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

Abstract

The crystal chemistry of complex structured pavonite homologue Cu x+y Bi 5−y S 8 (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 Cu x+y Bi 5−y S 8 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 Cu x+y Bi 5−y S 8 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 Cu 1.576 Zn 0.024 Bi 4.6 S 8 , which showed ∼30% enhancement over that of the pristine Cu x+y Bi 5−y S 8 compound.

High Thermoelectric Performance in Electron-Doped AgBi3S5 with Ultralow Thermal Conductivity

“…The thermoelectric performance of pristine AgBi 3 S 5 is quite low, with the maximum ZT ( ZT max ) of only ∼0.3 at 750 K. Cl doping at S sites significantly improves the ZT value of AgBi 3 S 5 , where a ZT max of ∼1 at 800 K is achieved in the x = 0.33% sample. As shown in Figure d, this value is higher than any other n-type eco-friendly metal sulfides − investigated so far, making AgBi 3 S 5 a competitive candidate for further investigations and possible midtemperature thermoelectric power generation when nontoxic materials are required.…”

“…(a) Thermal properties as a function of temperature for AgBi 3 (S 1– x Cl x ) 5 . (b) Comparison of lattice thermal conductivities between different metal sulfides. ,,− (c) Temperature dependence of ZT values for AgBi 3 (S 1– x Cl x ) 5 . (d) Comparison of ZT values between different n-type toxic-free metal sulfides. − …”

“…Lead or tin chalcogenides and their alloys with other chalcogenides have been the archetypal of advanced thermoelectric materials, not only because of their low thermal conductivities but also due to their complex electronic structures that allow for excellent electrical properties. − However, due to the perceived toxicity of Pb and/or the scarcity of Te and Se elements, scientists are searching for new compounds, especially those eco-friendly metal sulfides, as potential substitutes for IV–VI semiconductors. ,,− For example, Biswas et al reported a ZT value of ∼0.6 at 773 K in the n-type Cl-doped Bi 2 S 3 . Liu et al synthesized a nanostructured CoSbS compound by ball milling and obtained a maximum ZT of ∼0.5 at 873 K in the n-type Ni-doped sample .…”

“…Guin et al reported that AgBiS 2 is an interesting thermoelectric material with extremely low thermal conductivities ranging between 0.4 and 0.6 Wm –1 K –1 from 300 to 823 K, though the pristine sample displays a relatively small n-type ZT < 0.2 at 823 K . Cu–Bi–S ternary pavonite, reported by Ahn et al, is another narrow gap n-type semiconductor having an intrinsically low thermal conductivity of ∼0.5 Wm –1 K –1 that is almost temperature independent. Through doping and elemental alloying, the optimized ZT value of Cu–Bi–S compound reaches ∼0.3 at 673 K. Another successful example was given by Guilmeau et al, who reported that the n-type copper-intercalated TiS 2 layered compound displays a promising ZT of 0.45 at 800 K because of the concurrent optimization of electrical and thermal properties .…”

Phase‐Controlled Synthesis of High‐Bi‐Ratio Ternary Sulfide Nanocrystals of Cu1.57Bi4.57S8 and Cu2.93Bi4.89S9

“…Compounds composed of Cu−Bi−S have been reported with various phases according to their compositions, such as Cu 9 BiS 6 , Cu 3 BiS 3 ,22 CuBiS 2 , Cu 4 Bi 4 S 9 ,, and a theoretical phase of Cu 4 Bi 5 S 10 . Although some of phases of high Bi ratio Cu−Bi−S have been reported previously, such as CuBi 5 S 8 and its derivatives,, however the synthetic method for these compounds needs a tedious vacuum‐sealed high temperature melted flux process. Generally, ternary Cu−Bi−S sulfides could be synthesized with reactive sintering method, solvothermal method, polyol route, chemical bath deposition method,, vacuum‐sealed solid state reaction, and colloidal hot‐injection method ,.…”