Structures of hexagonal vanadium fluoride bronzes: A high-resolution electron microscopic study

Abstract

At least three and possibly four phases are shown to exist in the system A x VF 3, where A = K, Rb, Cs, and Tl and x = 0.19–0.32. The α I phase has the hexagonal tungsten bronze structure with a doubled c axis. For this structure there is one hexagonal tunnel containing two A sites per unit cell. Calculated electron microscope images, assuming that only one layer of A atom sites is occupied, simulate experimental images of this phase from both the [001] and [100] directions. The α II phase is an orthorhombic superstructure containing two tunnels and four A sites per unit cell. Comparison of observed and calculated images shows one tunnel to be more fully occupied than the other. The α IV phase has a c axis the length of six octahedra. Its existence is shown by diffraction patterns and fringe images. The existence of an α III phase, having four hexagonal tunnels per unit cell, is suggested on the basis of diffraction patterns.