A crystallographic re-investigation of Cu2Sb-related binary, ternary, and quaternary structures: how many structure types can exist upon the same topology of a unit cell?
Crystal chemical analysis of several large sets of topologically related structures confirms earlier suggestions that within the same cell content, space group, and Wyckoff site occupancies, there may exist several structure types of sometimes quite different coordination chemistry and bonding arrangement. The original Flahaut's suggestion of demarcating anti-Cu2Sb and PbFCl structure types has been extended to an analysis of structures of over 350 compounds in terms of various structural characteristics as free parameters. For the Cu2Sb family, structural field maps indicate formation of at least three distinct structure types: Cu2Sb, PbFCl, CeFeSi (anti-type of PbFCl) and UAs2 (which is not an anti-type of Cu2Sb). The formerly suggested ternary ZrSiS structure type is in fact an ordered version of UAs2. Extension of this approach to “filled” versions of these structure types permits clear demarcation of two other topologically related structure types, LaOAgS and HfCuSi2, the latter being a derivative of UAs2 but not Cu2Sb. Very similar relationships have been observed for several smaller structural families. Perspectives of using different structural parameters as free coordinates are discussed.