Temperature and Pressure Dependent Vibrational Properties of Pristine and Doped Vacancy-Ordered Double Perovskite

TL;DR

This work addresses how Sb doping alters the lattice dynamics of the lead-free vacancy-ordered double perovskite Cs$_2$TiCl$_6$ by integrating temperature-dependent Raman, high-pressure Raman, XRD, and photoluminescence. Sb incorporation reduces impurity-related Raman features and yields cleaner spectra with three principal TiCl$_6$ vibrational modes, while a dopant-specific M$_1$ mode emerges below 100 K, the origin of which remains unresolved. High-pressure Raman reveals continuous mode hardening without phase transitions up to 30 GPa, and PL remains dominated by broad self-trapped exciton emission, with Sb-doped samples showing broader FWHM indicative of enhanced disorder. The results demonstrate that Sb doping modulates vibrational properties and improves phase purity, guiding future low-temperature structural studies and theoretical modeling of dopant effects in lead-free vacancy-ordered perovskites.

Abstract

Understanding lattice dynamics and structural transitions in vacancy-ordered double perovskites is crucial for developing lead-free optoelectronic materials, yet the role of dopants in modulatingthese properties remains poorly understood. We investigate the vibrational and optical properties of pristine and Antimony(Sb)-doped Cs$_2$TiCl$_6$ vacancy-ordered double perovskite through temperature-dependent Raman spectroscopy (4-273 K), high-pressure studies (0- \~30 GPa), ambient powder XRD, and photoluminescence measurements. Sb doping improves phase purity, reducing impurity-related Raman modes present in pristine samples. Most notably, Sb-doped samples exhibit an anomalous Raman mode M$_1$ appearing exclusively below 100 K at 314-319 cm$^{-1}$, accompanied by changes in the temperature coefficient $χ$ and anharmonic constant $A$ across this threshold. This behavior is absent in pristine Cs$_2$TiCl$_6$. While these observations suggest possible structural changes at low temperature, the origin of the M$_1$ mode remains unclear and may arise from disorder-activated vibrations, symmetry breaking, or dopant-induced local distortions. Low-temperature structural characterization is needed to confirm the nature of this transition. Photoluminescence shows broad self-trapped exciton emission at 448 nm with broader FWHM in Sb-doped samples (164.73 nm) compared to Bi-doped samples (138.2 nm), consistent with enhanced structural disorder. High-pressure Raman measurements reveal continuous mode hardening to 30 GPa with no phase transitions. These results demonstrate that Sb doping modulates the vibrational properties of Cs$_2$TiCl$_6$, though further investigation is required to establish the underlying mechanisms.

Figures (18)

• Figure 1: General structure of a. $ABX_3$ and $A_2BX_6$ halide perovskite
• Figure 2: a. Ambient X-ray diffraction measurement using 0.154 nm X-rays. b. Ambient Raman Spectra using 532 nm laser for all synthesized samples
• Figure 3: Excitation and emission spectra of Cs$_2$TiCl$_6$ and doped systems, including Cs$_2$Ti$_{(1-x)}$Bi$_x$Cl$_6$ and Cs$_2$Ti$_{(1-x)}$Sb$_x$Cl$_6$ (3% Bi/Sb incorporation).
• Figure 4: Excitation dependent emission of Cs$_2$TiCl$_6$ and Cs$_2$TiCl$_6$ (3% Sb/Bi incorporated) systems
• Figure 5: Ambient FTIR measurement