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

 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, and .

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)

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