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 NbAl3 in their cubin L1(2), tetragonal D0(22), and naturally stable forms (i.e., the D0(19) structure for YAl3 and the D0(23) structure 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, ZrAl3 to NbAl3. This leads to a phase transition from the cubic L1(2) structure (for YAl3) to the tetragonal D0(22) structure (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-metal d and the metalloid p states.