Materials with multi-stabilities controllable by external stimuli are potential for high-capacity information storage and switch devices.Second-harmonic-generation(SHG)as a distinct optical phenomenon is particularly suitable for information storage and readout,nevertheless,pressure-induced multi-step SHG switching material has rarely been reported.A research team led by Dr. Yonggang Wang from HPSTAR in collaboration with Prof. Hongwei Huang from China University of Geosciences(Beijing)reported the first example of a pressure-driven two-step SHG switching in polar BiOIO3.These findings were recently published inAngew.Chem.Int. Ed.
Why are the pressure-induced SHG switching materials so rare?The first reason may be that the requirements for a“switching”material or device is really strict,for example,the property conversion should be sharp and a good chemical stability is the basic requirement.In fact,there are several papers about the pressure-responsive SHG phenomenon,but most of them are either of slow,fuzzy,incomplete on-off transitions,or have only one-step SHG switching.Cations with stereo-active lone-pair electrons are a promising material family with a large SHG response.Pressure is considered to suppress the lone-pair effect,and thus to influence the overall SHG property of materials.An iodate BiOIO3,in which both Bi3+ and I5+ cations contain the stereo-active lone-pair electrons,is promised to achieve a stepwise SHG switching under compression.
Another difficulty for the study of SHG materials under high pressure is the lacking of measurement technique.We built an in-situ high-pressure powder SHG and polarization-dependent single-crystal SHG measurement system in HPSTAR,and got the as-expected SHG-high,SHG-intermediate,and SHG-off three-state SHG switching of BiOIO3by this system.Structure analyses reveal two pressure-induced phase transitions in BiOIO3from the ambient noncentrosymmetric phase to a noncentrosymmetric phase and finally to a centrosymmetric phase.HP Raman,XAS combined with DFT calculations also reveal the dramatic increase of the coordination number of I5+ from 3 to 5 under compression.Finally,HP optical absorption spectra show two distinct bandgap discontinuities,which also come from the stepwise suppression of lone-pair electrons.All of the experimental evidences confirm the occurrence of the rare two-step phase transition and property switching in BiOIO3.“The discovery of pressure-induced two-step SHG switching by controlling the lone-pair electronsmakeagoodstarton the exploration of multi-step switching materials,”said Dr. Wang.
Caption:Pressure-driven two-step SHG switching in BiOIO3
可控多穩態轉換資料具有用於大容量資訊存儲和開關器件的潜力。二次諧波(SHG)既是一種方便讀取的光學效應,又能反映出晶態資料的對稱性,具備較大的原位高壓研究價值。同時,壓力誘導的二次諧波轉換資料又鮮有報導。在北京高壓科學研究中心王永剛課題組與中國地質大學(北京)黃洪偉課題組合作的一個最新研究中,他們發現一種基於孤對電子的極性資料BiOIO3在壓力下具有多步的二次諧波轉換現象。晶體結構分析揭示了該資料在壓力下歷經了從非中心對稱到非中心對稱再到中心對稱的兩次相變,伴隨著二次諧波訊號的强、中、無三種狀態轉換。局域結構分析表明,二次諧波轉換的微觀原因是[IO3]-單元上的孤對電子在壓力下被抑制,功能單元的配比特環境改變也導致了光學吸收邊的不連續變化。該工作的關鍵是一臺原位高壓偏振二次諧波測試系統的成功搭建。相關成果以“Pressure-Driven Two-Step Second-Harmonic-Generation Switching in BiOIO3”為題發表在Angew.Chem.Int. Ed.上。