AlB2-related intermetallic compounds -: a comprehensive view based on group-subgroup relations

46 structure types of binary and ternary intermetallic compounds have been derived from the well known, simple aristotype AlB2. Many compounds with the general compositions RX2, RT2, RTX, and R2TX3 (R = alkaline earth, rare earth or actinoid metal; T= transition metal; X = main group element) belong to this family. The structural similarities between these compounds are manifested by an ordering of the transition metal atoms and main group element atoms on the boron positions of the AlB2 type. Also an ordering of vacancies on the boron positions occurs in some compounds. Striking structural motifs of the RX2, RT2, RTX, and R2TX3 compounds are planar or puckered six-membered rings consisting of X-6, T-6, T3X3, and T2X4 separated by the respective R atoms. The crystal chemical relationships between these compounds are discussed on the basis of a group-subgroup scheme in the concise and compact Barnighausen formalism with respect to the aristotype AlB2. Two different branches of AlB2 related structures are discussed. In the first, the hexagonal/trigonal branch, six-membered rings carry at least a three-fold axis, while in the orthorhombic/ monoclinic branch the distortions are more pronounced. The pseudo-hexagons show significant tilting and a loss of hexagonal/trigonal symmetry. Significant variations of the chemical bonding occur for some structure types, and therefore they may only be considered to be isopointal rather than isotypic. These "isopointal conversions" are based on a gradual changeover from two- to three-dimensionally infinite polyanions. Besides the group theoretical group-subgroup scheme we also discuss chemical bonding in these AlB2 related intermetallic compounds.