Improvement in the thermoelectric performance of the crystals of halogen-substituted In ₄Se _3-xH _0.03 (H = F, Cl, Br, I): Effect of halogen-substitution on the thermoelectric properties in In ₄Se _3-x

We explored the thermoelectric properties of the crystals of halogen-substituted In ₄Se _3-xH _0.03 in an effort to understand the significant effects of halogen-substitution on both electrical and thermal transport properties of In ₄Se _3-x crystals as well as the origin of the high thermoelectric performance over a wide temperature range in the chlorine-substitued crystal. The X-ray diffraction patterns and typical infrared absorption spectra of the crystals of In ₄Se _3-xH _0.03 exhibit preferred oriented ac- or bc-planes of crystals with energy band gaps between 0.62 and 0.63 eV. The chlorine, bromine, and iodine-substituted In ₄Se _3-xH _0.03 crystals exhibit significantly higher room temperature electrical conductivities than the unsubstituted and fluorine-substituted crystals. Except for fluorine, the other halogen-substituted in the In ₄Se _3-xH _0.03 crystals show electron concentrations as comparable as the unsubstituted crystals. Thus, the substantial increase in electrical conductivity of the halogen-substituted crystals should result from a remarkable increase in Hall mobility. It is quite notable that the room temperature power factors of the halogen-substituted (such as Cl, Br, and I) crystals are significantly higher than that of the unsubstituted and F-substituted crystals, which is mainly due to the substantial increase in room temperature electrical conductivity. Finally, a relatively low lattice thermal conductivity combined with a high power factor results in a high ZT of ∼1.0 at ∼660 K for the crystal of In ₄Se _2.32I _0.03.