Structure-property relations of the recently discovered germanium allotrope $\mathit{oP}32$-Ge are examined using a variety of experimental high-pressure techniques and compared with the results of density functional theory (DFT) calculations. High-pressure single-crystal x-ray diffraction shows that the unit-cell parameters and atomic positions of $\mathit{oP}32$-Ge change continuously up to 6 GPa, while peak shape and diffuse-scattering features remain essentially unchanged. X-ray diffraction reveals a sluggish transformation of $\mathit{oP}32$-Ge to the $\beta$-Sn Ge-II polymorph, i.e., the transition begins around 8 GPa and is complete by 12 GPa. The reconstructive transition is associated with the breakdown of $\mathit{oP}32$-Ge crystals into polycrystalline aggregates, which were examined up to 32 GPa. Synchrotron infrared absorption measurements indicate an increase in the band gap of $\mathit{oP}32$-Ge prior to the transition at 8 GPa to the higher-pressure metallic phase, which is characterized by high optical reflectivity. The DFT calculations of the structural and electronic properties are in good agreement with the measurements. Our experimental work conclusively shows that, under room-temperature quasihydrostatic compression, $\mathit{oP}32$-Ge remains in its semiconducting phase up to approximately 8 GPa, transforming into the Ge-II structure only by 12 GPa, a stability interval comparable to that of diamond-structured Ge (Ge-I). Crucially, we observe no anomalous structural or bonding changes over the range of conditions explored, indicating that the previously reported low-pressure superconductivity (near 2 GPa) must stem from subtle structural or stress-induced modifications accessible under the nonhydrostatic, low-temperature conditions of those experiments.

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• Crystal structure
• High-pressure studies
• Phase transitions
• Superconducting phase transition