Structural Studies of the Natural Antimonian Pyrochlores: I. Mixed Valency, Cation Site Splitting, and Symmetry Reduction in Lewisite
✍ Scribed by Roland C Rouse; Pete J Dunn; Donald R Peacor; Liping Wang
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 364 KB
- Volume
- 141
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
A type sample of the natural pyrochlore-group mineral lewisite, (Ca,Mn 2؉ ,Na) 1.12 Sb 3؉ 0.65 (Sb 5؉ ,Ti 4؉ ,Fe 3؉ ,Al) 2 O 6 (OH) 0.91 , is cubic, F4 3m (pseudo-Fd3 m), with a ؍ 10.277(1) A s and Z ؍ 8. The structure has been solved and refined in Fd3 m (R ؍ 0.017), F4 3m (R ؍ 0.014), and F23 (R ؍ 0.014) using 218 (or 326) X-ray reflections [I obs > 2 (I)] from a four-circle diffractometer and compositional constraints from electron microprobe and infrared spectroscopic analyses. Antimony is present in two valence states (Sb 5؉ : Sb 3؉ ؍ 60 : 40), with Sb 5؉ ordered in the pyrochlore B site (16c in Fd3 m). To accommodate the different coordination requirements of Sb 3؉ (a lone-pair ion) and Ca 2؉ , the pyrochlore A site is split into 8-coordinated Ca 2؉ on A(16d ) and Sb 3؉ on an asymmetrically 5-coordinated site, A(96g), the A-A separation being ca. 0.5 A s . The deviation from Fd3 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 F43 m symmetry 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 twophase mixture is shown to be in error.