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Physics and Chemistry of Minerals

, Volume 33, Issue 10, pp 677–690 | Cite as

Fast ion conduction character and ionic phase-transitions in disordered crystals: the complex case of the minerals of the pearceite–polybasite group

  • L. Bindi
  • M. Evain
  • A. Pradel
  • S. Albert
  • M. Ribes
  • S. Menchetti
Original Paper

Abstract

The minerals of the pearceite–polybasite group, general formula (Ag,Cu)16 M 2S11 with M = Sb, As, have been recently structurally characterized. On the whole, all the structures can be described as a regular succession of two module layers stacked along the c axis: a first module layer (labeled A), with general composition [(Ag,Cu)6(As,Sb)2S7]2−, and a second module layer (labeled B), with general composition [Ag9CuS4]2+. In detail, in the B layer of the pearceite structure silver cations are found in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. We have shown for the first time that the observed structural disorder in the B layer is strongly related to the fast ion conduction character exhibited by these minerals. This paper reports an integrated XREF, DSC, CIS and EPMA study on all the members of the pearceite–polybasite group. DSC and conductivity measurements pointed out that the 222 members show ionic-transitions at 340 K (arsenpolybasite-222) and 350 K (polybasite-222), whereas the 221 members have transitions at lower temperature, that is, 310–330 K (arsenpolybasite-221) and 335 K (polybasite-221). For the 111 members (pearceite and antimonpearceite), the transition occurs below room temperature at 273 K. In situ single-crystal X-ray diffraction experiments showed that these minerals present the same high temperature structure and are observed at room temperature either in their high temperature (HT) fast ion conductivity form or in one of the low temperature (LT) fully ordered (222), partially ordered (221) or still disordered (111) forms, with transition temperatures slightly above or below room temperature. The pearceite–polybasite group of minerals can be considered as a homogeneous series with the same aristotype, fast ion conducting form at high temperature. Depending upon the Cu content, an ordering occurs with transition temperatures related to that content: the lower the Cu content, the higher the transition temperature from the fast ion conducting HT form to the non-ion conducting form.

Keywords

Pearceite Polybasite Conductivity spectra Ionic phase transition Fast ionic conductor 

Notes

Acknowledgments

The authors are grateful to Professor Paul G. Spry (Iowa State University, USA) for his help in electron microprobe analyses. This work was funded by C.N.R. (Istituto di Geoscienze e Georisorse, sezione di Firenze) and by M.I.U.R., P.R.I.N. 2005 project “Complexity in minerals: modulation, modularity, structural disorder”.

Supplementary material

269_2006_117_MOESM1_ESM.doc (265 kb)
Supplementary material

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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • L. Bindi
    • 1
  • M. Evain
    • 2
  • A. Pradel
    • 3
  • S. Albert
    • 3
  • M. Ribes
    • 3
  • S. Menchetti
    • 1
  1. 1.Dipartimento di Scienze della TerraUniversità di FirenzeFirenzeItaly
  2. 2.Laboratoire de Chimie des Solides, I.M.N.UMR 6502 CNRS–Université de NantesNantes Cedex 3France
  3. 3.Laboratoire de Physicochimie de la Matière CondenséeUMR CNRS 5617Montpellier Cedex 5France

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