ORIENTATIONAL DISORDER IN CRYSTAL STRUCTURES OF (NH4)3ZrF7 И (NH4)3NbOF6
A. A. Udovenko, N. M. Laptash
Keywords: ammonium fluorozirconates and fluoroniobates, crystal structure, pentagonal bipyramid, orientational disorder
Pages: 500-506
Abstract
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, а(II) = 9.3371(5) Å; V(I) = 835.50(5) Å3, V(II) = 814.02(8) Å3; 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.
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