Local Electronic Structure and Protonic Conductivity in Perovskite-Type Oxide, SrZrO₃

Abstract

Local electronic states around hydrogen and acceptor ions in SrZrO₃ are simulated by the DV-Xα 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.