NaCa4Nb5O17: a layered perovskite O3+2 compound

Sodium tetracalcium pentaniobium heptadecaoxide, NaCa4Nb5O17, corresponds to the n = 5 term of the homologous AnBnO3n+2 family of structures composed of ABX3 perovskite layers. The structure consists of perovskite-type blocks of n = 5 layers of NbO6 octahedra, separated by an interblock region. Successive blocks along the b axis are displaced by 1/2c with respect to each other. The deformation of the NbO6 octahedra is a minimum at the middle of each block, and increases along the direction of the axis to a maximum at each end of the block. Ca and Na share the same intrablock sites, coordinated by 12 O atoms, whereas only Ca atoms are found in the interblock cavities, at sites with different coordination geometries. (orig.)