Rotary modes in the antifluorite crystal (${\mathrm{ND}}_4$${)}_2$${\mathrm{SnCl}}_6$

A selection of normal modes of vibration and libration in a single crystal of (${\mathrm{ND}}_4$ ${)}_2$ ${\mathrm{SnCl}}_6$ is studied at several temperatures by means of thermal neutron inelastic scattering techniques. In particular, the behavior of the longitudinal rotary mode of the ${\mathrm{SnCl}}_6$ octahedra is investigated and compared with the ‘‘soft-mode’’ behavior of this libration in other antifluorite crystals. We also observe the lowest-lying librational mode of the ${\mathrm{ND}}_4$ ${\mathrm{}}^{+}$ ions, and find it to be dispersionless. Values are obtained for the elastic constants of (${\mathrm{ND}}_4$ ${)}_2$ ${\mathrm{SnCl}}_6$ from the slopes of the low-frequency acoustic-mode dispersion curves.