Diffusion in bcc Transition Metals

Abstract

THE transition metals are of great importance both from industrial as well as scientific view points. In the periodic table,the transition metals representexpansion of a group of 8 (s'p6 ) electron structures into anoth- er one of 18 (s2p "d10). The ground state of free atoms, as the series progresses , also exhibits a change from 4p 3d 4s to 4p 4s 3d state . Such a transition offers excellent opportunity for sd electron hybridization somewhere in the middle of the series . The possibility of spd hybridization also increases as 4p level moves closer to 4s level. Though the exact role of s p- and d - electronic interactions in determining the physical stab-ility of transition metals is a subject of discussion', it is conceded that the major contribution to binding energy comes from d -electrons . According to Engel " and Brewer''', the s-, p- electrons determine the stru-cture of the lattice . However, the strength of bonds in the lattice increases as the tendency for sd and spd hybridization increases ; the maximum being expected in the middle of a period. The `strengthening of bonds' is reflected in the nearest neighbour distances,the cohesive energy values , the melting points and selfdiffusion activation energy values of the metals as one moves along K-Cr-Cu, Rb-Mo-Ag or Cs-W-Pt series.