X-ray powder diffraction study on the modulated high temperature forms of SiO2 tridymite between 110 and 220 °C

 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 SiO_4/2 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