NEUTRON POWDER REFINEMENTS OF THE 3 STRUCTURAL FORMS OF RB2KSCF6

The three allotropic forms of Rb2KScF6 have been refined by the Rietve ld method from the neutron powder diffraction data collected at 150K, 240K and 300K. At 150K, the monoclinic variety of Rb2KScF6 (space grou p P12(1)/n1) is related to the cryolite-type structure. With increasin g temperature, rotational motions of the ScF6 octahedra and a shift of the rubidium atoms lead to the tetragonal variety (space group I4/m). At higher temperature, a greater disorientation of the ScF6 octahedra leads to the cubic elpasolite-type structure (space group Fm (3) over bar m). Both displacive and order/disorder models have been considere d in order to account for the the phase transition mechanisms.