Copper Hyper-Stoichiometry: the Key for the Optimization of Thermoelectric Properties in Stannoidite

Copper Hyper-Stoichiometry: The Key for the Optimization of Thermoelectric Properties in Stannoidite Cu 8+x Fe 3−xSn 2S12 V. Pavan Kumar, T. Barbier, V. Caignaert, B. Raveau, R. Daou, B. Malaman , G. Le Caër, P. Lemoine and E. Guilmeau [email protected] Journées Nationales de Thermoélectricité 2017, December 6-8 1 Introduction PbTe based – promising but toxic and expensive constituents Alternative – Copper based sulfides P-type n-type Cu/M≥1 Cu/M≤1 CuFe 1-xMxS2 (M = In, Zn) ( ZT = 0.22 ) Stannites (Cu ZnSnS ) ( ZT = 0.36 ) 2 4 CuFe 2S3 (ZT = 0.14 ) Tetrahedrites (Cu 12 Sb 4S13 ) ( ZT > 1.0 ) Bornite (Cu 5FeS 4)( ZT > 0.6 ) Cu-S conducting network is the key Colusite (Cu 26 T2X6S32 ; T= V, Nb, Ta & X= Sn, Ge) ( ZT > 0.9 ) Germanite (Cu 22 Fe 8Ge 4S32 ) ( ZT ~ 0.2 ) 2 Stannoidite Cu 8Fe 3Sn 2S12 Available as a natural mineral Superstructure of Stannite Edge-sharing CuS 4, SnS 4 and FeS 4 ordered tetrahedra Metallic atoms are distributed on seven different crystallographic sites. Stacking along the a axis of pure tetrahedral univalent copper layers alternating with mixed tetrahedral layers containing copper, tin and iron. The logical rationale for studying this material is supported by: -Derivative structure of stannite (ZT=0.36 @ 700 K)[1] -Larger number of atomic positions (point defects) -Pristine compound never synthesized in laboratory 3 [1] Liu, M. L.; Huang, F. Q.; Chen, L. D.; Chen, I. W.. Appl. Phys. Lett. 2009, 94 , 202103. Synthesis Process High energy ball-milling process: WC jar (20mL) and balls (7 x 10 mm) 600 rpm : 12 hours Stannite structure Broad peaks – not well crystallized 4 Stannoidite Cu 8Fe 3Sn 2S12 SPS conditions: WC dies - 150 Mpa under Vacuum After SPS 600°C 18’ 15’ 40’ RT RT Space group: I 222 Unit Cell: Orthorhombic a = 10.758 Å b = 5.397 Å c = 16.135 Å Superstructure diffraction peaks of the stannoidite Broad superstructure reflections indicate presence of atomic disorder or crystallographic defects. 5 V. Pavan Kumar et al. , J. Phys. Chem. C 121, 16454 (2017) Stannoidite Cu 8Fe 3Sn 2S12 Densely packed > 98 % Grain size of 150-200 nm. X=0.0 200 nm 6 Stannoidite Cu 8Fe 3Sn 2S12 119 Sn Mossbauer spectra (80 K) Isomer shift of 1.42 mm s -1 characteristic of tin atoms in a +IV oxidation state. Strongly covalent SnS 4 tetrahedron x = 0 7 V. Pavan Kumar et al. , J. Phys. Chem. C 121, 16454 (2017) Stannoidite Cu 8Fe 3Sn 2S12 57 Fe Mössbauer Spectroscopy Fe 2+ Fe 2+ Fe 3+ + 2+ 3+ 4+ 2- (Cu )8(Fe )(Fe )2(Sn )2(S )12 Outer doublet with large quadrupole splitting of 2.9 mm s -1 attributed to Fe 2+ Fe 3+ ions give rise to two different doublets. Site 2(a) is occupied by 1 Fe 3+ ion, while site 4(i) is occupied by 1 Fe 2+ and 1 Fe 3+ cations. [1] 8 [1] T. Yamanaka and T. Kato, Am. Mineral. 1976 , 61 , 260–265 Stannoidite Cu 8Fe 3Sn 2S12 + 2+ 3+ 4+ 2- (Cu )8+x(Fe )1-2x(Fe )2+x(Sn )2(S )12 Fe 2+ Fe 2+ Fe 3+ x = 0 x = 0.3 x = 0.5 Fe 2+ content decreases at the expense of the Fe 3+ content as expected when copper (Cu +) substitutes for iron. 9 V. Pavan Kumar et al. , J. Phys. Chem. C 121, 16454 (2017) Stannoidite Cu 8Fe 3Sn 2S12 Crossover from a semiconducting to a metal-like behavior Fe 3+/Fe 2+ species play the role of hole donors/acceptors (reservoirs) Cu 2+ + Fe 2+ ⇌ Cu + + Fe 3+ Increase of the copper concentration provides higher power factor 10 Stannoidite Cu 8Fe 3Sn 2S12 Kappa far below from those reported in related- stannite compounds (κL = 5 W/m K at RT in [1] Cu 2ZnSnS 4). Additional crystallographic sites favour phonon scattering. V. Pavan Kumar et al. , J. Phys. Chem. C 121, 16454 (2017) [1] Liu, M. L.; Huang, F. Q.; Chen, L. D.; Chen, I. W.. Appl. Phys. Lett. 2009, 94 , 202103. 11 Conclusions Stannoidite is prepared for the first time in the laboratory 57 Fe Mössbauer – Fe 2+ decreasing Semiconducting-metal crossover increase in PF Increase of Cu content Large number of atomic positions – low kappa Copper hyper stoichiometry – 5 fold improvement in ZT over pristine Maximum ZT of 0.35 at 630 K Very cheap constituents – with improvement in ZT- good TE material Future Perspectives: band structure and phonon calculations Callaway model studies are undergoing to understand κL 12 Acknowledgements Emmanuel Guilmeau Bernard Raveau Tristan Barbier Vincent Caignaert Pierric Lemoine (ISCR, Rennes) Bernard Malaman (IJL, Nancy) Gérard Le Caër (IPR, Rennes) 13.

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