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Title: Quartz: structural and thermodynamic analyses across the α ↔ β transition with origin of negative thermal expansion (NTE) in β quartz and calcite

The temperature variation,T, of the crystal structure of quartz, SiO 2, from 298 to 1235 K was obtained with synchrotron powder X-ray diffraction data and Rietveld structure refinements. The polymorphic transformation fromP3 221 (low-T, α quartz) toP6 222 (high-T, β quartz) occurs at a transition temperature,T tr= 847 K. TheTvariations of spontaneous strains and several structural parameters are fitted to an order parameter,Q, using Landau theory. The change in Si atom coordinate, Si x, givesT tr-T c= 0.49 K, which indicates an α ↔ β transition that is weakly first order and nearly tricritical in character (Q 4∝T). Strains give higherT tr-T cvalues (≃ 7 K). Other fitted parameters are the oxygen O zcoordinate, Si—Si distance, Si—O—Si and φ angles, and intensity of the (111) reflection,I 111. In α quartz, the Si—Si distance increases withTbecause 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 SiO 4tetrahedra. The Si—Si distances contract withTand 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, CaCOmore » 3, the short Ca—Ca distance expands withT, but the next-nearest Ca—Ca distance, which is of equal length to theaaxis, contracts withTand causes NTE along theaaxis. The thermal librations of the atoms in the rigid CO 3group increase withTalong thecaxis.« less
Publication Date:
OSTI Identifier:
Resource Type:
Journal Article
Resource Relation:
Journal Name: Acta Crystallographica. Section B, Structural Science, Crystal Engineering and Materials (Online); Journal Volume: 72; Journal Issue: 2
International Union of Crystallography
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States