Experimental and theoretical investigations on d and f electron systems under high pressure

The pressure-induced electron transfer from sp to d band in transition elements, and spd to f band in the light actinides significantly influences the stability of crystal structures in these metals. Although alpha -> omega -> beta phase transition with increasing pressure in group IV transition elements is well documented, the beta -> omega transition under pressure has not been reported until recently. Our experimental study on the beta-stabilized Zr-20Nb alloy reveals that it transforms to omega phase on shock compression, whereas this transition is not seen in a hydrostatic pressure condition. The platelike morphology of omega formed under shock compression is in contrast to the fine particle morphology seen in this system under thermal treatment, which clearly indicates that the mechanism of the beta -> omega transformation under shock treatment involves a large shear component. In this article, we have analyzed why the omega -> beta transition pressures in Ti, Zr, and Hf do not follow the trend implied by the principle of corresponding states. Our analysis shows that the omega -> beta transition depends on how the increased d population caused by the sp -> ad transfer of electron is distributed among various d substates. In Th, we have analyzed the role of 5f electrons in determining the mechanical stability of fcc and bct structures under hydrostatic compressions. Our analysis shows that the fcc to bct transition in this metal, which has been reported by high-pressure experiments, occurs because of softening of the tetragonal shear modulus C' = (C-11 - C-12)/2 under compression. From the total energy calculated as a function of specific volume, we have determined the 0 K isotherm, which is then used to deduce the shock Hugoniot. The theoretical Hugoniot compares well with the experimental data.