Crystal structures of group IVa metals at ultrahigh pressures

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/V₀ 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.