Acta Crystallographica Section B

Structural Science

Volume 54, Part 4 (August 1998)

research papers

Acta Cryst. (1998). B54, 391-398 [ doi:10.1107/S0108768198002833 ]

The Yttrium Oxide Fluoride Solid Solution Described as a Composite Modulated System

S. Schmid

Abstract: Single-crystal X-ray diffraction data (Mo K [alpha] radiation) are used to determine the structure of the [Delta] = 0.174 member of a (YO_{1 -}F)F_{1 +}, 0.12 [less-than or equal to] [Delta] 0.22, wide-range non-stoichiometric solid solution in the yttrium oxide fluoride system. The structure is refined as a composite modulated structure composed of two mutually incommensurable Q and H substructures with overall superspace-group symmetry Acmm(0,0,1.174..)0s0. The Q substructure has lattice parameters a = 5.415 (2), b_Q = 5.534 (2) and c_Q = 5.525 (2) Å, and superspace-group symmetry Acmm(0,0,1.174..)0s0. The H substructure has lattice parameters a = 5.415 (2), b_H = 1/2b_Q = 2.767 (1) and c_H = 4.696 (2) Å, and superspace-group symmetry Pmcm(0,1/2,0.8518...)s00. Refinement on 338 unique reflections converged to R = 0.025, while a previous conventional superstructure refinement led to R = 0.101. This lowering of the R factor goes hand-in-hand with a substantial reduction in the number of refined parameters.