Unusual Pressure Effect on the Shear Modulus in MgAl2O4 Spinel

velocities of polycrystalline MgAl2O4 spinel have been measured up to 14 GPa and 900 K using ultrasonic interferometry and in situ X-ray diffraction techniques. Here, we observed a weaker pressure dependence in shear modulus (G) for MgAl2O4 spinel, as compared to a stronger ∂G/∂P for magnesium silicate/germanate counterpart. Our first-principles calculations show that the tetragonal shear modulus CS = (C11−C12)/2 decrease with pressures, indicating acoustic mode softening, which further supports our observed experimental results. Using a finite strain equation of state approach the elastic bulk and shear moduli, as well as their pressure and temperature derivatives, are derived from the directly measured velocities and densities, yielding KS0 =196.0(9) GPa, G0 = 109.0(4) GPa, ∂KS/∂P = 4.60(9), and ∂G/∂P = 0.58(3) independent of pressure calibration. The temperature derivatives for the bulk and shear moduli were tightly constrained from acoustic velocity measurements as ∂KS/∂T = −0.022(3) GPa/K and ∂G/∂T = −0.014(1) GPa/K. In addition, the mechanism for the unusual pressure effect on the shear modulus in MgAl2O4 spinel has been addressed by the coupling between atomic displacements and shear strains, namely a better accommodation of the AlO6 octahedral distortion and shear strains, as well as the pressure-induced tilting/distortion and/or symmetry changes in MgAl2O4 spinel. I