Experimental and Theoretical Investigations on d and f Electron Systems under High Pressure

Abstract

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 a fi x fi b phase transition with increasing pressure in group IV transition elements is well documented, the b fi x transition under pressure has not been reported until recently. Our experimental study on the b-stabilized Zr-20Nb alloy reveals that it transforms to x phase on shock compression, whereas this transition is not seen in a hydrostatic pressure condition. The platelike morphology of x 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 b fi x transformation under shock treatment involves a large shear component. In this article, we have analyzed why the x fi b transition pressures in Ti, Zr, and Hf do not follow the trend implied by the principle of corresponding states. Our analysis shows that the x fi b transition depends on how the increased d population caused by the sp fi d 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.

On the structural stability of CeN at high pressures: Ab initio calculations

“…The elastic moduli of B1 phase at a given volume are determined using the total energy computed as a function of appropriate strain e. 24,25 The elastic moduli C ij are finally extracted by expressing the energy of the strained lattice as follows: 26…”

The α → ω Transformation in Titanium-Cobalt Alloys under High-Pressure Torsion

“…Under the applied pressure, the sp-bands rise faster in energy, causing electrons to be transferred into d-band [6]. This process is known as s-d transition and governs the structural properties of the transition metals.…”

“…Among this variety, very interesting are the phase transformations with formation of high-pressure phases. The α → ω allotropic transformation takes place at high pressures in titanium, zirconium and hafnium as well as in their alloys [3][4][5][6][7][8][9][10]. The transition pressure, the ability of high pressure ω-phase to retain after pressure release, and the pressure interval where α-and ω-phases coexist depend on the conditions of high-pressure treatment [2,[36][37][38][39][40][41][42][43][44][45][46][47][48].…”

Titanium Alloys at Extreme Pressure Conditions

“…Under pressure, the sp bands rise faster in energy, causing electrons to be transferred to the d bands (Gupta et al, 2008). This process is known as the s-d transition and it governs the structural properties of the transition metals.…”