Bandfilling and structural stability of trialuminides: YAl₃, ZrAl₃, and NbAl₃

The cohesive properties and electronic structures versus the structural stability of transition-metal trialuminides YAl₃, ZrAl₃, and NbAl₃in their cubic L1₂, tetragonal DO₂₂, and naturally stable forms (i.e., the DO₁₉structure for YAl₃and the DO₂₃structure for ZrAl₃) have been investigated using a total energy local-density approach. The variation of structural stability with transition-metal constituent can be simply understood in terms of the bandfilling of the bonding states in the rigid band sense, with the valence electrons gradually filling the bonding states on going from YAl₃, ZrAl₃to NbAl₃. This leads to a phase transition from the cubic L1₂structure (for YAl₃) to the tetragonal DO₂₂structure (for NbAl₃). It is argued that this criterion may also apply to explain the variation of the structural stability of other transition-metal compounds (such as transition-metal carbides, nitrides, silicides, etc.) that are dominated by covalent interactions between the transition-metaldand the metalloidpstates.