X-ray and neutron investigations of the phase instabilities in K2SnCl6

From X-ray and neutron diffraction work with single crystals and powders we found two phase transitions at T_c1 = 260.5 ± 1 K and at T_c2 = 255 ± 1 K. The transition at T_c1 is characterised by the occurence of superstructure reflections breaking the f.c.c. translational symmetry, whereas the second one at T_c2 is indicated by a monoclinic lattice distortion without any further loss of translational symmetry. Both transitions can be explained by the softening of optical branches at the X- and the Γ-point of the Brillouin zone, corresponding to librations of the SnCl₆-octahedra. Evidence for this low energy phonon branch is offered from the observation of diffuse streaks with X-rats and from their inelastic nature as revealed by the neutron triple axes technique.