Hf6Al7 and Hf4.44(1)Nb1.56(1)Al7—the First Fully Ordered Main Group Metal Containing W6Fe7 Type Compound and its Ternary Coloring Variant

Abstract

Attempts to synthesize Hf4NbAl7 led to the discovery of nominal Hf4Nb2Al7 instead of the envisioned product. It was identified based on powder diffraction data. The compound can be described as a substitutional/coloring variant of the rhombohedral W6Fe7 type structure (space group Rmathematical equationm). The formation of the ternary compound motivates the synthesis of the binary compound Hf6Al7. Different synthetic strategies lead to its discovery. The crystal structures of the binary as well as its ternary coloring variant are refined from single-crystal X-ray diffraction data showing both the same obverse–reverse twinning. For the Nb-containing phase, a composition of Hf4.44(1)Nb1.56(1)Al7 is refined, in agreement with the powder X-ray data. Solid-state 27Al NMR investigations indicate the formation of a crystalline compound; however, only one distinct signal can be observed in contrast to the two crystallographic Al positions. Magnetic susceptibility measurements confirm the expected Pauli-paramagnetic character. Quantum-chemical calculations alongside analyses of the chemical bonding with the LOBSTER program package show that a strong Hf–Hf interaction is present, which transforms into an even stronger Nb–Nb interaction in nominal Hf4Nb2Al7. For the latter, three substitutional variants are calculated, clearly indicating the experimentally observed one as the most stable.