Theoretical study of the structural properties and thermodynamic stabilityof the omega phase in the 4d-transition series

Using the full potential-linearised-muffin-tin-orbitals (FP-LMTO) method, w e have studied the structural properties and the thermodynamic stability of the AlB2 (C32) type-structure, so-called omega (Omega) phase, of Y, Zr, Nb , Mo, Tc, Ru, Rh, Pd and Ag. The equilibrium volume, lattice parameters, in teratomic distances, and density-of-electron-states at the Fermi level of t he stable or metastable Omega phase of these elements are reported, and com pared with those for the respective bce phase. The zero kelvin energy diffe rence between Omega and bce is determined as a function of the number of el ectrons per atom. The energetics of the bcc-->Omega phase transition in Y, Zr and Nb is studied by calculation, and a renewed contact is established w ith the Ho, Fu and Harmon picture of this transition, which is based on the softening of the 2/3 [1,1,1] longitudinal phonon mode of the bce phase. (C ) 1999 Elsevier Science S.A. All rights reserved.