The thermal expansion of ScAlO₃ — A silicate perovskite analogue

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 Mg_0.9Fe_0.1SiO₃ (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.