A New Look at Bonding in Trialuminides:  Reinvestigation of TaAl<sub>3</sub>

Single crystals of TaAl<sub>3</sub> were grown at high temperatures from an Al-rich, binary solution. TaAl<sub>3</sub> adopts the D0<sub>22</sub> structure type, space group <i>I</i>4/<i>mmm</i> with <i>a</i> = 3.8412(5) Å, <i>c</i> = 8.5402(17) Å, and <i>Z</i> = 2. The structure type, which is the preferred structure for all group 5 trialuminides and TiAl<sub>3</sub> as well as the high-temperature form of HfAl<sub>3</sub>, 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 TaAl<sub>3</sub> 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 D0<sub>23</sub> (ZrAl<sub>3</sub>-type) and the L1<sub>2</sub> (AuCu<sub>3</sub>-type), structures. A modified Wade's rule argument provides insights into the structural preferences.