Recent discoveries in high pressure science have revealed entirely unexpected chemical behavior of two-dimensional (2D) materials. However, there is still a lack of unambiguous insight on the pressure-driven behavior of in-plane bonds in the 2D layered structures. Layered germanium phosphide (GeP5) is a metal with honeycomb-like sheets structurally similar to semiconducting black phosphorous, but with electrical conductivity ten times higher than that of graphite. Here, we report a remarkable pressure-dependent structural transformation that includes lengthening of the main in-plane bonds under pressure, although practically high pressure usually leads to shorter stiffer bonds. In situ Raman measurements show that there is significant phonon mode softening through the 2D–3D structural reconstruction, correlating with the in-plane bond extensions in GeP5 upon compression. This is accompanied by unusually superconducting behavior, on both sides of the transformation. This superconductivity with a maximum transition value of 10.5 K at 13.5 GPa shows a robust character without obvious reduction up to 60 GPa and is accompanied by pressure-induced amorphization in GeP5. Our experimental results, together with those from first principles calculations, not only provide the detailed high-pressure phase diagram of GeP5 but also connect the pressure-dependent bond extension with enhanced superconductivity.
Jingyan Song, Ge Fei, Xiaobing Liu, Shuai Duan, Bingchao Yang, Xin Chen, David J. Singh, Yunxian Liu, aLiuxiang Yang, Jiangang Guode, Ping Zhang, Pressure-driven significant phonon mode softening and robust superconductivity in layered germanium phosphide, J. Mater. Chem. A, 2020, 8, 20054, DOI: 10.1039/d0ta07243e abstract
Structure and characterization of GeP5 single crystal. (a) Optical image of a produced GeP5 single crystal at 2 GPa and 1273 K. (b) TEM image and (c) SAED pattern for the grown GeP5 indicate a high-quality single crystalline structure. (d) Typical EDS data. Cu and C elements in the EDS data are from the TEM substrate. The analyzed composition for the produced sample is close to GeP5.52. Bottom right inset: SEM image of GeP5. (e) and (f) Represent elemental mapping for Ge and P elements, respectively.