| 초록 |
The crystal structure of Ba11W4O23, believed to be cryolite-related but remained unclear so far, has been solved for the first time. The crystal structure has been refined by powder neutron and x-ray Rietveld method (Fd3m, a=17.1881(1) Å, Z=8, 25°C). It is a remarkable example of A-site vacancy-ordered 4x4x4 superstructure of a simple perovskite ABO3. It may be written as (BaBa0.75↵0.25)(BaW)O5.75↵0.25, emphasizing the defects on metal-site as well as oxygen-site. The local structure of one of two asymmetric tungsten ions is WO6 octahedron, typical of perovskite. But that for the other, WO18/3 polyhedron, is extraordinary. Eighteen oxygen ions, three of which are asymmetric, are disordered and partially occupied (~30% on the average). The WO18/3 polyhedra form three-dimensional chains with the ordered cation vacancies in between along the way. Bond valence summations provided a support to the justification of the metal-site vacancy as well as large anisotropic thermal vibration amplitudes of some oxygen and barium ions of the complex and unique crystal structure. Since structure-property relation is important, it was quite obvious to expect the ionic conductivity of the compound. The total conductivities up to 1100°C in air were lower than those for the YSZ by an order of magnitude. However, it is anticipated that a number of new materials with various properties would be produced as the derivatives by substitution at Ba, W and/or oxygen sites. |
