Structural studies of the natural antimonian pyrochlores - I. Mixed valency, cation site splitting, and symmetry reduction in lewisite

A type sample of the natural pyrochlore-group mineral lewisite, (Ca,Mn2+,Na )(1.12)Sb-0.65(3+)(Sb5+,Ti4+,Fe3+,Al is cubic, F (4) over bar 3m (pseudo-Fd (3) over bar m), with a 10.277(1) Angstrom and Z = 8. The structure has be en solved and refined in Fd (3) over bar m (R = 0.017), F (4) over bar 3m ( R = 0.014), and F23 (R = 0.014) using 218 (or 326) X-ray reflections [I-obs >2 sigma(I)] from a four-circle diffractometer and compositional constraint s from electron microprobe and infrared spectroscopic analyses. Antimony is present in two valence states (Sb5+:Sb3+ = 60:40), with Sb5+ ordered in th e pyrochlore B site (16c in Fd (3) over bar m). To accommodate the differen t coordination requirements of Sb3+ (a lone-pair ion) and Ca2+, the pyrochl ore A site is split into 8-coordinated Ca2+ on A(16d) and Sb3+ on an asymme trically 5-coordinated site, A'(96g), the A-A' separation being ca. 0.5 Ang strom, The deviation from Fd (3) over bar m symmetry is small, as shown by the fact that only two of the symmetry-forbidden reflections (200 and 420) have statistically significant intensities. The finding of F4 (3) over bar m symmetry in lewisite supports the choice of this space group for certain anomalous synthetic pyrochlores that have been characterized by powder diff raction methods. A previous study that purports to discredit lewisite as a two-phase mixture is shown to be in error. (C) 1998 Academic Press.