Synthesis, structure, and electronic properties of 4H-germanium

Reinvestigation of the reaction of Li₇Ge₁₂ with benzophenone in tetrahydrofuran solution affords the metastable crystalline germanium allotrope allo-Ge, which transforms into another allotrope, 4H-Ge, upon annealing at temperatures between 150 and 300 °C. When annealing 4H-Ge above 400 °C the ground state modification α-Ge is obtained. The crystal structure of 4H-Ge was refined from powder X-ray diffraction data (space group P6₃/mmc (no. 194), a = 3.99019(4) and c = 13.1070(2) Å, Z = 8) and the sequence of phase transitions from allo-Ge to α-Ge was monitored by temperature-dependent powder X-ray diffraction experiments. Electrical resistivity measurements and quantum-mechanical calculations show that 4H-Ge is a semiconductor, which is in contrast to previous theoretical predictions. The Raman spectrum of 4H-Ge displays three bands at 299, 291, and 245 cm^-1 which are assigned to E_1g, E_2g and A_1g modes, respectively, and relate to the optic mode in α-Ge.