Page 63 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
60-61
The purity and crystallinity of the samples, as well as
the crystal structure of colusite—P4–3n, sulphur corner- sharing tetrahedral network (8e, and 24i) centred around Cu (6d, 8e, and 12f) and Sn (6c), with V atoms at interstitial position (2a), (inset of figure 1, [3, 4])— have been confirmed by combined Rietveld refinement of X-ray powder diffraction data and neutron powder diffraction data recorded on D1B with a wavelength
of 1.28 Å (figure 1). These latter data were valuable in accurately determining the atomic co-ordinates of
the colusite sample sintered at 873 K, which were
used as starting values for theoretical calculations. Moreover, a careful analysis of the patterns pointed to some minor differences in the positions and intensities of the superstructure diffraction peaks between the two samples. Despite testing several structural models to refine the diffraction data of the colusite sample sintered at 1023 K, the crystal structure remained ambiguous, suggesting the existence of local structural defects in
this sample and requiring further investigations by local probe techniques.
Transmission electron microscopy revealed no defect regions or superstructures for the sample sintered at 873 K. Conversely, analysis of the sample sintered at 1023 K revealed different structural defects/features. The [001] images (figures 2a and 2b) clearly showed the presence of 1D line defects as a result
of Sn substitutions with V and/or Cu. Given the sizes of the tetrahedra (V < Cu < Sn) and the chemical composition, the Sn−Cu anti-sites are expected to be kinetically more probable; this is supported by total energy calculations indicating that the formation of Cu−Sn anti-sites involving Cu 6d and Cu 8e are energetically favoured. In addition, in some other areas we observed a clear tendency to form larger 3D disordered regions, explaining the ambiguous crystal structure resolution of this colusite sample
from powder diffraction data only. These disordered (figure 2c) and ordered (figure 2d) areas are coherently intergrown with equivalent structural frameworks and unit cell parameters, and are in fair agreement with the different domains of ordered and disordered cation distributions observed in natural colusite minerals [3]. These results suggest that sulphur volatilisation occurring at high temperature during hot-pressing at 1023 K favours a balance between
an entropy-governed disordered phase and an internal energy-governed ordered phase.
Figure 3
Temperature dependencies of a) lattice thermal conductivity (κL) and b) ZT values of colusite Cu26V2Sn6S32 samples sintered at 873 K and 1023 K, respectively.
Figure 2
a) [001] HAADF-STEM image of the sample sintered at 1023 K. Red colour indicates 1D line defects distributed in the perfect crystal of Cu26V2Sn6S32.
b) Enlargement part of line defect. Red circle depicts Sn-site defects columns. [111] HAADF-STEM images showing the coexistence of
c) disordered and d) ordered regions in the same crystallite in the sample sintered at 1023 K.
Finally, by combining experiments and calculations (electronic band structure and vibrational properties), we concluded that the significant decrease in lattice thermal conductivity in the sample sintered at 1023 K (figure 3a) can mainly be attributed to the scattering effects induced by cationic disorder, which scatter short-wavelength phonons. In addition, the ordered and disordered
regions being coherently intergrown, their interfaces can also contribute to the scattering of phonons of medium wavelengths, at the same time maintaining good carrier transport without excessive scattering. Recent inelastic neutron scattering experiments on IN6-SHARP have confirmed experimentally the role of structure disordering on phonon scattering in Cu26V2Sn6S32 colusites. This leads to a huge enhancement of the figure of merit ZT, up to 0.93 at 675 K in the sample sintered at 1023 K (figure 3b).
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