Novel $\gamma$-Phase of Titanium Metal at Megabar Pressures

Group IV transition metals titanium, zirconium, and hafnium are expected to transform from an ambient hexagonal close packed (hcp, $\alpha$-phase) to a body centered cubic (bcc, $\beta$-phase) at high pressures. This transition path is usually facilitated by the occurrence of an intermediate hexagonal phase (distorted bcc, $\omega$-phase). The existence of a bcc phase in zirconium and hafnium at high pressures has been known for the past ten years; however, its occurrence in titanium has been theoretically predicted but never observed. We report a novel unexpected transformation in titanium metal from an omega phase to an orthorhombic phase (distorted hcp, $\gamma$-phase) at a pressure of $116\pm 4\mathrm{GPa}$.