Modelling the crystal structure of quartz as it changes with increasing temperature
What is it about?
Recent high-precision X-ray structural refinements of quartz at temperatures between 298 and 1235 K by Antao [Acta Crystallogr. B72, 249-263 (2016)] have provided a strong experimental basis for our work. We apply a geometrical transformation technique (PCRT) to follow the changes in distortion of the SiO4 tetrahedra with increasing temperature. Our work demonstrates the use of low-order polynomial curve-fitting in order to summarise the structural trends quantitatively and to allow the prediction of crystal structure at interpolated temperatures. The Landau function for a first-order phase transition is exploited in order to accommodate the non-linearity in the variation of crystal-chemical parameters with temperature.
Why is it important?
The work demonstrates the potential of the pseudocubic representation of tetrahedra (PCRT) as a technique in Crystal Chemistry. It provides a simple method of measuring tilt-angles of SiO4 tetrahedra from crystal structural data. It also allows the prediction of crystal structures at temperatures, pressures or compositions that have not been experimentally investigated. Although the distortions of the tetrahedra have generally been regarded as insignificant until now, it is shown that they are necessary at all temperatures above 566 K.