Isopointal intermetallics: the cP24, dca phases as a representative set of examples, along with their vacancy-ordered variants β-Mn and SrSi2

Abstract

The crystal-chemical relationship of the cubic Laves phase MgCu2 (space group F d 3 ‾ $d\bar{3}$ m, cF24) with the ternary phases Cd2Cu3In, Na2Au3Al, Mg2Rh3P, Li2Pd3B, Ag2Pd3S, Cu3Pt2B, Mo3Al2C, Mo3Ni2N, and V3Ga2N (subgroup P4132, cP24, dca) is discussed based on a group-subgroup scheme. The course of the lattice parameters and the free positional parameters show substantially different distortions and thus clear differences in chemical bonding, classifying these phases as isopointal rather than isotypic (usually they are all assigned to the Mo3Al2C type). The group-subgroup scheme further shows that the β-Mn and SrSi2 structures are vacancy-ordered variants of the cP24, dca phases. The structures of Mn3IrSi and LaIrSi (space group types P213; translationengleiche subgroups) are their ternary ordered versions.