Tuning Structure And Optical Properties In Cationic Lead Halide Hybrids Via High Pressure

Organometallic halides have potential applications in solid state lighting because of their excellent photoelectric properties and structural tunability.Among them,organic lead halide perovskites have received extensive attention due to their high luminous efficiency.However,the majority of organic lead halide hybrids exhibit labile structures based on the weak electrostatic interactions between organic cations and inorganic framework.Simultaneously,the hydrophilic of most organic ions make lead halide hybrids corroded by ambient moisture,leading to the dissociation or migration of inorganic structural units and restricting their applications.Recently,the cationic lead halide hybrids have emerged as an intriguing material for the excellent intrinsic stability and rare broadband luminescence properties.Instead of the conventional organic cationic hybrids as for perovskite,it utilizes anionic organic compounds with carboxylic acid groups as structural linkers to bind inorganic framework and organic components through coordination bonds.The cationic lead halide hybrids greatly improve the environmental stability by transforming from ionic structure to coordination expansion structure.The distorted inorganic network provides a platform for the generation of self-trapping exciton（STE）,endowing the materials with the rare broadband emission in three-dimensional metal halide systems.Nonetheless,the fluorescence is weak at ambient conditions because only a few excitons are captured by instantaneous lattice deformation for emission,resulting from the limited structural distortion.Pressure is an effective tool for regulating optical properties by shrinking the lattice and modifying bonding patterns without changing the chemical composition.Meanwhile,it provides a strategy to construct the relationship between structure and optical properties,and solve the dispute about the luminescence mechanism of some materials.The properties of materials may be optimized by preserving the thermodynamically metastable states with improved optical properties at high pressures to environmental conditions during recrystallization process treated through compression-decompression cycles.We selected cationic lead bromide hybrid hybrids[Pb₂Br₂][O₂C（CH₂）₄CO₂]with three-dimensional inorganic network formed by Pb Br₃O₃ octahedron,cationic lead bromide hybrid hybrids[Pb₃Br₄][O₂C（CH₂）₂CO₂]with three-dimensional inorganic network formed by Pb Br₄O₂ octahedron and Pb Br₂O₂tetrahedron and cationic lead chloride hybrid hybrids[Pb₂Cl₂][O₂C(C₆H₁₀)CO₂]with two-dimensional inorganic layer formed by Pb₂Cl₄O₄ dimer for high pressure research.Firstly,we improved the optical properties of cationic lead halides by pressure engineering,and then understanding the relationship between structure and optical properties based on high pressure experimental techniques and theoretical calculation.（1）For cationic lead bromide hybrid hybrids[Pb₂Br₂][O₂C（CH₂）₄CO₂],We achieved a remarkable emission enhancement by exploiting a diamond anvil cell.At12.5 GPa,the fluorescence intensity enhanced 100 times compared to the initial condition.Note that the fluorescence exhibited a redshift of 110 nm,accompanied by an increase in intensity upon complete pressure release.In situ angle dispersive synchrotron X-ray diffraction（ADXRD）,Raman spectroscopy and infrared（IR）spectroscopy clearly indicated that pressure-induced structural phase transitions from tetragonal structure to cubic structure to triclinic structure that enhanced the Pb Br₃O₃octahedral distortion and facilitated the electron-phonon coupling effects,which promoted the radiation transition of STEs.We were astonished to find that the metastable orthogonal structure formed by the first phase transition was preserved to the environmental conditions,which affected by the partial amorphous of the organic components.The optimization fluorescence of quenched[Pb₂Br₂][O₂C（CH₂）₄CO₂]was attributed to an irreversible structural phase transition that intensified the lattice distortion and increased lattice relaxation energy.This work provides an effective pathway to improve the optical properties of three-dimensional lead halide hybrid materials with stable structure.（2）For cationic lead bromide hybrid hybrids[Pb₃Br₄][O₂C（CH₂）₂CO₂],high pressure was implemented to achieve dual STE emission and 130-fold emission enhancement by inducing isostructural phase transition.We confirmed that the new fluorescence peak formed at 8.0 GPa was from STE emission based on variable excitation power density fluorescence and time-resolved fluorescence spectra measurements.Combined with the structural analysis,we found that the fluorescence emission under environmental conditions comes from the radiation recombination of STEs near the Pb Br₄O₂octahedron,and the pressure induced new fluorescence is ascribed to the radiation recombination of STEs from the Pb Br₂O₂ tetrahedra with the promoted distortion through the isostructural phase transition.Furthermore,the wide range of emission chromaticity can be regulated from“cold”white to“warm”white to bluish-white by controlling the distortion order of different polyhedral units upon compression.This research provides a new way to solve the luminescence controversy of some materials based on high pressure strategy.（3）In order to deeply unerstand the high-pressure regulation of the structure and optical properties of cationic lead halide materials,we selected cationic lead chloride hybrid material[Pb₂Cl₂][O₂C(C₆H₁₀)CO₂],whose inorganic components decreased from three-dimensional network structure to two-dimensional layer structure,as the high pressure research object to explore the influence and rule of high pressure on different dimensions.Based on the pressure contraction of organic ions and inorganic components,we realized the fluorescence regulation of the material under different excitation.At 303 nm excitation,the material has an intrinsic emission based on theπ*-n transition between organic ions.By reducing the distance between C and O atoms between adjacent organic ions through pressure,the interaction was effectively enhanced,resulting in an increased in thermal vibration relaxation and decreased in radiation energy,which was manifested as a blue shift in fluorescence from blue light（0.16,0.09）to"cold"white light（0.29,0.38）.Under the excitation of 355 nm,the distortion of Pb₂Cl₄O₄ dimer was greatly increased due to the pressure induced structural phase transition,which resulted in the enhancement of exciton localization and STE emission.In summary,we systematically study the relationship between the structure and optical properties of cationic lead hybrid hybrids under high pressure,and achieved a significant optimize in the luminescence properties of the materials,providing a potential strategy for development of organometallic halides with structurally stable and excellent broadband emission properties.Meanwhile,it also provides a new idea based on high pressure strategy to solve the controversy of atmospheric pressure luminescence of materials.