OXYGEN DIFFUSION IN DOPED LaGaO₃, A BOND VALENCE STUDY

Aniceta Skowron and Anthony Petric

Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, L8S 4L7 Canada,
Skowron@mcmail.mcmaster.CIS.ca

High oxygen ion conductivity, superior to that of Y/ZrO₂ at temperatures below 1000^oC, was recently discovered in LaGaO₃ doped with Sr and Mg [1, 2]. We studied the spatial and chemical aspects of the oxygen diffusion in LaGaO₃ using the bond valence method. We showed that to facilitate the oxygen passage from an occupied to a vacant site, relaxation of the neighbouring cations is necessary. The ease of oxygen passage, measured by its bond valence at the saddle point, is proportional to the degree of required relaxation.

In accord with the experimental findings [1, 3] we conclude that for high ionic conductivity, substitution of Sr for La is more favorable than substitution of Ca or Ba since the latter two cause more strain in the structure, lowering its stability, and may also be expected to act as vacancy repellant or sink, respectively. Sr substitution, on the other hand, facilitates the passage of the oxygen ion without significant strain. Substitution of Mg for Ga does not seem to enhance the ease of the oxygen diffusion. We discuss a model for a local lattice relaxation near the Mg substitution site.

In agreement with earlier studies [4], we show that the path of the migrating oxygen does not coincide with the edge of the Ga-O octahedron, curving away from it by approximately 0.4^o.

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