Orientational disorder in crystals of (NH4)3MoO3F3 and (NH4)3WO3F3.

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) species reported in the literature. Both complexes reveal a dynamic disordering: the MX(6) anions are disordered on 48 equivalent orientations and the N2H(4) groups have eight orientations.