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Zeitschrift für Kristallographie - Crystalline Materials

Ed. by Antipov, Evgeny / Bismayer, Ulrich / Huppertz, Hubert / Petrícek, Václav / Pöttgen, Rainer / Schmahl, Wolfgang / Tiekink, E. R. T. / Zou, Xiaodong

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Structural change of orthorhombic-I tridymite with temperature: A study based on second-order thermal-vibrational parameters

Kuniaki Kihara / Takeo Matsumoto / Moritaka Imamura

Citation Information: Zeitschrift für Kristallographie. Volume 177, Issue 1-2, Pages 27–38, ISSN (Online) 0044-2968, ISSN (Print) 1433-7266, DOI: 10.1524/zkri.1986.177.1-2.27, August 2010

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X-ray studies of orthorhombic-I tridymite, especially focussed on atomic mean-square displacements (m.s.d.'s) at 443, 493, 573, 653 and 693 K respectively, were carried out on the basis of the usual second-order refinement of the Dollase model.

The m.s.d.'s for O are highly anisotropic, and large in the planes perpendicular to Si – Si axes. Plots of temperature vs. m.s.d.'s point to a disorder of O atoms in the orthorhombic-I form. The transition, from the orthorhombic-I to the hexagonal high form or vice versa, is an example of a displacive structure transition from a disordered structure to another disordered one. When temperature falls from 693 K, any two SiO4 tetrahedra joined by O on the two-fold axes parallel to a move away from the hexagonal positions, rotating progressively around the two-fold axes, and then symmetry is lowered to orthorhombic. The pair-wise rotations are accompanied by anisotropic changes of the thermal ellipsoids for two kinds of O atoms forming the basal planes of silica tetrahedra, nearly normal to c; the thermal ellipsoids are distorted to elliptical shapes in the sections perpendicular to Si – Si axes when temperature falls. On the other hand, the thermal ellipsoid for the apical O atom is approximately circular in that section.

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