Reversible dc-Ge to (β-Sn)-Ge transformation under high shear

The pressure-synthesized phases of Ge have properties of technological interest. Such phases are generally formed after decompression from the metallic β-Sn structure of Ge above 10 GPa under hydrostatic compression. Here, we subjected diamond cubic Ge (dc-Ge) to high-pressure and high-shear environments using both regular diamond anvil cells with no pressure medium and a rotational diamond anvil cell. We report both a reversible (β-Sn)-Ge to dc-Ge pathway and a significant reduction, as low as 2 GPa, in the pressure required to form the (β-Sn)-Ge phase in high-shear conditions. This lowered transition pressure may be promoted by an increase in shear-induced defects, which act as nucleation sites for the transition to the metallic (β-Sn)-Ge phase. The metallic phase formed below 8 GPa shows reversible transformation back to the diamond cubic phase upon decompression, contrasting with metallic Ge formed above 10 GPa, which irreversibly transforms into several metastable phases. This work provides insights into the behavior of Ge under pressure and high-shear environments.