Conspicuous variation of the lattice unit cell in the pavonite homologous series and its relation with cation/anion occupational modulations

The pavonites is a homologous series of sulfosalts with galena-like modules of variable size. A survey of their unit cells reveals that they are severely constrained by the metrics of an underlying common average sublattice. The unit cell of any compound of the series accommodates with high precision an integer number of approximately equal subcells. This explains a peculiar non-uniform variation of the unit cell parameters within the series and evidences that the interface between the galena-like modules, despite having a very different topology, approximately maintains the subperiodicity of the modules, and must therefore be subject to strong steric restrictions. It also implies that cations and anions occupy the nodes of the observed common underlying average sublattice according to a striking alternate cation/anion occupational modulation. This is the starting point for a description of these materials as modulated structures, which can make a proficient use of the approximate atomic positional non-crystallographic correlations caused by their modular character. Under this approach only four parameters suffice to define a realistic approximate model of any member of the series. A full structural characterization of any of the compounds only requires the determination of additional small/smooth modulations. As an example, the case of the P-7 pavonite Ag3Bi6.2Pb0 8S12, is analyzed. (C) 2013 Elsevier Ltd. All rights reserved.