Orientational disorder in crystal structures of (NH4)3ZrF7 and (NH4)3NbOF6

Crystal structures of (NH₄)₃ZrF₇ (I) and (NH₄)₃NbOF₆ (II) are refined by X-ray diffraction at room temperature. The compounds are isostructural and belong to the structural type of elpasolite: space group F23; a(I) = 9.4185(3) Å, a(II) = 9.3371(5) Å; V(I) = 835.50(5) ų, V(II) = 814.02(8) ų; Z = 4; R(I) = 0.0145, and R(II) = 0.0138. The refinement of the structures in the space group Fm3m yields abnormally short X-X distances in the pentagonal bipyramid MX₇ (X = F, O). The oxygen atom in II is identified by Nb-X distances and occupies one of the axial vertices of the bipyramid. The Nb atom in II is statistically distributed over the position 24f, while Zr in I resides in the symmetry center. The pentagonal bipyramid MX₇ has six independent orientations in I and twelve in II. One of three crystallographically independent ammonium groups of the structures is disordered over six or twelve equivalent orientations.