Structural phase transition in the ordered fluorides MIIZrF6(MII=Co,Zn). I. Structural study

The mixed fluorides with formula M II ZrF 6 (M II =Co,Zn) exhibit an ordered ReO 3 -type structure: this structure is made of a three-dimensional arrangement of corner-shared octahedra with an ordered distribution of M II and Zr IV cations in the octahedra sites. These materials undergo around room temperature a first-order ferroelastic structural phase transition from a cubic phase (Fm3m) to a rhombohedral modification (R3), as established by neutron and X-ray diffraction experiments. From a structural determination of CoZrF 6 at different temperatures through the phase transition, three order parameters have been measured, which enable one to describe completely the extent of lattice distortion in the rhombohedral phase: the spontaneous strain e s , the coordinate of octahedra rotation around the threefold axis R and the octahedra internal deformation coordinate Q. Important thermal variations are observed for e s and R, but the coordinate Q always remains very small, showing that the Co(Zr)F 6 octahedra are essentially rigid bodies. X-ray diffuse scattering experiments on CoZrF 6 at room temperature (cubic phase) are consistent with the existence of structural disorder due to rotations of the Co(Zr)f 6 octahedra around the cubic fourfold axes.