The thermal expansion of ScAlO3 — A silicate perovskite analogue |
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| Authors: | R J Hill Ian Jackson |
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| Institution: | 1. CSIRO Division of Mineral Products, PO Box 124, 3207, Port Melbourne, Victoria, Australia
2. Research School of Earth Sciences, The Australian National University, GPO Box 4, 2601, Canberra, ACT, Australia
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| Abstract: | The crystal structure of ScAlO3 has been refined at temperatures up to 1100° C on the basis of x-ray powder diffraction data. The thermal expansion is adequately
described by a Grüneisen-Debye model with the elastic Debye temperature and an effective Grüneisen parameter of 1.6. The volumetric
thermal expansion of 3.0% between 10 and 1100° C, corresponding to a mean thermal expansion coefficient of 2.7 × 10−5 K−1, is entirely attributable to the expansion of the AlO6 octahedra. The interoctahedral angles, though not fixed by symmetry, do not vary significantly with temperature —indicating
that the expansivities of the constituent AlO6 and distorted ScO8 polyhedra are well matched. Similar considerations of polyhedral expansivity suggest thermal expansion coefficients of ∼2
× 10−5K−1 for cubic CaSiO3 perovskite and a value between 2 × 10−5 K−1 and 4 × 10−5 K−1 for MgSiO3 perovskite. The lower value is consistent with the reconnaissance measurements for Mg0.9Fe0.1SiO3 (Knittle et al. 1986) below 350° C, with low-temperature measurements of single-crystal MgSiO3 (Ross and Hazen 1989), and with the results of some recent calculations. The markedly greater expansivity ∼4 × 10−5 K−1 measured at higher temperatures (350–570° C) by Knittle et al. is inconsistent with the simple Grüneisen-Debye quasiharmonic
model and may reflect the marginal metastability of the orthorhombic perovskite phase. Under these circumstances, extrapolation
of the measured expansivity is hazardous and may result in the under-estimation of lower mantle densities and the drawing
of inappropriate inferences concerning the need for chemical stratification of the Earth's mantle. |
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