Correlation of the atomic and electronic structures and the optical properties of the Σ5(2 1 0)/[ 0 01] symmetric tilt grain boundary in yttrium aluminum garnet

A series of atomic models of the Σ5(2 1 0)/[0 0 1] symmetric tilt grain boundary in yttrium aluminum garnet (YAG) are constructed according to the coincident site lattice theory. Calculations performed using empirical potentials show that the O-termination configurations, namely G(1) and G(2), are the most energetically favorable boundary structures. First-principles density functional theory calculations have been further performed to understand the atomic and electronic structures, effective charge, potential distributions and optical properties of G(1) and G(2) grain boundaries. The simulated high-resolution transmission electron microscopy images of G(1) and G(2) are generally in good agreement with the experimental micrographs of the Σ5(2 1 0)/[0 0 1] grain boundaries. Results show that the effective charges of the atoms in the grain boundary region are related to their coordination numbers and bonding lengths. Moreover, the overall total density of states of G(1) and G(2) has been found to have similar features with the bulk YAG, with the exception of some defect states being introduced at the top of the valence band, resulting in the reduction of the band gap. The calculated optical properties show that the refractive indices of both grain boundary models are slightly larger than those of the bulk YAG. This is in agreement with the experimental observation that the refractive index of the polycrystalline YAG is higher than that of its single crystalline counterpart.