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; (a,K (0,T) /a,T) = aEuro'0.037 +/- 0.006 GPa K-aEuro'1; a = 2.98 +/- 0.14 x 10(-5) K-1 and b = 0.47 +/- 0.28 x 10(aEuro'8) K-aEuro'2, where alpha (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 alpha (0,T0) was determined to be 2.95 x 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 alpha (0) 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-Gruneisen-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, Gruneisen parameter (gamma (0)) = 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 GPa/K) was found to be consistent with experimental data.