Crystal structure, XANES and charge distribution investigation of krennerite and sylvanite: analysis of Au—Te and Te—Te bonds in Au1–xAgxTe2 group minerals

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Vol. 78; no. 2; pp. 117 - 132
• Main Authors: Kitahara, Ginga, Yoshiasa, Akira, Tokuda, Makoto, Nespolo, Massimo, Hongu, Hidetomo, Momma, Koichi, Miyawaki, Ritsuro, Sugiyama, Kazumasa
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England International Union of Crystallography 01-04-2022; Blackwell Publishing Ltd
• Subjects: Au–Te bond distances; Bonding strength; Charge distribution; Coordination numbers; Crystal structure; effective coordination number (ECoN); Gold; Intermetallic compounds; krennerite; Minerals; Silver; single‐crystal structure analysis; sylvanite; Tellurides
• ISSN: 2052-5206; 2052-5192; 2052-5206
• DOI: 10.1107/S2052520622000804

The structure refinement and XANES study of two gold–silver–tellurides [Au1+xAgxTe2, krennerite (x = 0.11–0.13) and sylvanite (x = 0.29–0.31)] are presented and the structures are compared with the prototype structure of calaverite (x = 0.08–0.10). Whereas the latter is well known for being incommensurately modulated at ambient conditions, neither krennerite nor sylvanite present any modulation. This is attributed to the presence of relatively strong Te—Te bonds (bond distances < 2.9 Å) in the two minerals, which are absent in calaverite (bond distances > 3.2 Å). In both tellurides, trivalent gold occurs in slightly distorted square planar coordination, whereas monovalent gold, partly substituted by monovalent silver, presents a 2+2+2 coordination, corresponding to distorted rhombic bipyramids. The differentiation between bonding and non‐bonding contacts is obtained by computation of the Effective Coordination Number (ECoN). The CHARge DIstribution (CHARDI) analysis is satisfactory for both tellurides but suggests that the Te—Te bond in the [Te3]2− anion is not entirely homopolar. Both tellurides can therefore be described as Madelung‐type compounds, despite the presence of Te–Te in both structures.
Single‐crystal XRD, XANES and charge distribution investigations of krennerite, sylvanite and calaverite are reported.