Thermoelastic properties of chromium oxide Cr₂O₃ (eskolaite) at high pressures and temperatures

A new synchrotron X-ray diffraction study of chromium oxide Cr₂O₃ (eskolaite) with the corundum-type structure has been carried out in a Kawai-type multi-anvil apparatus to pressure of 15 GPa and temperatures of 1873 K. Fitting the Birch–Murnaghan equation of state (EoS) with the present data up to 15 GPa yielded: bulk modulus (K_0,T0), 206 ± 4 GPa; its pressure derivative K′_0,T, 4.4 ± 0.8; (∂K_0,T/∂T) = ‒0.037 ± 0.006 GPa K^‒1; a = 2.98 ± 0.14 × 10⁻⁵ K⁻¹ and b = 0.47 ± 0.28 × 10^‒8 K^‒2, where α_0,T = a + bT is the volumetric thermal expansion coefficient. The thermal expansion of Cr₂O₃ was additionally measured at the high-temperature powder diffraction experiment at ambient pressure and α_0,T0 was determined to be 2.95 × 10⁻⁵ K⁻¹. The results indicate that coefficient of the thermal expansion calculated from the EoS appeared to be high-precision because it is consistent with the data obtained at 1 atm. However, our results contradict α₀ value suggested by Rigby et al. (Brit Ceram Trans J 45:137–148, 1946) widely used in many physical and geological databases. Fitting the Mie–Grüneisen–Debye EoS with the present ambient and high-pressure data yielded the following parameters: K_0,T0 = 205 ± 3 GPa, K′_0,T = 4.0, Grüneisen parameter (γ₀) = 1.42 ± 0.80, q = 1.82 ± 0.56. The thermoelastic parameters indicate that Cr₂O₃ undergoes near isotropic compression at room and high temperatures up to 15 GPa. Cr₂O₃ is shown to be stable in this pressure range and adopts the corundum-type structure. Using obtained thermoelastic parameters, we calculated the reaction boundary of knorringite formation from enstatite and eskolaite. The Clapeyron slope (with d P/ d T= - 0.014 GPa/K) was found to be consistent with experimental data.