A New Look at Bonding in Trialuminides:  Reinvestigation of TaAl₃

Single crystals of TaAl₃ were grown at high temperatures from an Al-rich, binary solution. TaAl₃ adopts the D0₂₂ structure type, space group I4/mmm with a = 3.8412(5) Å, c = 8.5402(17) Å, and Z = 2. The structure type, which is the preferred structure for all group 5 trialuminides and TiAl₃ as well as the high-temperature form of HfAl₃, is a binary coloring of the face-centered-cubic (fcc) arrangement. The distribution of Ta atoms creates a three-dimensional network of vertex and edge-sharing square pyramids of Al atoms. Temperature-dependent electrical resistivity and magnetic susceptibility measurements are consistent with TaAl₃ being a metallic compound with a relatively low density of states at the Fermi surface. Furthermore, tight-binding electronic structure calculations are utilized to describe the bonding in these compounds and to compare their stability with respect to the alternative fcc-related, e.g., the D0₂₃ (ZrAl₃-type) and the L1₂ (AuCu₃-type), structures. A modified Wade's rule argument provides insights into the structural preferences.