Studies of K₃Nb₇O₁₉ by high-resolution electron microscopy and X-ray powder diffraction

Abstract

An idealized structure model of the K3Nb7Ol9 compound has been deduced from high-resolution electron micrographs. For simplicity the structure was derived in a pseudo-monoclinic system with triclinic symmetry (space group P1) and unit-cell dimensions a = 13.777 (2), b =6-431 (1), c= 18.897 (6) A, a-~ 90, /3 = 98.06 (2) and y -~ 90 °, V = 1657.7 A 3, Z = 4. It has been verified both by simulated image calculations in two projections and by X-ray powder diffraction studies. The structure consists of a threedimensional framework which is built up of pairs of NbO6 octahedra having one edge in common in such a way that the structure can be described by means of{011} crystallographic shear in the ReO3 type structure. The pairs are linked by edge and corner sharing as in the {120} crystallographic-shear (CS) structure M702o. Two kinds of tunnels are formed, in which the K atoms are located. The structure thus independently derived agrees with that recently published by triclinic. We have refined the cell parameters given in that article: a = 14.1282 (9), b--9.9369 (9), c = 6.4553 (6) A, a= 106.44(1), /3=95.856(6) and y= 97.30 (1) °, V= 853.05 ~3, Z =2, M~ = 1071.64, and Dx = 4.17 g cm -3. However, in this paper the structure is described in a different manner and discussed in relation to neighbouring compounds of this system.