ELECTRON-MICROSCOPY STUDY OF A NEW CATION DEFICIENT PEROVSKITE-LIKE OXIDE - BA3MONBO8.5

A new ordering scheme in cationic-deficient perovskite-like phases of the Ba-n(Mo,Nb)(n-delta)O3n-x system has been described for the compos ition Ba3MoNbO8.5, which has an oxygen stoichiometry between those of the 9R polytype and the palmierite structure. An electron diffraction and high-resolution electron microscopy study shows that the oxygen de fect with respect to the 9R polytype (hhc)(3) leads to an ordered dist ribution of octahedra and tetrahedra in a 3:2 ratio in the cubic layer s. The flexibility of the (hhc)3 stacking sequence allows the formatio n of a modulated structure through an ordered shearing mechanism along c by introducing mixed BaO2.6 cubic layers in the 9R, polytype.