Thermal and pressure-dependent lattice dynamics of TlBiSe₂ and its chromium-doped variants

Abstract

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 Cr_0.06TlBi_0.94Se₂ show structural stability across the entire temperature and pressure ranges. However, long-range ordering in Cr_0.02TlBi_0.98Se₂ changes above 300 K. Additionally, Cr_0.02TlBi_0.98Se₂ 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.