Doped Apatites for Novel Solid Oxide Fuel Cell Applications

Abstract

The aim of this study is to gain insight into the local coordination of doped Apatites (e.g. La8Y2Ge6O27)using the GULP (General Utility Lattice Program) code. Apatite ceramics are believed to facilitate oxygen conduction by rearranging its local structure in the presence of oxygen excess. By understanding the oxygen conduction mechanism we can design materials with improved functionality.The objectives of this study are twofold: firstly, to investigate the energetics of doping in the Ge apatites and secondly, to further investigate the interstitial-oxide migration pathway and energy barriers involved in the migration. It has been found that isovalent doping with ions of smaller ionic radii than Ge4+, e.g. Si4+and Ti4+, are favourable, as is hypervalent doping on the Sr site, leading to an oxide ion excess in the structure. The excess oxide ion migration pathway takes place via a wheel mechanism with an energy barrier of 0.66 eV. This mechanism is different from the one proposed in Si-based apatites, which is done by a sinusoidal trajectory.