Acta Crystallographica Section B

Structural Science

Volume 64, Part 3 (June 2008)

research papers

Acta Cryst. (2008). B64, 305-311 [ doi:10.1107/S0108768108010288 ]

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

A. A. Udovenko and N. M. Laptash

Abstract: Triammonium trioxotrifluoromolybdate (NH₄)₃MoO₃F₃ (I) and trioxotrifluorotungstate (NH₄)₃WO₃F₃ (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 N2H₄ were determined. The slightly compressed MX₆ (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 [MO₃F₃]^n- species reported in the literature. Both complexes reveal a dynamic disordering: the [MX₆]^3- anions are disordered on 48 equivalent orientations and the N2H₄ groups have eight orientations.

Keywords: elpasolites; dynamic orientational disorder; vibrational spectra.


	Structure factor file (CIF format) (8.7 kbytes) [ doi:10.1107/S0108768108010288/bp5011Isup2.hkl ] Contains datablock I

	Structure factor file (CIF format) (8.4 kbytes) [ doi:10.1107/S0108768108010288/bp5011IIsup3.hkl ] Contains datablock II

Notes:

To open or display or play some files, you may need to set your browser up to use the appropriate software. See the full list of file types for an explanation of the different file types and their related mime types and, where available links to sites from where the appropriate software may be obtained.

The download button will force most browsers to prompt for a file name to store the data on your hard disk.

Where possible, images are represented by thumbnails.