Structural Studies of the Natural Antimonian Pyrochlores: I. Mixed Valency, Cation Site Splitting, and Symmetry Reduction in Lewisite

Abstract

A type sample of the natural pyrochlore-group mineral lewisite, (Ca,Mn2+,Na)1.12Sb3+0.65(Sb5+,Ti4+,Fe3+,Al)2O6(OH)0.91, is cubic,F43m(pseudo-Fd3m), witha=10.277(1) Å andZ=8. The structure has been solved and refined inFd3m(R=0.017),F43m(R=0.014), andF23 (R=0.014) using 218 (or 326) X-ray reflections [Iobs>2σ(I)] from a four-circle diffractometer and compositional constraints from electron microprobe and infrared spectroscopic analyses. Antimony is present in two valence states (Sb5+:Sb3+=60:40), with Sb5+ordered in the pyrochloreBsite (16cinFd3m). To accommodate the different coordination requirements of Sb3+(a lone-pair ion) and Ca2+, the pyrochloreAsite is split into 8-coordinated Ca2+onA(16d) and Sb3+on an asymmetrically 5-coordinated site,A′(96g), theA–A′ separation being ca. 0.5 Å. The deviation fromFd3msymmetry is small, as shown by the fact that only two of the symmetry-forbidden reflections (200 and 420) have statistically significant intensities. The finding ofF43msymmetry in lewisite supports the choice of this space group for certain anomalous synthetic pyrochlores that have been characterized by powder diffraction methods. A previous study that purports to discredit lewisite as a two-phase mixture is shown to be in error.