Local structural distortions and reduced thermal conductivity in Ge-substituted chalcopyrite Sahil Tippireddy 1 , Feridoon Azough 2 , Vikram 1 , Animesh Bhui 3 , Philip Chater 4 , Demie Kepaptsoglou 5,6 , Quentin Ramasse 5,7 , Robert Freer 2 , Ricardo Grau-Crespo 1 , Kanishka Biswas 3 , Paz Vaqueiro 1 , Anthony V. Powell 1 1 University of Reading, UK, 2 University of Manchester, UK, 3 Jawaharlal Nehru Centre for Advanced Scientific Research, India, 4 Diamond Light Source, UK, 5 SuperSTEM Laboratory, UK, 6 University of York, UK, 7 University of Leeds, UK Chalcopyrite (CuFeS2) offers a rare example of a promising n-type sulfide for thermoelectric applications. It possesses a moderate power factor (S2/ρ) but a high lattice thermal conductivity (κL). Substitution with Ge in CuFe1-xGexS2 (0.02 ≤ x ≤ 0.1) results in reductions of up to 60 % in lattice thermal conductivity, κL, compared to that of the parent CuFeS2 phase. The maximum figure-of-merit, zT ≅ 0.4 achieved at 723 K represents a more than 6-fold increase over that of CuFeS2, and is one of the highest values obtained in the CuFeS2 system in this temperature range. Preliminary structural studies using X-ray diffraction and scanning electron microscopy (SEM) indicate the retention of the chalcopyrite structure on Ge substitution, with no evidence for secondary phase(s) or precipitates. Mass fluctuation scattering of phonons alone is not enough to explain the substantial reduction of κL in the substituted samples. Moreover, the absence of secondary phase(s)/precipitates and micro/ nanostructural features in the SEM data, suggest that the origin of enhanced phonon scattering arises from local geometrical changes due to Ge substitution in the CuFeS2 phase. Pair-distribution function (PDF) analysis of the X-ray total scattering dataindicate a displacement of Ge atoms within the GeS4 tetrahedra, which induces a local structural distortion and introduces asymmetry into the chemical environment, thereby affecting the phonon modes and phonon transport. This leads to lattice softening and enhanced strain-field fluctuation scattering, also evidenced by a significant reduction in the measured sound velocities, Debye temperature and elastic moduli. Our results provide new insights into the dynamical properties of these materials and further our understanding of the behaviour of the lattice thermal conductivity in substituted chalcopyrite phases.
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© The Author(s), 2022
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