Abstract

The local structure and phase transformations of Ga-doped Ba2In2O5 are studied by cluster expansion (CE) calculations based upon density functional theory (DFT), and Monte Carlo (MC) simulations based upon CE. Further, we conducted x-ray diffraction experiments which confirmed our theoretical results. Our results show that Ba2In2O5 (BIO) has two phase transformations with increasing temperature, from an orthorhombic (ortho) phase to tetragonal (tet) and finally to a disordered cubic (cub) form, all of which is consistent with the results of Speakman et al. High oxygen ion conductivity occurs in tet and cub phases therefore these phases should be stabilized at lower temperatures for applicability in solid oxide fuel cells. We show with MC that increasing the Ga doping amount in Ba2(In1-xGax)2O5 (BIGO) results in a progressive reduction in the low- and high-temperature phase transformation temperatures. The ortho-tet phase transformation is even completely suppressed for BIGO at x>0.2 within quenched samples, as also experimentally shown by Yao et al. However, the present theoretical and experimental studies indicate phase separation of Ga-rich and -poor regimes for BIGO prepared at equilibrium conditions.