New research from a team of scientists co-led by Dr. Huiyang Gou from HPSTAR achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl₂ and ferroelectric-to-paraelectric phase transition in NbOI₂ under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. This study is published in the journ of Nature communications.

Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. Using the single-crystal X-ray diffraction analysis and SHG measurements at extreme conditions, together with DFT calculations, the research team discovered the intralayer ferroelectric (FE)-to-antiferroelectric (AFE) phase transition in NbOCl₂ and ferroelectric-to-paraelectric (PE) phase transition in NbOI₂ under certain pressures, which can in turn effectively manipulate the NLO properties in NbOX₂, which provide great opportunities for manipulating NbOX₂-based optoelectronic applications from optical sensing/computing/switch to BPVE.

“It is important to develop effective ways to control the NLO responses in NbOX₂. the Nb atoms shifted along the b direction can create a degree of freedom, which in principle may also generate different metastable AFE phases or PE phase with inversion symmetry that can effectively turn off the overall second-order NLO

responses.” said by Wenju Zhou, co-first author at HPSTAR. “single-crystal X-ray diffraction collected at 5.7 GPa reveals that NbOCl₂ undergoes an unusual novel FE-to-AFE phase transition rather than simply to the centrosymmetric PE phase without local spontaneous polarization. The off-center displacements of Nb atoms along the b direction redistribute the Nb atoms on both sides of the M_Cl plane, forming a centrosymmetric non-polarized structure with the twofold rotational symmetry retaining. This change of non-centrosymmetric structure to the inversionsymmetry-invariant structure has important impacts on all the second-order NLO responses.”

Caption: A reversible ferroelectric-to-antiferroelectric phase transition in NbOCl₂ and a reversible ferroelectric-to-paraelectric phase transition in NbOI₂ under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms.

Caption: Under high temperature or high pressure, the extra reflections suggest the doubled Z value of the crystal structures, implying the structural phase transitions of NbOCl₂.

范德瓦尔斯氧化铌二卤化物（NbOX₂）由于其微弱的层间电子耦合，较强的极性和各向异性以及可调谐的强二次谐波响应在传统和量子非线性光学领域都显示出巨大的应用潜力。发现并理解NbOX₂结构与非线性光学响应之间的联系可以帮助科学家们深入挖掘材料非线性光学响应变化的机理，设计并合成新的非线性光学响应材料，精准调控材料的NLO性质，为光学传感、光电开关、BPVE等光电器件的发展提供新的思路。北京高压科学研究中心（HPSTAR）缑慧阳团队与北京计算科学中心的黄兵团队等国内外团队合作，利用高压原位单晶X射线衍射和原位二次谐波信号检测以及理论计算，发现了压力下NbOCl₂和NbOI₂晶体中的结构相变，揭示了结构演变与非线性光学响应之间的关系，解明了氧化铌二卤化物NLO性质变化的微观机制。相关结果于近期以“Manipulation of nonlinear optical responses in layered ferroelectric niobium oxide dihalides”为题发表于Nature communications。