Crystal structures of some niobium and tantalum oxides: IX. K 3Nb 7O 19: A new potassium niobium oxide tunnel structure

Abstract

K 3Nb 7O 19 crystallizes in the triclinic system with unit cell dimensions (from single-crystal data) a = 14.143(3), b = 9.948(2), c = 6.463(2)Å, α = 106.45(2), β = 95.82(1), γ = 97.29(1)°, and space group P 1, z = 2 . The structure was solved using the Patterson (“P1” method) and Fourier techniques. Of the 4974 unique reflections measured by counter techniques 2722 with I ≥ 3 σ( I) were used in the least-squares refinement of the model to a conventional R of 0.049 ( R w 0.054). The structure of K 3Nb 7O 19 may be described as consisting of edge-shared pairs of octahedra that are corner shared to one another to form double strings seven pairs long. Each of these strings is corner shared to four other identical strings forming tunnels bounded by six octahedra. Two pairs of octahedra at each end of the strings of seven pairs are edge shared to adjacent strings forming a second series of tunnels approximately perpendicular to the first series. Potassium ions are located within both series of tunnels. The structure may also be described as one in which the oxygen ions are in approximately cubic close-packed layers perpendicular to [501] with potassium ions occupying anion sites in a regular manner.