Crystal structure of an unusual polytype: 7H-Ba7Nb4MoO20

A new cation deficient hexagonal perovskite with the unusual polytype 7H (h hchhcc) has been obtained in the Ba-Mo-Nb-O system for the Ba7Nb4MoO20 comp osition. It crystallizes in the P (3) over bar m1 space group (No. 164) wit h hexagonal lattice parameters a = 5.8644(2) Angstrom and c = 16.5272(4) An gstrom. The crystal structure has been studied by means of X-ray powder dif fraction, electron diffraction, high-resolution electron microscopy, and ne utron powder diffraction. The unit cell of Ba7Nb4MoO20 can be seen as an or dered intergrowth along the c-axis of the structural units corresponding to the 12R and the palmierite polytypes. Both types of blocks are isolated th rough the cationic vacancies, giving rise to a layered structure. The exist ence of two cationic empty sites per unit cell is probably the reason for t he 7H polytype containing two hh pairs being adopted.