Homoatomic Clustering in T4Ga5 (T = Ta, Nb, Ta/Mo): A Story of Reluctant Intermetallics Crystallizing in a New Binary Structure Type

• Published in: Inorganic chemistry Vol. 54; no. 3; pp. 821 - 831
• Main Authors: Fredrickson, Rie T, Kilduff, Brandon J, Fredrickson, Daniel C
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 02.02.2015
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic501966v

In the formation of binary compounds, heteroatomic interactions are generally expected to play the leading role in providing stability. In this Article, we present a series of gallides, T4Ga5 (T = Ta, Nb, and Ta/Mo), which appear to defy this expectation. Their complex crystal structures represent a new binary structure type (to the best of our knowledge),, which can be visualized in terms of a host lattice of T@T8 body centered cubic (bcc) clusters linked through face-capping Ga2 dumbbells to form a primitive cubic framework. The cubic spaces that result are alternately filled by distorted T pentagonal dodecahedra (sharing atoms with the host lattice) and dimers of bcc fragments, leading to a √2 × √2 × 2 supercell of the host framework structure. Ga tetrahedra and icosahedral units fill the remaining void spaces. Underlying these structural features is a strong tendency for homoatomic clustering of Ta and Ga, which is evident in all of the coordination polyhedra. Electronic structure calculations using density functional theory (DFT) and DFT-calibrated Hückel models reveal possible origins for this elemental segregation and the factors stabilizing the structure as a whole. A deep pseudogap is present at the Fermi energy of Ta4Ga5 (as well as at that of Nb4Ga5), corresponding to the near-optimization of Ta–Ta and Ta–Ga interactions. This pseudogap emerges as a result of the ability of extensive Ta–Ta bonding to provide local 18-electron configurations to the Ta atoms, despite the electron concentration being only 8.75 electrons per Ta atom. Support for these Ta–Ta interactions is provided by Ga bridging atoms, whose valence orbitals’ low number of angular nodes confers preferential stabilization to Ta–Ta bonding functions over antibonding ones. The observed spatial separation of the structure into Ta and Ga domains occurs as a consequence of the Ga atoms being pushed toward the periphery of the Ta clusters to play this supporting role.