Equation of state of alpha-Al2O3 (corundum) from quasi-harmonic atomistic simulations

A two-body interatomic potential function, including fractional atomic char ges and a shell model for oxygen, and supplemented by an Q-Al-O bond-angle energy term, was fitted to the structural, elastic and vibrational properti es of alpha-Al2O3, corundum, at ambient conditions. Full quasi-harmonic cal culations were then carried out on a p,T grid of 54 points in the domain 0- 40 GPa and 300-1700 K. The crystal structure was equilibrated at each point , taking into account the anisotropy of vibrational pressure and the therma l dependence of elastic constants, so as to obtain unit-cell edges, atomic coordinates, bulk modulus, thermal expansion coefficient and other thermody namic properties. Polynomial approximations were developed to represent the p,T dependence of these quantities. Comparison with experimental results f or the separate p (T = 300 K) and T (p = 0) behaviours shows very good agre ement, with average deviations of 0.1% for the unit-cell volume and 6% for the thermal expansion coefficient. The coupled p,T dependence of the proper ties of corundum is predicted to be very small for the bulk modulus (partia l derivative(2)K(T)/partial derivative p partial derivative T = 8.4 x 10-5 K-1), but not at all negligible for the volume [(1/V)partial derivative(2)V /partial derivative p partial derivative T in the range -1.2 to -7.5 x 10(- 7) GPa(-1) K-1 over the p,T domain explored].