MULTIPLE ORDER IN SULFIDES AND SELENIDES

Abstract

A novel type of crystal structure (structures with multiple order) has been found for the hexagonal compounds A1-pCr2X4-p (A = Ba, Sr, Eu, Pb ; X = S, Se ; p ≈ 0.29). The structure may be described as an intergrowth of three structural units which have a common hexagonal basal plane (a ≈ 21.5 A for X = S ; a ≈ 22.5 A for X = Se), but different — and mutually incommensurate — c axes. CrX6 octahedra sharing edges and faces form a three-dimensional framework of, composition Cr21X36 with c0 ≈ 3.45 A for X = S ; c0 ≈ 3.63 A for X = Se. This framework leaves wide channels around the sixfold and threefold axes. The channels about the sixfold axes contain a unit A6Cr2X6 per repeat distance c6 (c6 ≈ 5.7 A for X = S ; c6 ≈ 6.0 A for X = Se), the channels about the threefold axes a unit A3X per repeat distance c3 (c3 ≈ 4.2 A for X = S ; c3 ≈ 4.6 A for X = Se). The composition of the compounds is determined by the ratios c6/c0 and c3/c0. The mutual positions of the three structural units in the c direction are related in Ba1-pCr2Se4-p (which has a commensurate superstructure with c = 5 c0 = 4 c3 = 3 c6), but virtually uncorrelated in Pb1-pCr2S4-p ; in the other compounds the positions of the units about the sixfold axes and the framework are not correlated (resulting in diffuse reflections), but the units about the threefold axes are correlated with the framework and modulated by it (resulting in sharp reflections and satellites). Structural parameters are given for Eu1-pCr2Se4-p.