Schmahl, Wolfgang W.; Swainson, I. P.; Dove, M. T.; Graeme-Barber, A.
LANDAU FREE-ENERGY AND ORDER PARAMETER BEHAVIOR OF THE ALPHA/BETA PHASE-TRANSITION IN CRISTOBALITE.
In: Zeitschrift fur Kristallographie, Vol. 201, No. 1-2: pp. 125-145
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Cristobalite, the high-temperature phase of silica, SiO2, stable between1743 K and the melting point at 1898 K, exists in metastable form atlower temperatures. The cristobalite tetrahedral framework distorts fromcubic Fa3mBAR to tetragonal P4(1)2(1)2 symmetry at a strongly firstorder phase transition near 533 K with a latent heat of 1256 Jmol-1. Thetransition involves a large (5\%) volume discontinuity such that singlecrystals tend to shatter. We measured the thermal evolution of thestructural order parameter of the alpha/beta-cristobalite transition andthe unit cell strain by neutron time-of-flight and X-ray Guinier powderdiffractometry. The system is well described by a single-domain approachwith only one active order parameter component, and there is no evidencefor a coupling mechanism between the six possible order parametercomponents. The transition follows a Landau free energy expansion up to