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Science 16 December 1988:
Vol. 242. no. 4885, pp. 1539 - 1541
DOI: 10.1126/science.242.4885.1539
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Articles

Dynamics of a Second-Order Phase Transition: P&1macr; to I1 Phase Transition in Anorthite, CaAl2Si2O8

Subrata Ghose 1, G. Van Tendeloo 2, and S. Amelinckx 2

1 Mineral Physics Group, Department of Geological Sciences, University of Washington, Seattle, WA 98195
2 University of Antwerp, Rijksuniversitair Centrum Antwerpen, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

Electron microscopic study of the reversible P1 to I1 phase transition in anorthite (transition temperature Tc = 516 Kelvin) shows that the antiphase boundaries (APBs) with the displacement vector R = 1/2[111] become unstable at Tc, and numerous small APB loops are formed. These interfaces are highly mobile, and their vibration frequency increases strongly with temperature. These observations suggest that close to Tc, breathing-motion—type lattice vibrations of the framework cause the two different configurations around the calcium atoms, which are related by a translation of R ap 1/2[111], to interchange dynamically through an intermediate I1 configuration. The high-temperature I1 structure is interpreted as a statistical-dynamic average of highly mobile antiphase domains of primitive anorthite.

Submitted on July 25, 1988
Accepted on October 24, 1988


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