Synthesis, Structure, and Bonding of Orthorhombic R₅Au₂Te₂ (R = Lu, Ho, Dy, Y). Electronic Structure of the Binary Parent Valence Compound Eu₅As₄

Four examples of R5Au2Te2 (vec = 29 e–; R = Lu, Ho, Dy, Y) have been synthesized by high-temperature solid-state techniques, isotypic examples of Tm5Sb2Si2 (vec = 33 e–) and binary Eu5As4 (vec = 30 e–). The crystal structure was established for Lu5Au2Te2, (orthorhombic Cmce (No. 64), a = 15.056(2), b = 7.749(1), c = 7.754(1) Å, and Z = 4), in which pairs of tellurium layers alternate with two-dimensional (2D) Lu5Au2 slabs that are aggregated in such a way that each Au2-centered bi-trigonal prism (BTP) of Lu interconnects four other identical units, with the remaining cavities filled by nominal body-centered Lu cubes. The metal–metal aggregation in this structure provides a novel building unit in ternary rare-earth-metal-rich tellurides. Linear-muffin-tin-orbital (LMTO) electronic structure calculations and COHP analyses reveal that Lu5Au2Te2 is a poor metal with Au2 dimers and strong polar Lu–Au and Lu–Te interactions. The first theoretical analysis of the binary parent structure Eu5As4 (vec = 30 e–) provides a simpler description of the equivalent orbital interactions and a closed shell gap in terms of the idealized (Eu2+)5(As24–)(As3–)2 representation, particularly for the explicit filled As2 levels σs, σs*, σp, π, π*, plus empty σp*. Crystal Orbital Hamilton Population (−COHP) data illuminate the prominent roles that polar bonding of Eu–As or Lu–Te and Lu–Au and relativistic effects with gold play in these, the former corresponding to 83% and 86% of the total Hamilton population for Eu5As4 and Lu5Au2Te2, respectively.