Strain Effects on the Electronic and Thermoelectric Properties of n(PbTe)-m(Bi ₂Te ₃) System Compounds

Abstract

Owing to their low lattice thermal conductivity, many compounds of the n(PbTe)-m(Bi ${}_2$ Te ${}_3$ ) homologous series have been reported in the literature with thermoelectric (TE) properties that still need improvement. For this purpose, in this work, we have implemented the band engineering approach by applying biaxial tensile and compressive strains using the density functional theory (DFT) on various compounds of this series, namely Bi ${}_2$ Te ${}_3$ , PbBi ${}_2$ Te ${}_4$ , PbBi ${}_4$ Te ${}_7$ and Pb ${}_2$ Bi ${}_2$ Te ${}_5$ . All the fully relaxed Bi ${}_2$ Te ${}_3$ , PbBi ${}_2$ Te ${}_4$ , PbBi ${}_4$ Te ${}_7$ and Pb ${}_2$ Bi ${}_2$ Te ${}_5$ compounds are narrow band-gap semiconductors. When applying strains, a semiconductor-to-metal transition occurs for all the compounds. Within the range of open-gap, the electrical conductivity decreases as the compressive strain increases. We also found that compressive strains cause larger Seebeck coefficients than tensile ones, with the maximum Seebeck coefficient being located at −2%, −6%, −3% and 0% strain for p-type Bi ${}_2$ Te ${}_3$ , PbBi ${}_2$ Te ${}_4$ , PbBi ${}_4$ Te ${}_7$ and Pb ${}_2$ Bi ${}_2$ Te ${}_5$ , respectively. The use of the quantum theory of atoms in molecules (QTAIM) as a complementary tool has shown that the van der Waals interactions located between the structure slabs evolve with strains as well as the topological properties of Bi ${}_2$ Te ${}_3$ and PbBi ${}_2$ Te ${}_4$ . This study shows that the TE performance of the n(PbTe)-m(Bi ${}_2$ Te ${}_3$ ) compounds is modified under strains.