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Bindi, L., Evain, M. & Menchetti, S. (2006) Temperature dependence of the silver distribution in the crystal structure of natural pearceite, (Ag,Cu)(16)(As,Sb)(2)S-11. Acta Crystallogr. Sect. B-Struct. Sci. 62 212–219.
Added by: Florent Boucher (2016-05-12 13:21:37) |
| Type de référence: Article DOI: 10.1107/S010876810600108X Numéro d'identification (ISBN etc.): 0108-7681 Clé BibTeX: Bindi2006a Voir tous les détails bibliographiques  |
Catégories: MIOPS Mots-clés: a(7)pse(6), ag, ionic conductors, phase-transitions, polybasite, series Créateurs: Bindi, Evain, Menchetti Collection: Acta Crystallogr. Sect. B-Struct. Sci. |
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| Résumé |
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The crystal structure of the mineral pearceite, (Ag, Cu)(16)(As, Sb)(2)S-11, has been solved and refined at 300, 120 and 15 K. At room temperature pearceite crystallizes with trigonal symmetry, space group P (3) over bar m1; the refinement of the structure leads to a residual factor of R = 0.0464 for 1109 independent observed reflections and 92 variables. The crystal structure consists of sheets stacked along the c axis. The As atoms form isolated (As, Sb) S3 pyramids, which typically occur in sulfosalts, copper cations link two S atoms in a linear coordination, and the silver cations are found in a fully occupied position and in various sites corresponding to the most pronounced probability density function locations (modes) of diffusion-like paths. These positions correspond to low-coordination (2, 3 and 4) sites, in agreement with the preference of silver for such environments. d(10) silver-ion distribution has been determined by means of a combination of a Gram-Charlier description of the atomic displacement factors and a split-atom model. To analyse the crystal chemical behaviour of the silver cations as a function of temperature, a structural study was carried out at 120 K (R = 0.0450). The refinement indicates that the mineral exhibits the same structural arrangement as the room-temperature structure (space group P (3) over bar m1) and shows that the silver cations are still highly disordered. In order to investigate a possible ordering scheme for the silver cations, a data collection at ultra-low temperature (15 K) was performed. The structural skeleton was found to be similar to that of the room-temperature and 120 K atomic structures, but the best solution was achieved with a fully split-atom model of five silver positions, giving an R value of 0.0449 for 651 observed reflections and 78 parameters. Although the silver cation densities condense into better defined modes, the joint probability density function still exhibits a strong overlapping of neighbouring sites.
Added by: Florent Boucher |