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American Mineralogist; November 2007; v. 92; no. 11-12; p. 1912-1918; DOI: 10.2138/am.2007.2585
© 2007 Mineralogical Society of America
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High-pressure structure and bonding in CaIrO3: The structure model of MgSiO3 post-perovskite investigated with time-of-flight neutron powder diffraction

C. David Martin1,*, Ronald I. Smith2, William G. Marshall2 and John B. Parise1,3

1 Geosciences Department, 255 Earth and Space Sciences Building, Stony Brook University, Stony Brook, New York 11794-2100, USA
2 ISIS Neutron Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
3 Chemistry Department, Stony Brook University, Stony Brook, New York 11794-3400, USA

Correspondence: * E-mail: martin{at}anl.gov

The structure of CaIrO3 (Cmcm) has been refined at high pressure and at low temperature using time-of-flight neutron powder diffraction data. Evidence supporting deviation from space group Cmcm to Cmc21 is inconclusive. As CaIrO3 (Cmcm) unit-cell volume changes, refinements indicate deformation of cation-centered coordination polyhedra, rather than tilting. Structure models demonstrate Ca2+-centered polyhedra are an order of magnitude more compressible than Ir4+-centered octahedra. Bond valence sums show significant chemical strain (over-bonding) of calcium and oxygen at ambient conditions. Implications for structure change in MgSiO3 post-perovskite are discussed and a method for predicting the Clapeyron slope between perovskite and post-perovskite phases is proposed based on extrapolation of the volume-ratio between cation-centered polyhedra.

Key Words: Post-perovskite • high pressure • structure • neutron diffraction • Rietveld refinement • bond valence • D'' layer • CaIrO3






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