Structural phase transition in the ordered fluorides M^IIZrF₆ (M^II=Co,Zn). II. Brillouin and Raman scattering study

For pt.I see ibid., vol.2, p.7373 (1990). The first-order ferroelastic Fm3m to or from R3 structural phase transition occurring in ordered M^IIZrF₆ fluorides (M^II=Co,Zr) is studied by means of Brillouin and Raman scattering experiments. The Brillouin data obtained with a CoZrF₆ single crystal show that the elastic constant C₄₄ does not change in the cubic phase, when the transition temperature (T_c=272+or-1 K) is approached from above. The Raman spectra of CoZrF₆ and ZnZrF₆ in the cubic phase are consistent with group-theoretical predictions; in the rhombohedral phase, the spectra exhibit a splitting of the nu ₅ triply degenerate bending mode of the M^II(Zr)F₆ octahedra and show the existence of two soft rotatory modes of these octahedra. From these results and group-theoretical considerations, it is concluded that the phase transition can be considered as improper ferroelastic, driven by octahedra rotations; possibly, an intermediate behaviour between true displacing and order-disorder regimes takes place in these materials.