Landau free energy and order parameter behaviour of the α/β phase transition in cristobalite

Cristobalite, the high-temperature phase of silica, SiO2, stable between 1743 K and the melting point at 1898 K, exists in metastable form at lower temperatures. The cristobalite tetrahedral framework distorts from cubic Fdanti 3m to tetragonal P41212 symmetry at a strongly first order phase transition near 533 K with a latent heat of 1256 Jmol-1. The transition involves a large (5%) volume discontinuity such that single crystals tend to shatter. We measured the thermal evolution of the structural order parameter of the α/β-cristobalite transition and the unit cell strain by neutron time-of-flight and X-ray Guinier powder diffractometry. The system is well described by a single-domain approach with only one active order parameter component, and there is no evidence for a coupling mechanism between the six possible order parameter components