Single crystal x-ray diffraction, synchrotron infrared spectroscopy, and first-principles calculations of oP32-type Ge under pressure*

Single-crystal diffraction of the oP32-structured allotrope of Ge was performed under hydrostatic and quasihydrostatic pressures up to 6 GPa. Analysis of unit cell parameters, systematic absences, and structure factors show no evidence for structural transitions, as a continuous compression of the structure was observed. In addition, occurrence of multiple domains as well as diffuse scattering rods between Bragg peaks along some directions are observed, which could be the result of stacking disorder. The phase transforms to the β-Sn type structure near 9 GPa, as indicated by powder diffraction which was conducted to at least 32 GPa. Synchrotron infrared and optical absorption measurements up to 8.5 GPa indicate widening of the band gap of the op32 phase prior to collapse to the highly reflecting metallic β-Sn type phase. The increased band gap energy with pressure of oP32 Ge and the phase transition to the b-Sn phase agree with first-principles density functional theory calculations. The effects of pressure on the structural and electronic properties of oP32 Ge will also be discussed.