Different crystal phases of semiconductors are of high interest due to having useful optical and electronic properties. Conventionally, optical and electronic properties are controlled by doping the base material. By having different starting structures, a wider range of properties become accessible. High pressure is one of the methods to alter a material's crystal structure (phase). This is typically achieved through the use of diamond anvil cells (DACs), which allow direct observation of the structure through X-ray diffraction, and in situ optical measurements. After being subjected to high pressures, semiconductors like silicon and germanium can form altered metastable phases which persist at ambient conditions.

Silicon and germanium share very similar phase transformation pathways, with a key difference being that germanium can form a metastable tetragonal phase (st12) on unloading, which is unobserved in silicon. This st12 phase is notably only formed during non-hydrostatic decompression/unloading (Non-isotropic forces in all directions).

However, the effect of non-hydrostatic conditions in germanium during loading remains relatively unexplored. Using in situ Raman spectroscopy and powder X-ray diffraction, the effect of various types of non-hydrostatic loading on the transformation pathway were investigated.

The findings reveal that highly non-hydrostatic environments are capable of driving the transformation process at significantly lower pressures than the literature reports. Through use of non-hydrostatic compression, the formation of the st12-Ge phase has been directly observed using Raman spectroscopy at pressures as low as 2 GPa. Formation of -Sn-Ge is observed in x-ray diffraction under torsion at ~2-4 GPa, where a strong threshold effect was observed. The transformation from dc-Ge to -Sn-Ge rapidly increases at ~3-4 GPa. Additionally, on unloading, the torsion sample -Sn-Ge reverted back to dc-Ge. The mechanisms and reasoning to the difference in pathway observed in these experiments are discussed within.

The findings in this work provide new evidence for inducing phase transformation in Ge at significantly lower pressures than described in literature. Additionally, evidence is provided indicating that the dc-Ge to -Sn-Ge transformation is reversible under specific stress conditions.