Transformations of the stuffed cristobalites, K2MSiO4, M=Mg, Zn, Co, Cd, with temperature and composition |
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Authors: | W A Dollase |
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Institution: | (1) Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA Fax: +(310)825-2779, US |
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Abstract: | The effects of composition and of temperature on the orthorhombic, Pca2
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to cubic, F4ˉ3m transition of the stuffed cristobalite structure are reported. A distorton index which measures the departure of the orthorhombic
unit cell from a metrically cubic cell shows that at room temperature, distortion increases in the progression K2CdSiO4 <K2MgSiO4 <K2ZnSiO4≈K2CoSiO4. High temperature X-ray powder measurements document an apparently discontinuous transition to a structure of F4ˉ3m symmetry. Differential scanning calorimetry shows a sharp, reversible, first order transition to the high temperature phase
at about 500–600 °C for these compounds. Measured transformation enthalpies in the range of 7 to 16 J/g correlate roughly
with the distortion index. The transformation involves tetrahedral rotation to an orientationally disordered cubic structure
which retains an ordered M2+/Si distribution.
Received: 8 November 1996 / Revised, accepted: 14 October 1997 |
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