The crystal and electronic structure of RE23Co6.7In20.3 (RE = Gd–Tm, Lu): A new structure type based on intergrowth of AlB2- and CsCl-type related slabs

New ternary rare-earth indides RE23Co6.7In20.3 (RE = Gd–Tm, Lu) have been synthesized by arc-melting the elements under argon and subsequent annealing at 870 K for 1200 h. Single-crystal X-ray diffraction revealed Er23Co6.7In20.3 to crystallize in a new structure type in oP100, space group Pbam and Wyckoff sequence h11g13da with a = 23.203(5), b = 28.399(5), c = 3.5306(6) Å. The crystal structures of RE23Co6.7In20.3 (RE = Tb, Ho, Er and Tm) were determined from single crystal and powder X-ray diffraction data and further investigated by DFT methods. The compounds belong to a large family of ternary rare-earth indides with intergrowth of the AlB2- and CsCl-type related slabs. In the Er23Co6.7In20.3 structure, four types of fragments REIn and RET of CsCl-type, as well as RET2 and REIn2 of AlB2-type, are present simultaneously. A simple Python tool was developed to determine the coordination number for each crystallographic site with various methods and tested on the complex structure of RE23Co6.7In20.3. [Display omitted] •RE23Co6.7In20.3 (RE = Gd–Tm, Lu) were prepared by arc-melting under argon and subsequent tempering at 870 K for 1200 h.•RE23Co6.7In20.3 crystallize in a new orthorhombic structure type composed of CsCl- and AlB2-type related slabs.•Multiple methods have been applied to analyze highly irregular coordination environments.•This new compound combines structural motifs usually observed for intermetallics with B, Si, P, As, etc.•The quantum chemical calculations confirmed the formation of short oligomeric motifs via In-In bonding.