Four-dimensional description of the structure and phase transitions of KFeF4 (phase II and superstructure phase III).

The structural phase transitions of ${\mathrm{KFeF}}_{4}$ have been investigated by x-ray powder and monocrystal techniques. Dilatometric experiments have allowed us to characterize the transition between the so-called phase II and the twofold superstructure phase III. It is accompanied by a contraction of the a and b parameters, which is compatible with a rotation of the ${\mathrm{FeF}}_{6}$ octahedra around the c axis, and by an anomaly along the c direction, 10 K above the previous transition temperature. To investigate the phase transition II\ensuremath{\rightarrow}III, structural characterization of the two phases II and III has been achieved. Preliminary classical studies had shown that the related symmetry space groups were, respectively, Amma and Pmcn. A new structural four-dimensional investigation, which considers phase III as a modulated structure of the average of phase II, has confirmed these first results.