Pressure‐Induced Phase Transition, Jahn‐Teller Suppression, Optical and Electronic Property Evolutions in Ruddlesden‐Popper Perovskites Rb₂CuCl_4‐xBr_x

Abstract

Transition-metal containing halides with Ruddlesden-Popper (RP) perovskite structures have received extensive attention owing to their emerging and anisotropic photoelectric functionalities. Among them, A 2 CuX 4 (A = alkali metal or organic cations, X = Cl, Br, I) series are particular, because of the Jahn-Teller distortion of Cu 2 + sensitive to external stimuli such as temperature and pressure. In this article, we report the structure evolution and physical property responses of RP perovskites Rb 2 CuCl 4-x Br x (x = 1, 2) to external pressure. Dramatic structural phase transitions from orthorhombic to monoclinic occur around 3.0 GPa in both materials regardless of their distinct compositions. Structure analyses reveal the suppression and final vanishing of the Jahn-Teller distortion of Cu 2 + cations under compression and crossing the phase transition, respectively. Rb 2 CuCl 4-x Br x perovskites exhibit abrupt bandgap narrowing (from reddishbrown to black) along with the structural phase transition, and an overall bandgap narrowing of 75% up to ~27 GPa but still keeping semiconductive. During the compression processes, the resistances of Rb 2 CuCl 4-x Br x have been greatly reduced by 5-orders of magnitude. Moreover, all of the pressure-induced phenomena in Rb 2 CuCl 4-x Br x perovskites are reversible upon decompression and no obvious difference is observed for the pressure responses between [CuCl 4 Br 2 ] and [CuCl 4 (Cl,Br) 2 ] coordination environments. The impact of pressure on the structural and physical properties in twodimensional Rb 2 CuCl 4-x Br x provides in-depth understanding on the structure design of functional halide perovskites at ambient conditions.

Enhanced Structural Stability and Pressure‐Induced Photoconductivity in Two‐Dimensional Hybrid Perovskite (C6H5CH2NH3)2CuBr4

“…In this work, it is striking that the photoresponse of the (PMA) 2 CuBr 4 sample is clearly observed at 10 GPa, and exists throughout the entire measured pressure range up to 40 GPa. The maximum value of light conductivity can reach up to 5.0×10 −3 S cm −1 at 28 GPa, setting a new pressure record in 2D halide perovskites for the structural stability and the obtained optimal photoelectric characteristics [36, 45, 52] . Stimulated by such results, 2D Cu‐based perovskite materials (such as CuBr 4 , CuCl 4 , CuBr x Cl 4−x ) composed of bulky volume (such as C 6 H 5 C 2 H 4 NH 3 and C 6 H 5 CH 2 NH 3 ) or long‐chain (such as CH 2 ) 6 O 2 (NH 3 ) 2 ) organic cations are also expected to exhibit good photoconductivity behavior at high pressures, which would be worth further investigation.…”

“…Figure 5c shows the calculated value of the pressure‐dependent photoconductivity (difference between the light and dark conductivity). The enhancement of photoconductivity at relatively low pressures is the comprehensive effect of narrowing the band gap, reducing the exciton binding energy and increasing the carrier mobility with the increase of external strains [36, 45, 52–56] . The maximum value at 28 GPa is 5.61×10 −4 S cm −1 .…”

“…The maximum value of light conductivity can reach up to 5.0 × 10 À 3 S cm À 1 at 28 GPa, setting a new pressure record in 2D halide perovskites for the structural stability and the obtained optimal photoelectric characteristics. [36,45,52]…”

“…The enhancement of photoconductivity at relatively low pressures is the comprehensive effect of narrowing the band gap, reducing the exciton binding energy and increasing the carrier mobility with the increase of external strains. [36,45,[52][53][54][55][56] The maximum value at 28 GPa is 5.61 × 10 À 4 S cm À 1 . The reduction of photoconductivity above 28 GPa can be attributed to the increase of deformation and defects in the perovskite-like structure, as well as to a certain extent to the increasing external strains.…”

Enhanced Structural Stability and Pressure‐Induced Photoconductivity in Two‐Dimensional Hybrid Perovskite (C6H5CH2NH3)2CuBr4

“…Figure 5c shows the calculated value of the pressure‐dependent photoconductivity (difference between the light and dark conductivity). The enhancement of photoconductivity at relatively low pressures is the comprehensive effect of narrowing the band gap, reducing the exciton binding energy and increasing the carrier mobility with the increase of external strains [36, 45, 52–56] . The maximum value at 28 GPa is 5.61×10 −4 S cm −1 .…”