Understanding of isoelectronic alloying induced energy gap variation for a large enhancement of thermoelectric power factor

Band -structure engineering is an important strategy that can improve the properties of functional materials or even bring new features to existing systems. Band gap (or energy gap, Eg ) plays a crucial role in deciding the electronic or optical properties of one material. Isoelectronic and isostructural alloys usually exhibit similar electronic band structures, but the related effect of Eg variation was found to be distinct and sometimes controversial. Herein, we provided a deep understanding of the origin of band tuning in isoelectronic alloys based on experimental characterizations and theoretical calculations. The prerequisites of an isoelectronic alloy system with an Eg that is sensitive to composition are thoroughly disclosed by studying (Sb , Bi) 2 Te 3 and Pb(Se,Te). In the promising Mg 3 (Sb , Bi) 2 thermoelectric materials, it is found that Bi isoelectronic substitution can significantly decrease the Eg , due to the combination of the higher energy of Bi 6 p orbit than that of Sb 5 p orbit and the varied bond strength induced by lattice expanding. A high room -temperature power factor ( PF ) value of 38 . 5 mu W cm - 1 K - 2 is obtained in nearly zero -gap Mg 3 . 2 Sb 0 . 3 Bi 1 . 7 samples, which rationally guides the design of thermoelectric materials in the aspect of electronic transport.