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
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)
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