Intense Broadband Emission in the Unconventional 3D Hybrid Metal Halide via High‐Pressure Engineering

Abstract Developing hybrid metal halides with self‐trapped exciton (STE) emission is a powerful and promising approach to achieve single‐component phosphors for wide‐color‐gamut display and illumination. Nevertheless, it is difficult to generate STEs and broadband emission in the classical and widely used 3D systems, owing to the great structural connectivity of metal‐halogen networks. Here, high pressure is implemented to achieve dual emission and dramatical emission enhancement in 3D metal halide of [Pb3Br4][O2C(CH2)2CO2]. The pressure‐induced new emission is ascribed to the radiation recombination of STEs from the Pb2Br2O2 tetrahedra with the promoted distortion through the isostructural phase transition. Furthermore, the wide range of emission chromaticity can be regulated by controlling the distortion order of different polyhedral units upon compression. This work not only constructs the relationship between structure and optical behavior of [Pb3Br4][O2C(CH2)2CO2], but also provides new strategies for optimizing broadband emission toward potential applications in solid‐state lighting.


Material synthesis
The [Pb3Br4][O2C(CH2)2CO2] crystals were synthesized according to the method that has been previously reported in the literature.Lead bromide (PbBr2, 2.0 mmol, 0.7340g, Adamas, 99.9%), succinic acid disodium salt (NaO2C(CH2)2CO2Na, 4.0 mmol, 0.6482g, Sigma-Aldrich, 99.0%), perchloric acid (HClO4, 5.56 mmol, 460 μL, Greagent, 70%), and deionized water (16 mL) were loaded into a 25 mL Teflon-lined autoclave reactor.The solution was stirred for 15 min for sufficient dispersion.The autoclave was then sealed into a stainless steel vessel and heated at 175 °C for 48 h.After incubation, slow-cooling of the autoclaves at the rate of 10° C/h to room temperature yielded colorless block-shaped crystals.The crystals were washed with ethanol and deionized water, and dried overnight under 30 Pa pressure with vacuum.

In situ high-pressure experiments
High-pressure experiments were conducted using a diamond anvil cell (DAC) with 400 μm diameter culets.A T301 stainless steel gasket with a 150 μm hole and 45 μm thickness was served as the sample chamber.The sample was loaded into the sample chamber along with a ruby ball to determine pressure according to the ruby fluorescence technique.In situ high-pressure optical absorption, photoluminescence (PL) and X-ray diffraction (XRD) experiments, silicone oil was applied as pressure-transmitting medium.
The in situ high-pressure PL measurements were measured using the 355 nm line of a UV DPSS laser.In situ high-pressure UV-Vis absorption measurements were performed by a deuterium-halogen light source.The PL and absorption microphotographs were captured with a camera (Canon Eos 5D mark II) equipped on a microscope (Ecilipse TI-U, Nikon).The optical fiber spectrometer is an Ocean Optics QE65000 spectrometer.
The in situ high-pressure time-resolved PL measurements were performed using a 375 nm pulsed diode laser (LDH-P-C-375B, 40 ps) as excitation source.A 20× ultraviolet objective lens was utilized to project the incident laser onto the sample and collet the backscattered emission signal.The PL signal was directed into the 500 mm focal length grating spectrograph (HRS-500 MS), where a PMT together with a time correlated single photon counting electronics (TimeHarp 260 PICO) was used to detect the PL kinetics.The time resolved PL decay curves were fitted by the double exponential function: The average lifetime τ was calculated by the follow equation: The in situ high-pressure angle-dispersive XRD measurements (λ = 0.6199 Å) were performed at BL15U1 at the Shanghai Synchrotron Radiation Facility (SSRF).Before the experiments, CeO2 was used for geometry calibration.All the high-pressure experiments were conducted at room temperature.The diffraction patterns were integrated into one-dimensional profile using Fit2D program.The Reflex module combined in Materials Studio was applied for Rietveld refinement.The pressure-volume data were fitted by the third-order Birch-Murnaghan equation of state as follows: where  0 is the zero-pressure volume,  0 is the bulk modulus at ambient pressure, and ′ 0 is a parameter for the pressure derivative.
The in situ high-pressure Raman measurements were carried out by a Raman spectrometer (iHR 550, Syncerity, Horiba Jobin Yvon) with a 785 nm laser excitation.Liquid nitrogen was selected as the pressure transfer medium for the experiments.The in situ high-pressure IR absorption measurements were conducted using a Bruker Vertex 70 V FT-IR spectrometer (BRUKER OPTIK GMBH, Germany).Potassium bromide is used as a pressure transfer medium for testing.
To evaluated the distortion degree of the Pb2Br4O2 octahedra and Pb2Br2O2 tetrahedra based on the polyhedral variance   2 by the Pb-X (X = Br and O) bond lengths.
Where n is the number of Pb-X lengths in an octahedron or tetrahedron,   is the Pb-X length and  is the average bond length of octahedra and tetrahedra.
First-Principle Calculations.Electronic band structures were calculated using pseudopotential plane-wave methods based on density functional theory implemented in the CASTEP package.The starting structure was obtained from the Cambridge Structure Database.The plane-wave cutoff energy of 500 eV and Monkhorst-Pack grid for the electronic Brillouin zone integration was 3 × 3× 2. The self-consistent field (SCF) tolerance was set as 5.0 × 10 -6 eV/atom.The convergence thresholds between optimization cycles for maximum force, maximum stress and maximum displacement are set as 0.01 eV/Å, 0.02 GPa, and 5.0 ×10 -4 Å, respectively.To track the evolution of the vibrational modes of [Pb3Br4][O2C(CH2)2CO2], we carried out high-pressure Raman experiment.The lowfrequency Raman vibrational modes (50-100 cm -1 ) are assigned to the Pb-Br inorganic structure. [1]With the increase of pressure, Pb-Br vibrational modes showed evident blueshifts and the lattice modes below 50 cm -1 moved towards the detectable region, ascribed to the lattice contraction.Upon compression to 7.7 GPa, the shift rates of all vibration bands began to slow down without profile variation, indicating the increased rigidity of inorganic structure and the consequent isostructural phase transition.Under further compression to 16.0 GPa, most vibration peaks were widened and gradually disappeared, which should be ascribed to the seriously distorted inorganic structure.High-pressure infrared absorption spectrometry was measured to investigate the behaviors of organic layer upon compression.The infrared absorption peaks were mainly concentrated in 600-1800cm -1 , including the out of plane C=O deformation vibration (617 and 650 cm -1 ), the O-H bending mode (880 and 900 cm -1 ), the C-O stretching mode (1150 and 1180 cm -1 ), and the C-H bending mode (1400 cm -1 ). [2]During the compression process, the peaks continued to move towards the high wavenumber region.After 7.0 GPa, the movement rates of infrared absorption peaks slowed down, accompanied by the peaks broadening and weakening.It is consistent with the change trend of inorganic structure, indicating that the behavioral changes of organic parts are closely related to the structural changes of inorganic structure.S25).The CBM was less sensitive to the shrinkage of structure due to the nonbonding characteristic of Pb 6p orbitals. [3]

Figure S4 .
Figure S4.(a) PL location, (b) intensity and (c) full width at half-maximum (FWHM) of peak Ⅰ with increasing pressure from 1 atm to 7.0 GPa.

Figure S6 .
Figure S6.PL intensity of peak Ⅰ and peak Ⅱ upon compression from 8.0 GPa to 12.0 GPa.

Figure S7 .
Figure S7.(a) PL Location and (b) Full width at half-maximum (FWHM) of peak Ⅰ and peak Ⅱ with increasing pressure from 8.0 GPa to 12.0 GPa.

Figure S8 .
Figure S8.(a) PL spectra and (b) CIE coordinates between ambient condition and decompression.(The signal marked with * comes from a diamond).

Figure S9 .
Figure S9.PL intensity of Peak Ⅰ and Peak Ⅱ at (a) 8.0 GPa, and (b) 12.5 GPa as a function of Power density, which is the "k" means the slope of PL intensity as a function of power density.

Figure S10 .
Figure S10.Normalized time-resolved PL decay curves of Peak Ⅰ and Peak Ⅱ at 12.5 GPa.

Figure S16 .
Figure S16.Pressure-dependent relative compression rates of (a) a axis, (b) b axis, (c) c axis.

Figure S21 .
Figure S21.Schematic illustration of Br-Pb-O unit under pressure.

Figure S22 .
Figure S22.Evolution of bond angle of Br-Pb-O upon compression.

Figure S23 .
Figure S23.Evolution of bond lengths of (a) Pb-Br and (b) Pb-O as a function of pressure.

Figure S24 .
Figure S24.Evolution of the theoretical band gap values with increasing pressure.

Table S1 .
Type of emission and ratio of the maximum PL intensity via pressure (It) to the initial PL intensity (I0) in 3D metal halides.