Local electronic structure and protonic conductivity in perovskite-type oxide, SrZrO3

Local electronic states around hydrogen and acceptor ions in SrZrO3 are simulated by the DV-X alpha molecular orbital method to examine their effects on protonic conductivity. The calculated ioncities of the acceptor dopant ion, M. and the surrounding six oxygen ions, O-(i) (i = 1-6) are found to change largely with M in the doped oxide, where M's are Yb, Y, In, Al and Ga. There is a clear tendency that the protonic conductivity decreases as these ionocities around the dopant ion. M, deviate further from the ones around the Zr ion in un-doped oxide. Also, in a geometrical viewpoint, the bond order between M and O-(i) (i = 1-6) ions is another indication to control the protonic conductivity. The presence of the slightly weaker M-O-(i) bond than the Zr-O bond causes small expansion of the MO6 octahedron, and then gives a nearly symmetrical position for proton to move readily to the neighboring oxygen sites. These results are also found in the other oxides, BaZrO3 and CaZrO3. Both the ionicity and the bond order are indeed useful parameters for the design of perovskite-type oxides with high protonic conductivity.