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

An idealized structure model of the K3Nb7O19 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, α≅90, β=98.06(2) and γ≅900, V=1657.7 A3, 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 three-dimensional 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 M7O20. Two kinds of tunnels are formed, in which the K atoms are located. The structure thus independently derived agrees with that recently published by Fallon, Gatehouse and Guddat, which was determined from single-crystal X-ray data, although their unit cell is triclinic. We have refined the cell parameters given in that article: a=14.1282(9), b=9.9369(9), c=6.4553(6) A, α=106.44(1), β=95.856(6) and γ=97.30(1)0, V=853.05 A3, Z=2, Mr=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. The JCPDS Diffraction File No. for K3Nb7O19 is 38-1499. (orig.)