Crystal structures of group IVa metals at ultrahigh pressures

Abstract

High-pressure energy-dispersive x-ray-diffraction studies were carried out on the group IVa metals hafnium (Hf) to 252 GPa (to a volume compression V/${\mathit{V}}_0$ of 0.48) and titanium (Ti) to 87 GPa using a synchrotron source. Two new structural phase transformations between an hcp phase and an ω phase and between the ω phase and a bcc phase (isostructural with group V transition metals) were observed during both compression and decompression in Hf. Equilibrium transformation pressures for Hf are 38±8 and 71±1 GPa for the hcp-to-ω and ω-to-bcc phase transformations, respectively, at room temperature. In Ti, only the hcp-to-ω phase transition was observed above 8 GPa, and the ω phase was stable to 87 GPa. The crystal structure sequences of Ti, Zr, and Hf at ultrahigh pressures are discussed.