Processes of ordering of structural elements, critical and noncritical parameters of phase transitions in the [.sub.3]W[O.sub.3][F.sub.3] crystal

The structure of the low-temperature triclinic phase of the [(N[H.sub.4]).sub.3]W[O.sub.3][F.sub.3] crystal has been determined and the structure of the cubic phase of this crystal has been refined from data of an X-ray diffraction experiment performed 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 obtained experimental data. It has been found that the symmetry transformation in the crystal can be schematically represented in the following form: Fm3m (Z = 4) → P1(Z = 1) → P1(Z = 6). This transformation is accompanied by the complete ordering of W[O.sub.3][F.sub.3] polyhedra and the displacement of N[H.sub.4] ions.