The mixed fluorides with formula MIIZrF6 (MII=Co,Zn) exhibit an ordered ReO3-type structure: this structure is made of a three-dimensional arrangement of corner-shared octahedra with an ordered distribution of MII and ZrIV 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 CoZrF6 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 es, the coordinate of octahedra rotation around the threefold axis R and the octahedra internal deformation coordinate Q. Important thermal variations are observed for es and R, but the coordinate Q always remains very small, showing that the Co(Zr)F6 octahedra are essentially rigid bodies. X-ray diffuse scattering experiments on CoZrF6 at room temperature (cubic phase) are consistent with the existence of structural disorder due to rotations of the Co(Zr)f6 octahedra around the cubic fourfold axes.