acta Cryst. B74, 165-181] is applied to quartz (SiO₂) and its structural analogue germanium dioxide (GeO₂). In α-quartz and GeO₂, the pseudocubes are defined by three length parameters, a_PC, b_PC and c_PC, together with an angle parameter α_PC. In β-quartz, α_PC has a fixed value of 90°. For quartz, the temperature evolution of parameters for the pseudocubes and the silicon ion network is established by reference to the structural refinements of Antao [Acta Cryst. (2016), B72, 249-262]. In α-quartz, the curve-fitting employed to express the non-linear temperature dependence of pseudocubic length and Si parameters exploits the model of a first-order Landau phase transition utilized by Grimm & Dorner [J. Phys. Chem. Solids (1975), 36, 407-413]. Since values of tetrahedral tilt angles about ⟨100⟩ axes also result from the pseudocubic transformation, a curve for the observed non-monotonic variation of α_PC with temperature can also be fitted. Reverse transformation of curve-derived values of [Si+PC] parameters to crystallographic parameters a, c, x_Si, x_O, y_O and z_O at interpolated or extrapolated temperatures is demonstrated for α-quartz. A reverse transformation to crystallographic parameters a, c, x_O is likewise carried out for β-quartz. This capability corresponds to a method of structure prediction. Support for the applicability of the approach to GeO₂ is provided by analysing the structural refinements of Haines et al. [J. Solid State Chem. (2002), 166, 434-441]. An analysis of trends in tetrahedral distortion and tilt angle in α-quartz and GeO₂ supports the view that GeO₂ is a good model for quartz at high pressure.