Chemical Expansion in Barium Zirconate

Abstract

The formation volume of several types of point defects in BaZrO3 has been investigated using DFT and represented in terms of a defect strain tensor. The ionic radius for the vacancy and the hydroxide ion are both found to be smaller than the oxygen ionic radius. The radius for the substitutional yttrium ion is found to agree very well with experimental values, as is the formation volume of 0.28 A3 per unit cell upon hydration. The strain tensor was found to be non-isotropic but diagonal for both the vacancy and the hydroxide ion while a combination of point defects caused shear strain. The strain tensor in a finite concentration is thus not a linear combination of the strain tensors for the individual defects in the dilute limit. The cubic perovskite structure of BaZrO3 was also found to be unstable in the LDA and the I4/mcm symmetry was found to exhibit a lower energy although the phonon spectrum still included some imaginary modes.