Elasticity of single-crystal MgAl₂O₄ spinel up to 1273 K by Brillouin spectroscopy

The adiabatic single-crystal elastic constants, C_ij, of stoichiometric magnesium aluminate spinel (MgAl₂O₄) have been measured up to 1273 K by highresolution Brillouin spectroscopy, using a 6-pass tandem Fabry-Pérot interferometer and an argon ion laser (514.5 nm). Two platelet samples were employed for probing the acoustic phonons along [100] and [110] directions by platelet and backscattering geometries. The measured temperature dependences of the elastic moduli show a distinct anomaly at 923 K in the shear modulus C_s = (C₁₁-C₁₂)/2 (along [110] direction) and the longitudinal modulus C₁₁ (along [100] direction). This anomaly is consistent with the order-disorder phase transition, resulting from the atomic exchange between Mg at the tetrahedral site and Al at the octahedral site, which has been well documented recently (Peterson et al. 1991; Millard et al. 1992) by neutron powder diffraction and ²⁷Al magic-angle spinning NMR. The values of the temperature derivatives of v_p, v_s, and K_s, in the temperature range 300-923 K, calculated by the Voigt-Reuss-Hill approximation are -0.40ms^-1 K^-1, -0.26ms^-1 K^-1, and -1.89 x 10^-2GPaK^-1.