INFLUENCE OF PSEUDOCORE VALENCE-BAND HYBRIDIZATION ON THE CRYSTAL-STRUCTURE PHASE STABILITIES OF TRANSITION-METALS UNDER EXTREME COMPRESSIONS

We demonstrate that the conventional picture of crystal structure stab ilities of transition metals, namely, as being determined by the degre e of filling of the ''canonical'' d bands, breaks down at very high pr essures. We show, by means of first-principles calculations, that for extreme compressions the pseudocore p states become broad and start to hybridize with the valence states. This results in a modification of the electronic structure and consequently unexpected crystal structure s become stabilized. Simple model calculations using p- and d-canonica l bands, which are allowed to hybridize with each other, predict that the bcc structure is the most stable crystallographic phase at high pr essure for most of the transition elements, in agreement with our firs t-principles calculations.