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Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction,thermal expansion and heat capacity |
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| Authors: | X Liu Y Wang R C Liebermann P D Maniar A Navrotsky |
| Institution: | 1. Center for High Pressure Research, USA 2. Mineral Physics Institute and Dept. of Earth and Space Sciences, State University of New York at Stony Brook, 11794, Stony Brook, NY, USA 4. Department of Geological and Geophysical Sciences, Princeton University, 08544, Princeton, NJ, USA |
| Abstract: | High-temperature x-ray powder diffraction study by the full pattern Rietveld method of orthorhombic CaGeO3 (Pbnm at ambient condition) perovskite confirms the previously observed phase transition at Tc=520 K. The measured volumetric thermal expansion coefficients are 3.1 x 10-5 (K-1) below Tc and 3.5x 10-5 (K-1) above Tc. The space group at T>Tc has been tentatively identified as Cmcm. Such a transition involves the disappearance of one of the two octahedral rotations in the (001) plane, and the doubling of the unit cell volume, with c axis unchanged. Although this transition should be of first order from symmetry considerations, the distortion of the Pbnm phase decreases continuously as the temperate approaches Tc and there is no observable volume discontinuity at Tc. The measured heat capacity places an upper limit on the enthalpy of transition of 50 J/mol, which is quite reasonable in terms of the crystallographic nature of this phase transition.A National Science Foundation Science and Technology Center |
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