Study of alpha-Al2O3 and the role of Y in YAlO3 and Y3Al5O12 by first principles electronic structure calculations

The ground state electronic properties of the alpha-Al2O3, YAlO3 and Y3Al5O12 (YAG garnet) systems have been studied using the tight-binding linearized muffin-tin orbital (TB-LMTO) method. We have also performed studies of the three compounds under isotropic compressions and expansions. For YAlO3, we have varied a, b and c lattice parameters to get the ground state. The calculated equilibrium lattice parameters for all these systems and the energy gap for Al2O3 agree very well with experimental and earlier theoretical results. We have calculated the cohesive energy and bulk modulus for them and a comparison with the earlier results (available only for Al2O3 and YAlO3) shows good agreement. Our calculations correctly account for the insulating property of these systems. The gap is found to be direct for Al2O3 and YAG and indirect for YAlO3. Moreover, the band gap values are found to be decreasing from Al2O3 to YAlO3 to Y3Al5O12, perhaps due to the Y atom as well as due to the increased number of atoms per primitive cell. The band gap for all these systems increases monotonically with pressure, suggesting that the gap may due to a strong Al/Y s/p-O p hybridization in these systems. Our calculated density of states (DOS) for Al2O3 compares very well with the theoretical and experimental results and for the rest, experiments are yet to come to confirm our results. The calculated P-V relation for Al2O3 compares very well with earlier calculations; however, it deviates slightly for YAlO3 due to the difference in the calculated equilibrium lattice parameters from the experimental values. (C) 2004 Elsevier B.V. All rights reserved