Structural transformations and the critical and noncritical parameters during the phase transition in the (NH4)2KWO3F3 oxyfluoride

The structures of two phases of the (NH4)(2)KWO3F3 crystal have been determined from X-ray diffraction data obtained for a powder sample. The profile and structural parameters have been refined according to the procedure implemented in the DDM program. The results obtained have been discussed with invoking the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical and noncritical atomic orderings and allows one to interpret the experimental data. It has been found that the symmetry transformation in the crystal can be schematically represented in the form Fm (3) over barm (10-10((X) over bar5)) -> ((eta, 0, 0, 0, 0, 0)) Pmmn.This transformation is accompanied by the ordering of WO3F3 polyhedra and the displacement of NH4 and K ions.