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Pressure-Induced Phase Transition, Jahn-Teller Suppression, Optical and Electronic Property Evolutions in Ruddlesden-Popper Perovskites Rb2CuCl4-xBrx.

Transition-metal contained halides with Ruddlesden-Popper (RP) perovskite structures have received extensive attention owing to their emerging and anisotropic photoelectric functionalities. Among them, A2CuX4 (A = alkali metal or organic cations, X = Cl, Br, I) series are particular, because of the Jahn-Teller distortion of Cu2+ sensitive to external stimuli such as temperature and pressure. In this article, we report the structural evolution and physical property responses of RP perovskites Rb2CuCl4-xBrx (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 Cu2+ cations under compression and crossing the phase transition, respectively. Rb2CuCl4-xBrx perovskites exhibit abrupt bandgap narrowing (from reddish-brown 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 Rb2CuCl4-xBrx have been greatly reduced by 5-orders of magnitude. Moreover, all of the pressure-induced phenomena in Rb2CuCl4-xBrx perovskites are reversible upon decompression and no obvious difference is observed for the pressure responses between [CuCl4Br2] and [CuCl4(Cl,Br)2] coordination environments. The impact of pressure on the structural and physical properties in two-dimensional Rb2CuCl4-xBrx provides in-depth understanding on the structure design of functional halide perovskites at ambient conditions.