Elastic Properties of Calcium Oxide Perovskites
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Elastic Properties of Calcium Oxide Perovskites Nancy L. Ross, Ross J. Angel, Jennifer Kung1, and Tracey D. Chaplin2 Department of Geological Sciences, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, USA 1 Department of Geosciences, State University of New York, Stony Brook, NY 11794, USA 2 Department of Chemistry, University College London, Gower Street, London WC1E 6BT, U.K.
ABSTRACT The equations of state and axial moduli of the CaBO3 perovskites (B=Zr,Sn,Ti,Ge) and CaFeO2.5 with the brownmillerite structure have been determined using high-pressure, singlecrystal X-ray diffraction. The bulk modulus-specific volume relationship for the Ca-perovskites is nonlinear, with CaSnO3 and CaZrO3 displaying anomalous stiffening (higher bulk moduli) than previously reported and predicted [1,2]. The axial moduli of the a- and c-axes decrease steadily by ~30% from the least-distorted of the Pbnm perovskites, CaGeO3, to the most distorted, CaZrO3, while the b-axis shows little change. The net result is a threefold increase in the anisotropy of the axial moduli of CaSnO3 and CaZrO3 (~21%) relative to CaGeO3 and CaTiO3 (~ 4-8%). The bulk modulus of CaFeO2.5 falls significantly below the trend for the stoichiometric perovskites. The introduction of 1/6 vacancies on the oxygen positions softens the perovskite structure by 25%. INTRODUCTION Many ABO3 compounds with the perovskite structure exhibit orthorhombic Pbnm symmetry under ambient conditions and are isotypic with GdFeO3. Of this group of perovskites, the A2+-B4+ perovskites are of particular interest to earth scientists because the lower-mantle inventory of Mg2+ and Ca2+ is believed to be contained within the silicate perovskites, MgSiO3 and CaSiO3. The degree of distortion in these orthorhombic perovskites from the ideal cubic structure can be described in terms of the tilting of the [BO6] octahedra [3,4]. This tilting varies systematically with the size-ratio of the cations occupying the dodecahedral site (A2+ cation) and octahedral site (B4+ cation) [5]. The series of CaBO3 perovskites, where B=Zr,Sn,Ti,Ge,Si, include Pbnm perovskites with different degrees of distortion and CaSiO3 perovskite which has structure close to the ideal cubic prototype. A related phase, CaFeO2.5 , has the brownmillerite structure and can be described as a structure in which oxygens have been removed from a hypothetical CaFeO3 perovskite to produce a framework structure with ordered oxygen vacancies. This paper summarizes the equations of state and axial moduli of these phases, determined, with the exception of CaSiO3 perovskite, from high-pressure, single-crystal X-ray diffraction [2,6-8]. CaSiO3 perovskite becomes amorphous upon quenching to ambient conditions and its equation of state can only be studied in situ at high pressures and temperatures, which has been done using powder X-ray diffraction [9,10]. The elastic properties vary systematically with degree of distortion and the presence of vacancies affects both the bulk modulus and degree of anisotropy. D2.4.1 Dow
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