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Local structural distortions and reduced thermal conductivity in Ge-substituted chalcopyrite

DOI: 10.1039/D2TA06443J DOI Help

Authors: Sahil Tippireddy (University of Reading) , Feridoon Azough (University of Manchester) , Vikram Vikram (University of Reading) , Animesh Bhui (Jawaharlal Nehru Centre for Advanced Scientific Research) , Philip Chater (Diamond Light Source) , Demie Kepaptsoglou (SuperSTEM Laboratory; University of York) , Quentin Ramasse (SuperSTEM Laboratory; University of Leeds) , Robert Freer (University of Manchester) , Ricardo Grau-Crespo (University of Reading) , Kanishka Biswas (Jawaharlal Nehru Centre for Advanced Scientific Research) , Paz Vaqueiro (University of Reading) , Anthony V. Powell (University of Reading)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of Materials Chemistry A , VOL 6

State: Published (Approved)
Published: October 2022
Diamond Proposal Number(s): 30162

Open Access Open Access

Abstract: Chalcopyrite, CuFeS2 is considered one of the promising n-type thermoelectric materials with high natural abundance as a mineral. In this work, partial substitution of germanium in materials CuFe1−xGexS2, (0.0 ≤ x ≤ 0.10), leads to an almost six-fold enhancement of thermoelectric properties. X-Ray photoelectron spectroscopy (XPS) reveals that germanium is present in two oxidation states: Ge2+ and Ge4+. The stereochemically-active 4s2 lone-pair of electrons associated with Ge2+ induces a local structural distortion. Pair-distribution function (PDF) analysis reveal that Ge2+ ions are displaced from the centre of the GeS4 tetrahedron towards a triangular face, leading to pseudo-trigonal pyramidal coordination. This distortion is accompanied by lattice softening and an increase of the strain-fluctuation scattering parameter (ΓS), leading to a decrease in thermal conductivity. Phonon calculations demonstrate that germanium substitution leads to the appearance of resonant phonon modes. These modes lie close in energy to the acoustic and low-energy optical modes of the host matrix, with which they can interact, providing an additional mechanism for reducing the thermal conductivity. The weak chemical bonding of germanium with sulphur also leads to localized electronic states near the Fermi level which results in a high density-of-states effective mass, enabling a relatively high Seebeck coefficient to be maintained, despite the reduced electrical resistivity. This combination produces an almost three-fold improvement in the power factor, which when coupled with the substantial reduction in thermal conductivity, leads to a maximum figure-of-merit, zT ∼ 0.4 at 723 K for CuFe0.94Ge0.06S2.

Subject Areas: Materials, Chemistry, Energy

Instruments: I15-1-X-ray Pair Distribution Function (XPDF)

Added On: 03/11/2022 10:06


Discipline Tags:

Quantum Materials Energy Physical Chemistry Energy Materials Chemistry Materials Science Thermoelectrics Inorganic Chemistry

Technical Tags:

Diffraction X-ray Powder Diffraction