Synthesis of GeSe with robust ferroelectricity under extreme conditions - Dr. Kuo Li & Dexiang Gao

SEPTEMBER 10, 2018

By combing with the high pressure and high temperature synthesis as well as density functional theory and evolutionary algorithms calculation, scientists from HPSTAR and The University of Hong Kong explored the pressure-induced phase transitions of GeSe systemically. Two novel phases (R3m and Fm-3m) are discovered which show robust ferroelectricity and a 3D topological crystalline insulator, respectively. The R3m phase was synthesized by compressing the a-GeSe under high temperature. This research greatly enrichs our knowledge of IV-VI compounds. The results were published on NPG Asia Materials .

The IV–VI binary compounds have many promising applications in photovoltaic and thermoelectric fields. As a promising IV–VI compound, GeSe is a narrow band gap semiconductor and possesses an orthorhombic Pnma crystal structure at ambient conditions. Most existing studies focus on this laminar Pnma structure, whereas the properties and structural phase transitions of GeSe under pressure are largely unexplored.

The research team systematically investigated the structural phase transitions of GeSe compound under high pressure by performing the extensive evolutionary algorithm searches and the laser-heated diamond anvil cell (DAC) experiments combing with the X-ray diffraction. Two new phases with space groups of R3m and Fm-3m were predicted to be stable in the pressure range between the well known α and β phases. By laser-heated DAC experiments, the GeSe was compressed to 2 GPa and heated to 1600 K. The XRD pattern of heated sample provided the evidence for the existence of the newly predicted R3m phase.

These two new phases show remarkable difference of physical properties. The R3m-GeSe has a layered crystal structure and exhibits robust ferroelectricity with a large spontaneous polarization. The Fm-3m-GeSe was predicted to be stable at the higher pressure than R3m-GeSe and it is predicted that this phase is a topological crystalline insulator under a hydrostatic pressure of 8 GPa.

The systematical investigation on the phase transitions of GeSe under high pressure and intriguing properties of the newly discovered GeSe phases further enrich our knowledge on the high-pressure behaviors of IV–VI compounds.

Caption: Left: Enthalpies of competing GeSe high-pressure phases and the crystal structures; Right: Rietveld refinement of the GeSe sample recovered from 2 GPa and 1600 K.

GeSe是一种窄带隙半导体，在光伏和热电领域具有许多应用前景。在常温常压下GeSe具有正交（Pnma）晶体结构。大多数现有的研究都聚焦在这种层状Pnma结构上，而其在压力下的结构相变和性质则研究的很少。近日，北京高压科学研究中心的李阔研究员和香港大学的陈粤教授合作，通过计算和实验的结合系统地研究了GeSe高压下结构相变，发现其在0到16 GPa有两个新相，R3m-GeSe和Fm3m-GeSe。R3m-GeSe具有层状晶体结构，并表现出强大的铁电性和非常大的自发极化。在高温高压条件下（2GPa, 1600 K）， 该相可以被成功的合成。Fm3m-GeSe则在8GPa的静水压力下表现出拓扑绝缘体性。这一成果近期发表在NPG Asia Materials上，研究生高德祥参与了相关的实验工作。