Compressibility of brownmillerite (Ca2Fe2O5): effect of vacancies on the elastic properties of perovskites |
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| Institution: | 1. Crystallography Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, USA;2. Bayerisches Geoinstitut, Universität Bayreuth, D95440 Bayreuth, Germany;1. Scripps Translational Science Institute, The Scripps Research Institute, La Jolla, CA 92037, USA;1. National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093, China;2. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China;1. Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA;2. Department of Earth Sciences, University of Cambridge, Cambridge, UK;3. School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK;4. Department of Astronomy, University of California, Berkeley, CA 94720, USA |
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| Abstract: | The evolution with pressure of the unit-cell parameters brownmillerite (Ca2Fe2O5), a stoichiometric defect perovskite structure, has been determined to a maximum pressure of 9.46 GPa, by single-crystal X-ray diffraction measurements at room temperature. Brownmillerite does not exhibit any phase transitions in this pressure range. A fit of a third-order Birch–Murnaghan equation-of-state to the P–V data yields values of KT0=127.0(5) GPa and K0′=5.99(13). Analysis of the unit-cell parameter data shows that the structure compresses anisotropically. Compressional moduli for the axes are Ka0=141(1) GPa, Kb0=118(3) GPa and Kc0=122.2(2) GPa, with Ka0′=8.9(3), Kb0′=6.2(6) and Kc0′=4. The stiffest direction (i.e. along a) coincides with the direction of the FeO4 tetrahedral chains. Comparison of these data with the elasticity systematics of Ca-perovskites shows that the presence of oxygen vacancies in the brownmillerite structure softens the structure by ～25% and that the ordering of vacancies in the perovskite structure increases the anisotropy of compression. |
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