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Evain, M., Luca, B. & Menchetti, S. (2006) Structural complexity in minerals: Twinning, polytypism and disorder in the crystal structure of polybasite, (Ag, Cu)(16)(Sb, As)(2)S-11. Acta Crystallogr. Sect. B-Struct. Sci. 62 447–456. 
Added by: Florent Boucher (2016-05-12 13:21:37)
Type de référence: Article
DOI: 10.1107/S010876810600975X
Numéro d'identification (ISBN etc.): 0108-7681
Clé BibTeX: Evain2006c
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Catégories: MIOPS
Mots-clés: a(7)pse(6), ag, coordination, ionic conductors, pearceite, phase-transitions, series, silver
Créateurs: Evain, Luca, Menchetti
Collection: Acta Crystallogr. Sect. B-Struct. Sci.
Consultations : 12/403
Indice de consultation : 2%
Indice de popularité : 0.5%
Résumé     
The crystal structures of 222- and 221-polybasite [(Ag,Cu)(16)(Sb,As)(2)S-11] crystals have been solved and refined by means of X-ray diffraction data (collected at 100 and 120 K, respectively) from twinned crystals. Both structures consist of the stacking of [(Ag, Cu)(6)Sb2S7](2-) and [Ag9CuS4](2+) module layers in which Sb forms isolated SbS3 pyramids typically occurring in sulfosalts; copper links two S atoms in a linear coordination and silver occupies sites with coordination ranging from quasi- linear to almost tetrahedral. An Ag -{>} Cu substitution in the [(Ag,Cu)(6)Sb2S7](2 -) module layer is observed in both structures, the substitution amount being larger in the 221- than in the 222- polybasite. A pattern of the possible mechanism regulating the type of unit cell that is stabilized is proposed: starting from the hypothetical stoichiometric and fully ordered Ag15CuSb2S11 222- polybasite structure, with a low C2/c monoclinic symmetry and a large 222 supercell, the disorder introduced by the substitution of Cu for Ag increases the symmetry with a cell reduction along the c axis yielding the 221 supercell and a trigonal crystal system. A further increase of the substitution gives rise to a folding of the cell along the a and b axes and the 111-pearceite structure, space group P (3) over bar m1.
Added by: Florent Boucher  
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