Crystal Structure Refinements of Ge(tP12), Physical Properties and Pressure-induced Phase Transformation Ge(tP12) ↔ Ge(tI4)

Abstract

Single crystals of the modification Ge(tP12) are prepared by compressing semiconductor-grade germanium to pressures above 11(1) GPa and heating to temperatures between 1200(100) and 1500(150) K before a stepwise cooling precedes a complete pressure release. The tetragonal crystal structure of Ge(tP12) is refined by means of single crystal X-ray diffraction data collected at ambient conditions (a = 592.81(2), c = 698.03(3) pm, space group P43212). The atomic arrangement comprising interconnected spiral chains of fourfold symmetry bears a structural similarity to the high-pressure modification S(t I16). High-pressure ambient-temperature powder X-ray diffraction measurements reveal a significant anisotropy of the compressibility compatible with the selected crystal structure description. The determined bulk modulus of Ge(tP12) amounts to 70(1) GPa which is in good agreement with theoretical calculations and similar to experimental values of other four-coordinated germanium allotropes. Ge(tP12) is a diamagnetic semiconductor with χ0 = −3.1(3) ·10−7 emu g−1 and ρ300 K ≈ 6 Ωm at 300 K. At 10.1(3) GPa, the beginning of the formation of Ge(t I 4) indicates the onset of a structural transition. The two-phase region extends up to a maximum pressure of 13.0(5) GPa in the direction of increasing pressures. Upon stepwise decompression, the phase Ge(t P12) is reformed from Ge(t I4) at 9(1) GPa.