High-Temperature Structural Evolution in the Ba 3 Mo (1- x) W x NbO 8.5 System and Correlation with Ionic Transport Properties.

The evolution of the hybrid structure between 9R hexagonal perovskite and palmierite in the entire Ba3 Mo(1- x) W x NbO8.5 solid solution (where x = 0, 0.25, 0.5, 0.75, and 1) was probed in the 100-900 K range by synchrotron high-resolution powder diffraction. Each sample exhibits a chemical-dependent structural model in the low-temperature regime (from 100 to 500 K) in which 9R and palmierite structures compete each other, the former being progressively favored as tungsten replaces molybdenum. Above 500 K, unit cell parameters and metal site occupancies start to converge toward a similar structural arrangement that is completely reached at 900 K. In fact, at this temperature, the entire solid solution discloses comparable unit cell and an almost enterely occupied M1 site, with a structure that is much closer to palmierite rather than 9R polytype. The present crystallographic results well explain the behavior of the material's bulk ionic conductivity, whose temperature evolution for different compositions depends from the contribution of tetrahedral units proper of the palmierite structure.