Electron microscopic methods were used to elucidate the effect of structural modulations upon the conductivity of defect-fluorite solid solutions, xY2O3-(1-x)Ta2O5, where x ranged from 0.75 to 0.8. Diffuse circular reflections were observed near to the reciprocal lattice points, ½½½. Weak diffuse reflections were also observed at any hkl reciprocal lattice points which were forbidden for the Fm3m space group. Neither type of diffuse reflection was observed for specimens with x = 0.8; which were known to have much higher conductivities than Y3TaO7. The relatively low conductivity in the latter material was attributed to a local ordering of the structure; that gave rise to diffuse reflections. The ordered region was suggested to have a slab structure in which 2 types of slab, one containing pairs of associated O vacancies and the other containing no vacancies, alternated along <111> directions.

T.Tanaka, N.Ishizawa, F.Marumo, M.Yoshimura, Y.Tabira: Report of the Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 1995, 20, 47-51