Crystal structures of two new polytypes of cadmium iodide with uncommon space-group symmetry

Abstract

Crystal structures of two newly discovered polytypes, 8H~ and 24Hg, of cadmium iodide have been determined. The structures are (121)2 and (2222211)z, respectively, in Zhdanov symbols. The polytypes show hexagonal symmetry (space group P63mc), so far shown only by the basic type 4H. The rest of the 15 known cadmium iodide structures have been found to show the lower trigonal symmetry (space group P3ml, P3ml or R3m) which is usually expected for close-packed layer structures. The two polytypes also exhibit a transformation of the structure during crystal growth.