X-ray powder diffraction study on the modulated high temperature forms of SiO2 tridymite between 110 and 220?°C |
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| Authors: | H Graetsch |
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| Institution: | Institut für Geologie, Mineralogie und Geophysik Ruhr-Universit?t Bochum, 44780 Bochum, Germany Tel.: 0234 32 23516; Fax: 0234 32 14433 e-mail: heribert.graetsch@ruhr-uni-bochum.de, DE
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| Abstract: | The crystal structure of intermediate incommensurate tridymite was refined at 150 °C from powder data. Upon cooling from
above 220 °C, the basic structure with space group symmetry C2221 is gradually distorted from orthorhombic to monoclinic symmetry. With decreasing temperature, the monoclinic angle γ smoothly
opens up to 90.3°, while a displacive modulation with temperature-dependent wavelength develops. The 3 + 1 dimensional superspace
group of the incommensurate phase is C1121(αβ0). The modulation mainly consists of two sinusoidal transverse displacement waves for the silicon atoms coupled to rotations
of the rigid SiO4/2 tetrahedra. The wave vector is r=0.1192(1)a* − 0.0043(1)b* at 150 °C. Below 150 °C tridymite discontinuously transforms to another orthorhombic phase and the modulation partially
locks in at the wave vector r
1=1/3a*. Simultaneously, an additional incommensurate modulation with r
2= 0.0395(1)b* − 0.3882(1)c* is formed. The two-dimensional modulation does not vary significantly with the temperature.
Received: 13 September 2000 / Accepted: 29 January 2001 |
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| Keywords: | Tridymite Silica Incommensurate modulation Displacive phase transitions Lock-in |
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