Electron-microscope study of the crystal structures of mixed oxides in the systems Rb₂O-Ta₂O₅, Rb₂O-Nb₂O₅ and K₂O-Ta₂O₅ with composition ratios near 1:3. I. Stacking characteristics of MO₆ layers

The crystal structures of phases, previously designated as 11L, 9L and 16L, which were commonly found in the title systems are suggested from their high-resolution two-dimensional electron-microscope images. Two types of blocks are found which are composed of five and six layers of pentavalent-metal oxide octahedra and are closely related to the pyrochlore structure. The 11L structure is of rhombohedral stacking of the former type of block, and the 9L structure is of hexagonal stacking of the latter type of block. The 16L structure is found to be an alternate stacking of the two types of blocks. The method of stacking of the blocks is the same for the three structures and is found to be favorable from the point of view of local charge balance.