Soft mode and structural phase transition in the cubic elpasolite Cs2NaNdCl6

Inelastic neutron scattering measurements have been performed on the elpasolite Cs₂NaNdCl₆ in the cubic phase. It is found that the cubic-to-tetragonal phase transition at 137 K is driven by a zone-center soft phonon mode. The mode is of symmetry Λ^′₁₅ and corresponds to a rotation of the Cl₆ octahedra. The characteristics of the phase transition are similar to those observed in some compounds of the perovskite and antifluorite structures.