Experimental determination of thermal expansivity of calcium ferrite-type MgAl₂O₄ and its application to thermodynamical assessment of thermoelastic parameters

High-temperature X-ray diffraction measurements of calcium ferrite (CF)-type MgAl₂O₄ were performed in a temperature range of 300–673 K at atmospheric pressure. From temperature dependence of the unit cell volume, thermal expansivity (α) was determined to be α(T) = (2.46 ± 0.13) × 10^–5 + (1.2 ± 0.3) × 10^–8 T in 1/K. Thermoelastic parameters of isothermal bulk modulus at zero pressure (K_T0), its pressure derivative (K_T′) and temperature derivative [(∂K_T0/∂T)_P] of MgAl₂O₄ CF were optimized by iteration calculation combining the least squares fitting of a third-order Birch–Murnaghan equation of state to previous P–V–T data with α calculation using the Grüneisen relation equation, α = γ_thC_V/(K_T0V) where γ_th and C_V are thermal Grüneisen parameter and isochoric heat capacity, respectively. γ_th was constrained by the α measured in this study. When pressure data were rescaled by Au equations of state which are different from that adopted in the previous study and temperature data were corrected using pressure dependence of electromotive force of a W–Re thermocouple, K_T0, K_T′ and (∂K_T0/∂T)_P were assessed to be 216(4) GPa, 3.9(3) and − 0.027(3) GPa/K, respectively. It was suggested that the optimized α was about 17% lower than that determined by the previous study at 2000 K.