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

TL;DR

This work demonstrates that pressure can be used to tune the structure and optoelectronic properties of the lead-free vacancy-ordered double perovskite Rb2TeBr6. The cubic Fm-3m phase remains stable up to ~6.8 GPa, where subtle inter-octahedral rotations enhance radiative recombination and cause a dramatic photoluminescence boost, peaking near 2.4 GPa. As pressure increases, the material undergoes symmetry-lowering transitions to orthorhombic Pnnm and monoclinic P2_1/m, followed by amorphization around 25.5 GPa, while the optical bandgap continuously narrows from ~2.07 eV to well below 2 eV, correlating with Te–Br bond dynamics. The observation that a magnetic field further enhances PL suggests spin-dependent recombination pathways, underscoring strong lattice–electronic coupling and establishing Rb2TeBr6 as a model for pressure-tunable, lead-free optoelectronics.

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

Figures (7)

• Figure 1: Rietveld refinement of the XRD pattern of Rb$_2$TeBr$_6$ at (a) ambient in cubic (Fm-3m), and (b) 8.0 GPa in orthorhombic (Pnnm) structure. The black balls represent experimental data. Red, green, and navy lines are Rietveld fit to the experimental data, background, and difference between experimental and calculated data, respectively. The magenta vertical lines show the Bragg peaks of the sample. The silver peaks at 8.0 GPa are identified with stars. The broadening of (200) peak and splitting of (400) peak at 8.0 GPa are indicated by arrows. Schematic representations of the unit cell at (c) ambient conditions, and (d) 8.0 GPa.
• Figure 2: (a) and (b) PL spectra of Rb$_2$TeBr$_6$ at some selected pressure points. (c) and (d) The PL spectra of Rb$_2$TeBr$_6$ at 1.0 GPa and 5.0 GPa respectiveley in the absence and presence of an external magnetic field (0.4 Tesla). The applied magnetic flux induces a noticeable enhancement in the PL intensity.
• Figure 3: Relative PL intensity of Rb$_2$TeBr$_6$ under pressure. The relative PL intensity at each pressure is defined as the PL intensity at that pressure divided by the PL intensity at ambient conditions
• Figure 4: (a)Evolution of the unit cell volume of Rb$_2$TeBr$_6$ as a function of applied pressure in the cubic phase. The red line represents the fit to the third-order Birch–Murnaghan equation of state. The inset shows a plot of normalized pressure (H) vs Eulerian strain (f$_E$).(b)The relative volume compression of the unit cell, RbBr$_{12}$ cavity, and TeBr$_6$ octahedron under pressure in the cubic phase of Rb$_2$TeBr$_6$. The relative volume compression at each pressure is defined as the volume at that pressure (V$_p$) divided by the volume at ambient conditions (V$_0$)
• Figure 5: (a) Schematic representation of the Rb$_2$TeBr$_6$ structure at ambient pressure and 3.5 GPa, illustrating the emergence of slight octahedral rotation under compression within the cubic Fm-3m framework. (b) Pressure-induced octahedral rotational deviation in Rb$_2$TeBr$_6$ within the cubic Fm-3m phase.