Bandfilling and structural stability of trialuminides: YAl 3 , ZrAl 3 , and NbAl 3
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A. J. Freeman Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3112, and Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received 20 December 1990; accepted 14 February 1991)
The cohesive properties and electronic structures versus the structural stability of transition-metal trialuminides YA13, ZrAl 3 , and NbAl3 in their cubic LI2, tetragonal DO22, and naturally stable forms (i.e., the D0 19 structure for YA13 and the DO23 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 YA13, ZrAl3 to NbAl 3 . This leads to a phase transition from the cubic L l 2 structure (for YA13) to the tetragonal DO22 structure (for NbAl 3 ). 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.
I. INTRODUCTION The study of the early transition metal (TM) trialuminides TMA13 (TM = Sc, Ti, V, Y, Zr, Nb, etc.) has both technological and scientific significance. The TM aluminides (such as TiAl3, ZrAl3, etc.) are attractive as potential structural materials for use in the high temperature environment1 due to relatively high melting points, light weight, and good oxidation resistance or as thermally stable (i.e., low coarsening rate) precipitates for developing so-called super "alumalloy"2 due to small lattice mismatch with the aluminum matrix. Recently, thin-film TM aluminides are also of great interest in the microelectronics industry3 due to the use of transition metals as diffusion barriers to suppress hillock formation, and to the increase of electromigration resistance. Since some of their applicability depends strongly upon their structural properties (e.g., the ductility of these intermetallics may possibly be improved substantially by control of ordered crystal structures), it is scientifically interesting to note the observed variation of the crystal structures of the TM trialuminides across each early transition-metal series4 (cf. Table I): The stability of the cubic L l 2 phase decreases from the left (top) to the right (bottom) for each row (column) of the transition metals; on the other hand, the stability of the tetragonal DO22 structure increases from left to right. In the middle of Table I, ZrAl3 possesses both cubic L l 2 and tetragonal D0 23 structures as metastable and stable forms. In 1188
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J. Mater. Res., Vol. 6, No. 6, Jun 1991
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TABLE I. Observed variation of the crystal structure in transition metal-trialuminides across the early-transition metal series. HT and RT d
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