Thermal and Pressure-Dependent Lattice Dynamics of TlBiSe₂ and Its Chromium-Doped Variants

Topological insulator (TI) materials, which are conductive at the surface but insulate in bulk, have drawn significant attention in the past decade due to their fascinating properties and potential applications in spintronics, quantum computing, and topological superconductivity. Among three-dimensional TIs, thallium (Tl)-based III–V–VI2 chalcogenides stand out due to their simple electronic band structure near the Fermi level. The study of lattice dynamic properties is crucial for the practical application of any material. In this work, we report the synthesis and lattice dynamics of TlBiSe2 and Cr-doped TlBiSe2. The long- and short-range ordering of the materials was investigated upon Cr doping by powder X-ray diffraction (XRD), X-ray absorption spectra (XAS), and Raman scattering. Temperature-dependent XRD (123–500 K) and Raman scattering (100–500 K), as well as pressure-dependent XRD up to 15 GPa, were carried out to understand lattice dynamics. Both pristine TlBiSe2 and Cr0.06TlBi0.94Se2 show structural stability across the entire temperature and pressure ranges. However, long-range ordering in Cr0.02TlBi0.98Se2 changes above 300 K. Additionally, Cr0.02TlBi0.98Se2 undergoes a monoclinic phase transition at a lower pressure (∼5.0 GPa) compared to that of pristine TlBiSe2 (∼6.8 GPa). This anomalous behavior regarding local structural distortion in Se atoms upon Cr doping at the Bi atomic site is understood.