POSITIONAL AND THERMAL DISORDER IN THE TRIGONAL BIPYRAMID OF MAGNETOPLUMBITE STRUCTURE TYPE SRGA12O19

The crystal structure of SrGa12O19 has been refined at 473 K, 298 K and 160 K to R-values of 0.025, 0.023 and 0.030, respectively. The temperature dependence of the Debye-Waller factors shows that five-fold coordinated Ga3+ is randomly displaced from the center of its trigonal bipyramidal coordination polyhedron along positive and negative directions of the c-axis. The magnitude of displacement increases with the temperature. Although the Madelung part of lattice energy favours central location, this is prevented by repulsion of the three equatorial oxygen atoms.