Y:BaZrO3 Perovskite Compounds I: DFT Study on the Unprotonated and Protonated Local Structures

Y-doped BaZrO3 derivatives were studied by Density Functional Theory calculations, investigating local arrangements of octahedral site environments embedded in Pm-3m cubic frameworks. Single and double substitutions of zirconium by yttrium were considered, including eventually a nearby oxygen-vacancy. Although the structural symmetries of undoped barium zirconate are not modified after yttrium doping, the presence of yttrium induces differences in the oxygen sites around it, according to the local geometrical arrangement of yttrium in the host matrix. The differences of such oxygen sites are shown, as an example, in presence of a proton. In this case, different stabilization energies characterize protonated fragments. Only in those structures where two yttrium atoms are neighbors (i.e. forming Y-O-Y moieties), relative energy-differences of the corresponding proton stable sites were in agreement with the correct order of magnitude of the experimental proton-hopping activation energy value. Distribution of such energy-differences suggested to group the oxygen atoms into three sets, owning peculiar structural features not easily deducible by an inspection of their topologies. The existence of proton traps were also discussed on the basis of the energy-difference distribution above.