Preferred ion diffusion pathways and activation energies for Ag in the crystal structure of stephanite, Ag₅SbS₄

Abstract

The crystal structure of stephanite fromthe type locality, Freiberg District, Saxony, Germany, was refined in the space group Cmc21, up to a final R index of 0.0427. Unit-cell parameters are: a 7.8329(6) Å, b 12.458(1) Å, c 8.5272(7) Å, V 832.1(1) Å3; Z = 4. The previously reported structural model is confirmed, but a higher-precision refinement was achieved herein by the introduction of thirdorder non-harmonic Gram-Charlier tensors for one Ag atom. In the structure of stephanite, Sb forms isolated SbS3 pyramids, which typically occur in sulphosalts, and Ag occupies sites with coordination ranging fromtriangular to almost tetrahedral. Both the Sb–S and Ag–S bond distances closely match the values commonly observed in the structures of other Ag sulphosalts and sulphides.The use of non-harmonic parameters for Ag allowed a better description of the electron density related to Ag, which is usually difficult to refine in good ionic conductors. A careful analysis of the energy barriers between the Ag sites defines preferred ion-diffusion pathways within the crystal structure of stephanite. The diffusion of Ag ions occurs preferentially along the sites Ag1 and Ag2, giving rise to two-dimensional nets of Ag atoms in which the ion conduction probably takes place.

A crystallographic excursion in the extraordinary world of minerals: the case of Cu- and Ag-rich sulfosalts

“…Although Leitl et al (2009) confirmed the main structural features previously reported, they also introduced new aspects. For example, one of the silver positions was refined by using non-harmonic parameters of the third order according to Gram-Charlier (Zucker & Schulz, 1982).…”

“…The latter authors used this mineral as an example to show to the mineralogical community that the non-harmonic approach, based upon a Gram-Charlier development of the atomic displacement factors, can be useful in mineral sciences for the determination of still unknown structures. The same approach was then used by Leitl et al (2009) to describe the crystal structure of stephanite, Ag 5 SbS 4 .…”

“…To verify the structural model given by Ribá r & Nowacki (1970) for stephanite and to possibly provide information on the diffusion pathways of silver in the structure, Leitl et al (2009) applied the non-harmonic approach. Indeed, the crystal structure of stephanite (Fig.…”

“…Along the latter direction, the highest activation energy of almost 1.0 eV is obtained, which is necessary for the silver atom to jump to the next noble metal site. After an analysis of the energy barriers between the silver sites, the preferred ion diffusion pathways within the crystal structure of stephanite were defined (Leitl et al, 2009) ions happens preferably along pathways with low activation energies, which for stephanite are the routes along the sites Ag1 and Ag2 with distances of about 3.0 Å .…”

Progress in the knowledge of ‘ruby silvers’: New structural and chemical data of pyrostilpnite, Ag3SbS3

“…Ag + and Cu + ions), which can result in the formation of various complex asymmetric coordinations often producing overlapping sites (Gaudin et al ., 2001). Classic examples of this kind of mineral are samsonite, Ag 4 MnSb 2 S 6 (Bindi and Evain, 2007), stephanite, Ag 5 SbS 4 (Leitl et al ., 2009), pearceite–polybasite group minerals, [(Ag,Cu) 16 M S 11 where M = Sb or As] (i.e. Bindi et al .…”