LANDAU FREE-ENERGY AND ORDER PARAMETER BEHAVIOR OF THE ALPHA/BETA PHASE-TRANSITION IN CRISTOBALITE

Schmahl, Wolfgang W.; Swainson, I. P.; Dove, M. T. und Graeme-Barber, A. (1992): LANDAU FREE-ENERGY AND ORDER PARAMETER BEHAVIOR OF THE ALPHA/BETA PHASE-TRANSITION IN CRISTOBALITE. In: Zeitschrift fur Kristallographie, Bd. 201, Nr. 1-2: S. 125-145

Abstract

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

• Dokumententyp: Zeitschriftenartikel
• Fakultät: Geowissenschaften > Department für Geo- und Umweltwissenschaften > Kristallographie und Materialwissenschaft
• Themengebiete: 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften, Geologie
• Sprache: Englisch