THE CRYSTAL STRUCTURE OF A SYNTHETIC ORTHORHOMBIC N = 8 MEMBER OF THE LILLIANITE HOMOLOGOUS SERIES

Abstract

The new compound, ideally Ag 2.25 Pb 2.5 Bi 4.25 S 10 , has been synthesized at 850°C by reacting simple sulfides of Bi, Pb and Ag. It crystallizes in the orthorhombic system, space group Cmcm , with a 4.084(1), b 13.453(4), c 33.932(9) A, V 1864.4(9) A 3 and Z = 4. The crystal structure has been refined to R 1 = 3.66% for 1132 unique [ F o > 4σ( F o )] reflections collected on a Bruker single-crystal diffractometer with a CCD detector and monochromated Mo K α X-radiation. The structure contains one Pb site with a trigonal prismatic coordination, positioned on the reflection planes of the space group, four distinct octahedral sites, of which one is a mixed (Pb,Ag) site and two are (Bi,Ag) sites, and six anion sites. The new compound is the member number N 1,2 = 8,8 of the lillianite homologous series. It is the homologue of lillianite richest in silver known to date. The present synthesis and investigation of the lillianite homologue 8,8 L allow us to contribute a new structural interpretation for the silver-rich portions of the system Pb–Bi–Ag–S. The existence of Ag-rich phases with N = 8 is caused by a viability of ordered slabs of Ag–Bi–Pb octahedra in the homologues that have favorable Ag:Bi:Pb proportions.