High-temperature phase transitions in dense germanium

Through a series of high-pressure x-ray diffraction experiments combined with in situ laser heating, we explore the pressure–temperature phase diagram of germanium (Ge) at pressures up to 110 GPa and temperatures exceeding 3000 K. In the pressure range of 64–90 GPa, we observe orthorhombic Ge-IV transforming above 1500 K to a previously unobserved high-temperature phase, which we denote as Ge-VIII. This high-temperature phase is characterized by a tetragonal crystal structure, space group I4/mmm. Density functional theory simulations confirm that Ge-IV becomes unstable at high temperatures and that Ge-VIII is highly competitive and dynamically stable at these conditions. The existence of Ge-VIII has profound implications for the pressure–temperature phase diagram, with melting conditions increasing to much higher temperatures than previous extrapolations would imply.