Importance of crystal chemistry with interstitial site determining thermoelectric transport properties in pavonite homologue Cu–Bi–S compounds

The crystal chemistry of complex structured pavonite homologue Cux+yBi5−yS8 (1.2 ≤ x ≤ 1.4, 0.4 ≤ y ≤ 0.55) compounds with various crystallographic atomic sites was investigated in the context of their thermoelectric properties. We clarified the origins of the electronic and thermal transport properties of Cux+yBi5−yS8 compounds based on the change in the composition, which is strongly correlated with the occupancy of each atomic site. Ab initio calculations revealed that the narrow gap n-type semiconducting nature of Cux+yBi5−yS8 compounds originates from the presence of interstitial Cu ions. Structural refinements combined with transport measurements revealed that asymmetrical disorders of interstitial Cu ions have a large anisotropic thermal displacement factor, leading to an intrinsically low value (∼0.49 W m−1 K−1) and temperature-independent behavior of lattice thermal conductivity. Comprehensive structural analysis provided an elemental doping strategy focusing on interstitial sites. Thermoelectric properties were significantly enhanced by the simultaneous increase of power factor and decrease of lattice thermal conductivity. It is noted that structural factors, such as occupancy and thermal displacement parameter, of interstitial sites among the various crystallographic sites should be considered as primary characteristics in the crystal chemistry of complex structured crystals. Correspondingly, a peak ZT for the system was obtained in Cu1.576Zn0.024Bi4.6S8, which showed ∼30% enhancement over that of the pristine Cux+yBi5−yS8 compound.