SUB- AND SUPERSTRUCTURE OF AgPbBr₃. H.-L. Keller, H.Ullmann, Chemistry Department/Inorganic Chemistry, University of Dortmund, Germany

Substructure and superstructure of AgPbBr₃ are discussed.

The compound AgPbBr₃ exists beside the compounds AgPb₂Br₅ and AgPb₃Br₇ in the system AgBr-PbBr₃ [1].

AgPbBr₃ melts congruently at 282C. The crystal structure was determined from a single crystal with X-ray methods. AgPbBr₃ crystallizes in the orthorhombic spacegroup type Cmcm (No. 63) with four formula units per unit-cell. Silver is surrounded by a slightly distorted octahedron of bromine. These octahedra are interconnected by two trans-edges and two trans-corners. For that reason AgBr_6/2-layers are formed parallel to the [010]-plane. The octahedra within the layers are arranged in a way that lead is coordinated trigonal-prismatic by bromine and two side-planes of the prisms are capped by additional bromines. This coordination around lead corresponds to the coordination in the PbCl₂-structuretype.

The structure refinement for both the lead as well as the silver positions shows unsatisfying large thermal displacement parameters.

X-ray film measurements of longer annealed crystals lead to a number of additional weak reflections. Indexing of X-ray powder diffractions shows a new metric in the same crystalsystem as before, but with the short axis three times enlarged. The superstructure was refined with data received from powder diffraction using Rietveld-methods in the original spacegroup type C mcm (a = 3a).

[1] Y. Otsubo, Y. Tanaka, M. Miyahara

Nippon Kagaku Zasshi 92(8) (1971) 735