Structure and stability of alkali gallates structurally reminiscent of hollandite

Abstract

AbstractSingle crystals of CsGa7O11, RbGa7O11, and RbGa4In5O14 were grown from alkali halide melts and their structures were characterized by single crystal and powder X‐ray diffraction. CsGa7O11 and RbGa7O11 adopt the same structure type, reminiscent of the hollandite structure type, as it contains nearly rectangular channels made up of two dimers of edge‐sharing GaO6 octahedra, and two corner‐sharing octahedron/tetrahedron pairs. The structure of RbGa4In5O14 is more complex and is comprised of indium octahedra, gallium trigonal bipyramids, and gallium tetrahedra, and contains similar sized tunnels as CsGa7O11 and RbGa7O11. CsGa7O11 and RbGa4In5O14 were further characterized by TGA, ion exchange experiments, and DFT studies revealing that both structures are thermodynamically stable up to 850°C; however, CsGa7O11 decomposes to GaO(OH) xH2O when heated in warm aqueous solutions. CsGa7O11 undergoes ion exchange in both an aqueous solution of RbCl and a RbNO3 melt, as predicted by DFT studies, where the ion exchange is more extensive in the RbNO3 melt.