First (3 + 2)-dimensional superspace approach to the structure of levyclaudite-(Sb), a member of the cylindrite-type minerals.
The structure of synthetic levyclaudite-(Sb), approximately (Pb(1 - y)Sb(y)S)(1.357)[Sn(1 - x)(Cu(2))(x)S(2)], has been determined by single-crystal X-ray diffraction on the basis of the (3 + 2)-dimensional superspace approach. This misfit-layer compound, of the cylindrite type, results from the combination of two heavily modulated triclinic Q and H subsystems with a common q wavevector and only one shared reciprocal axis (stacking direction). The Q pseudo-tetragonal layer, approximately (Pb(0.70)Sb(0.30)S), derived from the NaCl archetype, is positively charged; the H pseudo-hexagonal layer, approximately (Sn(0.85)Cu(0.30)S(2)), derived from the CdI(2) archetype, is negatively charged, owing to the replacement of Sn(4+) in an octahedral coordination by Cu(+) pairs in an opposite triangular coordination. The analysis shows a strong transverse displacive modulation of the two layers, referred to as a 'mondulation', correlated to a maximal Sb site occupation factor in the concavity of the Q layer undulation. The wavelength control of the ;mondulation' obeys the vernier principle (14c(Q) congruent with 13c(H)), which would correspond to an energy minimization through a charge transfer density modulation wave, common to all two-dimensional misfit-layer inorganic compounds.
Evain
Michel
M
Laboratoire de Chimie des Solides, IMN, UMR 6502 CNRS, Université de Nantes, 2 rue de la Houssinière, BP 32229, 44322 Nantes CEDEX 3, France. michel.evain@cnrs-imn.fr
Petricek
Vaclav
V
Moëlo
Yves
Y
Maurel
Colette
C