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The temperature variation, T, of the crystal structure of quartz, SiO2, from 298 to 1235 K was obtained with synchrotron powder X-ray diffraction data and Rietveld structure refinements. The polymorphic transformation from P3221 (low-T, α quartz) to P6222 (high-T, β quartz) occurs at a transition temperature, Ttr = 847 K. The T variations of spontaneous strains and several structural parameters are fitted to an order parameter, Q, using Landau theory. The change in Si atom coordinate, Six, gives TtrTc = 0.49 K, which indicates an α ↔ β transition that is weakly first order and nearly tricritical in character (Q4T). Strains give higher TtrTc values (≃ 7 K). Other fitted parameters are the oxygen Oz coordinate, Si—Si distance, Si—O—Si and φ angles, and intensity of the (111) reflection, I111. In α quartz, the Si—Si distance increases with T because of cation repulsion, so the Si—O—Si angle increases (and φ decreases) and causes the thermal expansion of the framework structure that consists of corner-sharing distorted rigid SiO4 tetrahedra. The Si—Si distances contract with T and cause negative thermal expansion (NTE) in β quartz because of increasing thermal librations of the O atom in the Si—O—Si linkage that occur nearly perpendicular to the Si—Si contraction. In calcite, CaCO3, the short Ca—Ca distance expands with T, but the next-nearest Ca—Ca distance, which is of equal length to the a axis, contracts with T and causes NTE along the a axis. The thermal librations of the atoms in the rigid CO3 group increase with T along the c axis.

Supporting information


Crystallographic Information File (CIF)
Sample CIF of data set 0 at 298 K


Zip compressed file
Zipped file containing all CIFs named as data sets 0-66 at temperatures between 298 and 1235 K

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