The crystal structure of Cu1.78Bi4.73Se8, an N = 3 pavonite homologue with a Cu-for-Bi substitution

Cu1.78Bi4.73Se8, synthesized in a dry phase system Cu—Bi—Se at 450 °C, is monoclinic, a= 13.759 Å, b= 4.168 Å, c= 14.683 Å, γ= 115.61°, space group C2/m. It is an N = 3 member of the pavonite homologous series, with the composition limits Cu1.96Bi4.67Se8—Cu1.77Bi4.60S8—Cu1.25Bi4.91Se8. The crystal structure is composed of two types of alternating slabs, with configurations typical for the above series. The first kind of slabs contains paired square pyramidal columns of Bi3, interconnected by octahedral columns with three distinct statistical copper sites (flattened tetrahedral sites in the octahedral interior and a tetrahedral site between adjacent octahedra). They add up to nearly 1.5 Cu. The second kind of slabs contains central octahedra of Bi1, flanked by marginal distorted octahedra of Bi2. These Bi sites are partly replaced by statistically occupied Cu1-3 sites, in flat-tetrahedral and irregular trigonal-planar coordinations. Details of the Cu-for-Bi substitutions are discussed and comparisons with other N = 3 homologues of pavonite are made in the paper.