Non-hydrostatic pressure induced a to b phase transition in group IV–VI monochalcogenide GeSe

The family of group IV–VI monochalcogenides shows great potential for advanced applications in the fields of ferroelectrics, electronics and thermoelectrics. Here we report the direct observation of a to b phase transition in two-dimensional van der Waals GeSe. Specifically, b-GeSe is obtained by applying high pressure to a-GeSe and remains stable under ambient conditions. The crystal structures of a- and b-GeSe were confirmed by in situ Raman and STEM characterization. The experimental results indicate that applying a high pressure of up to 24 GPa is critical to induce an intermediate Cmcm phase, which is necessary for facilitating the transition from the a to the b phase. This interpretation is strongly supported by first-principles calculations. The driving force behind this phase transition procedure is attributed to the shear stress induced by non-hydrostatic pressure generated within this high-pressure regime. Our work reveals the feasibility of inducing phase transformation in GeSe via application of high pressure. This offers an approach for the synthesis of different GeSe phases with specific crystal structures and advances our understanding of the high-pressure phase transitions in monochalcogenides.