Vibrational spectra of Pb2Bi2Te3, PbBi2Te4, and PbBi4Te7 topological insulators: temperature-dependent Raman and theoretical insights from DFT simulations

Abstract

Herein, using Raman spectroscopy, we have presented the investigation of a temperature-dependent frequency shift and the line broadening of phonon modes by inserting the atomic layers of Pb and PbTe in the prototype 3D topological insulator Bi₂Te₃. Good quality single crystals of Pb₂Bi₂Te₃, PbBi₂Te₄, and PbBi₄Te₇ were grown using the modified Bridgman technique. The Raman modes show progressive blue-shift with the decrease in temperature from 298 K to 93 K in Pb₂Bi₂Te₃, PbBi₂Te₄, and PbBi₄Te₇ due to the anharmonic vibrations of the lattice as well as the increase in the strength of Bi–Te covalent interactions. The experimental results were complemented by extensive first principles calculations, where a reasonable matching between the experimental and computational data was found. Chemical pressure, induced by the insertion of Pb and PbTe layers in Bi₂Te₃, modified the interactions at the boundaries of the quintuple-layers, which was evident from the evolution of the A²_1u mode. The enhancement in the out-of-plane Bi–Te vibrations with respect to the in-plane Bi–Te vibrations was observed at low temperatures. The temperature coefficients of the Raman modes were useful in determining the thermal conductivity, which is a key design parameter for the fabrication of spintronic devices using topological insulators. The estimated first order temperature coefficient (χ′) for Pb₂Bi₂Te₃ signified the decrease in the thermal conductivity relative to Bi₂Te₃, which was caused by the insertion of the Pb layers in the van der Waals gaps of Bi₂Te₃.