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

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 A21u 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.