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

Abstract

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 Fm 3 m , 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 [ M O 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.