Electronic structure and chemical bonding effects upon the bcc to Omega phase transition: Ab initio study of Y, Zr, Nb, and Mo

The bce -->Ohm phase transition is diffusionless: two-thirds of the atoms i n the (111) planes of the bcc phase collapse into double layers, whereas th e third remains a single layer. This transformation is observed in the grou p-IV elements Ti, Zr, and Hf at high pressure, but it may be induced at zer o pressure by quenching the samples at room temperature in alloys of these elements with other transition metals (TM's). This paper presents a systema tic theoretical study of the bcc-->Ohm transformation in Y, Zr, Nb, and Mo using the full-potential linearized-augmented-plane-wave method. Equilibriu m volumes, total-energy differences, density of electron states, and the ba nd structure of Y, Zr, Nh, and Mo in the stable and metastable bcc and Ohm structures are presented. In addition, the bcc-->Ohm energy difference is s tudied as a function of the specific lattice distortion as well as of press ure. These results are related to a picture that involves the softening of the (2/3,2/3,2/3) longitudinal phonon mode. Charge-density calculations are performed for both the bcc and Ohm phases, and the electronic contribution s that stiffen the bcc lattice against the Ohm distortion are identified. T he physical picture of the electronic and chemical bonding effects emerging from the present work should be useful in understanding the observed trend s in the relative stability of the Ohm phase in the TM's and their alloys.