Page 35 - ILL Annual Report 2019

SCIENTIFIC HIGHLIGHTS
32-33
responsible for the sharp reduction in the lattice thermal conductivity of this material. This leads to a material with the ZT value reaching unity. However, the power factor of colusite remains modest (figure 1) and significantly lower than those of state-of-the-art, intermetallic thermoelectric compounds [2]. The objective of our study was to optimise the power factor by substituting the pentavalent V5+ cations with hexavalent ones (i.e. Cr6+, Mo6+ and W6+) and
to understand the impact of the crystal structure on the thermoelectric properties of colusite.
In this context, high-purity polycrystalline Cu26T2Ge6S32
(T = V, Cr, Mo, W) samples were synthesised by combining mechanical alloying and spark plasma sintering. The crystal structures of these synthetic colusites (P43n space group) were studied using combined Rietveld refinements of X-ray powder diffraction and high-resolution neutron powder diffraction data recorded on D2B with
a wavelength of 1.594 Å (figure 2). From these
results we have shown that the ordered ‘Cu26Ge6S32’ sphalerite-derivative framework of colusite, constructed of corner-sharing GeS4 and CuS4 tetrahedra (figure 3a), can accommodate hexavalent d0 cations at ‘interstitial’ T position, leading to TS4 (T = Cr, Mo, W) tetrahedra sharing edges with CuS4 tetrahedra (Figure 3b), a structural feature rarely observed in sulphides.
Moreover, we have demonstrated that the nature of the ‘interstitial’ cation can drastically modify the electronic properties to reach high power factors (figure 1).
The maximum value of 1.94 mW m-1 K-2 at 700 K for Cu26Cr2Ge6S32 is comparable with those of the best state-of-the-art thermoelectric materials [2]. Thanks to combined structural analyses and electronic structure and transport calculations, we have shown that the electronic transport properties of the conductive ‘Cu26S32’ framework (figure 3c) are governed by the presence of mixed tetrahedral-octahedral [TS4]Cu6 complexes in the colusite structure (figure 3d). In such complexes, the T cations are underbonded to sulphur and form metal–metal interactions with copper. In the case of Cr-colusite, the smaller size and lower electronegativity of hexavalent chromium cations introduces T-Cu interactions that limit the distortion of the
Figure 2
Combined Rietveld refinement of the high-resolution NPD (top) and XRPD (bottom) patterns of the Cu26Cr2Ge6S32 sample recorded at room temperature. Second sets of diffraction peaks on the NPD pattern are related to the vanadium sample-holder contribution.
conductive ‘Cu26S32’ network and the perturbation of the electronic properties. The key role of the chemical bonds at the core of the mixed tetrahedral–octahedral complex over the transport properties was recently confirmed in Cu26Cr2-xMoxGe6S32 and Cu26Cr2-xWxGe6S32 solid solutions [4].
Finally, the nature of the ‘interstitial’ cation does not influence the thermal conductivity of these synthetic colusites. It leads to a dimensionless figure of merit ZT up to 0.86 at 700 K for un-optimised pristine Cu26Cr2Ge6S32, and an average ZT near 0.5 over the full temperature range. The latter is, to the best of our knowledge, the highest average ZT for pristine sulphide materials without specific optimisation.
Figure 3
Representation of the a) ordered sphalerite framework ‘Cu26Ge6S32’ resulting from the omission of the T(2a) atoms; b) Cu26T2Ge6S32 colusite structure;
c) conductive ‘Cu26S32’ framework (i.e. ordered sphalerite framework by simply omitting the Ge atoms); and d) mixed tetrahedral–octahedral complex [TS4]Cu6—the Cu, T, Ge, and S atoms are depicted in red, green, blue and yellow, respectively.
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