DETERMINATION OF THE LITHIUM SITES IN LiGa5O8 AND LiAl7B4O17
Stefan Borg and Göran Svensson
Department of Inorganic Chemistry, Chalmers University of
Technology, SE-412 96 Göteborg, Sweden,
E-mail: borg@inoc.chalmers.se
Keywords: lithium, LiGa5O8, LiAl7B4O17,
powder diffraction, neutron
Lithiumpentagalliumoctaoxide, LiGa5O8 has been widely used in optical studies as a carrier compound of d elements in tetrahedral sites 1. Doped with small amounts of Co2+ it is a possible photo-echo material or a host material for tunable lasers in the infrared region. In order to receive full understanding of the optical processes in Co2+ -doped LiGa5O8 and the results of circular dichroism studies 2 a precise structure determination is required.
The structure of LiGa5O8 has been determined by X-ray single crystal diffraction 3. It crystallizes as an invers spinel in the cubic space group P4332 (or P4132) with a = 8.2023(6) A and four formula units per cell. The gallium atoms are both in tetrahedral and octahedral sites while lithium is octahedrally coordinated. Ideally, the chains of edge-sharing octahedra have every fourth octahedron occupied by lithium and the rest occupied by gallium. It was found that the actual chains were disordered with lithium positions occupied by gallium and vice versa. This distortion is much greater for LiGa5O8 compared to the isostructural compounds LiAl5O8 4 and LiFe5O8 5.
The structure determination of LiAl7B4O17 gave the tetragonal space group I4/m with unit cell parameters a = 10.5454(7) A and c = 5.6246(4) A and two formula units per cell 6. The structure is built up of chains of edge-sharing AlO6 octahedra along one direction. The chains are cross linked in two ways, by boron atoms and assemblies of four AlO5 trigonal bipyramides, forming channels throughout the structure in the c-direction. The lithium atoms are located in the channels, which have a diameter of approximately 3.5 A, not occupied by the AlO5 groups. Lithium is disordered in at least two positions coordinated by five respectively six oxygen atoms with occupancy approximately 2/3 and 1/3.
In order to determine the extent of the disorder of lithium for LiGa5O8 and the lithium positions and occupancy for LiAl7B4O17 neutron powder diffraction data has been collected. These results will be presented.