Copper Hyper-Stoichiometry: The Key for the Optimization of Thermoelectric Properties in Stannoidite Cu_8+xFe_3–xSn₂S₁₂

Abstract

A univalent copper hyper-stoichiometric stannoidite Cu8+x Fe3–x Sn2S12 with 0 ≤ x ≤ 0.5 has been synthesized using mechanical alloying followed by spark plasma sintering. The X-ray diffraction analysis combined with 57Fe and 119Sn Mössbauer investigations has allowed the charge distribution of the cationic species on the various sites to be established and suggests the possibility of a small tin deficiency. The transport properties show a remarkable crossover from a semiconducting to a metal-like behavior as the copper content increases from x = 0 to x = 0.5, whereas correlatively the Seebeck coefficient decreases moderately, with S values ranging from 310 to 100 μV/K. The thermal conductivity decreases as the temperature increases showing low values at high temperature, far below those reported in related stannite materials. The investigation of the thermoelectric properties shows that the ZT figure of merit is dramatically enhanced by the copper hyper-stoichiometry by a factor of 5 going from 0.07 for x = 0 to 0.35 for x = 0.5 at 630 K. This thermoelectric behavior is interpreted on the basis of a model involving the Cu–S framework as the conducting electronic network where the Fe2+/Fe3+ species play the role of hole reservoir.

Chalcogenide Quaternary Cu2FeSnS4 Nanocrystals for Solar Cells: Explosive Character of Mechanochemical Synthesis and Environmental Challenge

“…The mechanochemical synthesis as a version of solid-state approach has been applied for various materials [13][14][15][16][17][18][19][20]. The combination of mechanical alloying and spark plasma sintering has documented the possibility to synthesize high-performance thermoelectric materials with the possibility to scale the synthesis process.…”

Copper Sulfides: Earth‐Abundant and Low‐Cost Thermoelectric Materials

“…Stannoidite (Cu 8 Fe 3 Sn 2 S 12 ) was also one of the natural minerals, and its thermoelectric properties were recently studied by Pavan Kumar et al The samples were prepared by a mechanical alloying followed by SPS. A series of samples with more copper stoichiometry were prepared to have compositions of the form of Cu 8+ x Fe 3− x Sn 2 S 12 with varying x from 0 to 0.5.…”

“…The composition‐dependent zT values of Cu 8+ x Fe 3− x Sn 2 S 12 as a function of temperature. Reproduced with permission . Copyright 2017, American Chemical Society. …”

Structure, microstructure and thermoelectric properties of germanite-type Cu22Fe8Ge4S32 compounds

“…[2] Beyond the promising results obtained for telluride-based materials which exhibit high performances, [3][4][5] the need to conciliate efficiency with environmental and cost constraints has triggered research toward copper-based sulfides, which, for most of them, benefit from containing eco-friendly and abundant elements. Tetrahedrite Cu 12 Sb 4 S 13 , [6][7][8][9][10][11][12][13][14][15] colusite Cu 26 V 2 Sn 6 S 32 , [16][17][18][19][20][21][22][23] stannoidite Cu 8 Fe 3 Sn 2 S 12 , [24][25][26] and bornite Cu 5 FeS 4 [27][28][29][30] are a few of the naturally occurring Cu-based sulfide minerals that have been reported to date for their promising thermoelectric properties.…”