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 Cs2TiCl6 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 M1 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 Cs2TiCl6. While these observations suggest possible structural changes at low temperature, the origin of the M1 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 Cs2TiCl6, though further investigation is required to establish the underlying mechanisms.
@article{arxiv.2512.21810,
title = {Temperature and Pressure Dependent Vibrational Properties of Pristine and Doped Vacancy-Ordered Double Perovskite},
author = {Aalok Tiwari and Karamjyoti Panigrahi and Mrinmay Sahu and Sayan Bhattacharyya and Goutam Dev Mukherjee},
journal= {arXiv preprint arXiv:2512.21810},
year = {2026}
}