Pressure-Driven Phase Evolution and Optoelectronic Properties of Lead-free Halide Perovskite Rb$_2$TeBr$_6$

Abstract

The structural, vibrational, and optical properties of Rb$_2$TeBr$_6$ have been investigated under high pressure using synchrotron X-ray diffraction, Raman spectroscopy, photoluminescence (PL), and optical absorption measurements. At ambient conditions, Rb$_2$TeBr$_6$ crystallizes in the cubic Fm-3m structure, which remains stable below 8.0 GPa. Within this pressure range, subtle inter-octahedral rotations develop, producing a gradual localized deviation from the ideal cubic framework. This local reorientation facilitates radiative recombination, leading to a pronounced enhancement of PL intensity with pressure up to 2.4 GPa. Beyond this pressure point, enhancement of nonradiative relaxation channels result in gradual PL quenching. Additionally, the PL intensity increases upon the application of an external weak magnetic field. A structural transition to the orthorhombic Pnnm phase occurs at around 8.0 GPa, followed by a monoclinic P$2_1/m$ phase above 10.7 GPa, and eventual amorphization beyond 25.5 GPa. Optical absorption spectra reveal continuous band-gap narrowing upon compression. These findings demonstrate the strong coupling among lattice dynamics, electronic structure, and optical response in Rb$_2$TeBr$_6$, underscoring its potential as a pressure-tunable optoelectronic material