Orientational disorder in crystals of (NH(4))(3)MoO(3)F(3) and (NH(4))(3)WO(3)F(3)

Triammonium trioxotrifluoromolybdate (NH(4))(3)MoO(3)F(3) ( I) and trioxotrifluorotungstate (NH(4))(3)WO(3)F(3) (II) were synthesized in a single-crystal form and their structures were refined by X-ray diffraction. These two isostructural compounds belong to the elpasolite-type structure ( cubic system, space group Fm3m, Z = 4). O and F atoms are randomly distributed in two independent positions [24( e) and 96( j)] of the cubic unit cell, and the central atoms and the ammonium cation containing N2 are shifted from the symmetry centers into the 32(f) position. As a consequence, O and F atoms in the equilibrium structure were identified on a local scale by the metal - ligand distance and hydrogen atomic coordinates of the disordered ammonium cation N2H(4) were determined. The slightly compressed MX(6) ( M = Mo, W; X = O, F) octahedron has a fac configuration with the central atom shifted toward the face occupied by three O atoms. The true geometry of this polyhedron permits us to explain the observed vibrational spectra of the compounds examined and to eliminate the contradiction in interpretation of the vibrational spectra of [MO(3)F(3)](n-) species reported in the literature. Both complexes reveal a dynamic disordering: the [MX(6)](3-) anions are disordered on 48 equivalent orientations and the N2H(4) groups have eight orientations.