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Instruments / Technical Area	Publication Details	Published	Actions
I15-1-X-ray Pair Distribution Function (XPDF)	Local structural distortions and reduced thermal conductivity in Ge-substituted chalcopyrite Sahil Tippireddy , Feridoon Azough , Vikram Vikram , Animesh Bhui , Philip Chater , Demie Kepaptsoglou , Quentin Ramasse , Robert Freer , Ricardo Grau-Crespo , Kanishka Biswas , Paz Vaqueiro , Anthony V. Powell Journal Of Materials Chemistry A 6 DOI: 10.1039/D2TA06443J Diamond Proposal Number(s): [30162] Open Access Show Abstract ▼ 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.	Oct 2022
I11-High Resolution Powder Diffraction	Jahn–Teller driven electronic instability in thermoelectric tetrahedrite Sebastian O. Long , Anthony V. Powell , Stephen Hull , Fabio Orlandi , Chiu C. Tang , Andrew R. Supka , Marco Fornari , Paz Vaqueiro Advanced Functional Materials 73 DOI: 10.1002/adfm.201909409 Diamond Proposal Number(s): [20141] Show Abstract ▼ Abstract: Tetrahedrite, Cu12Sb4S13, is an abundant mineral with excellent thermoelectric properties owing to its low thermal conductivity. The electronic and structural origin of the intriguing physical properties of tetrahedrite, including its metal‐to‐semiconductor transition (MST), remains largely unknown. This work presents the first determination of the low‐temperature structure of tetrahedrite that accounts for its unique properties. Contrary to prior conjectures, the results show that the trigonal–planar copper cations remain in planar coordination below the MST. The atomic displacement parameters of the trigonal–planar copper cations, which have been linked to low thermal conductivity, increase by 200% above the MST. The phase transition is a consequence of the orbital degeneracy of the highest occupied 3d cluster orbitals of the copper clusters found in the cubic phase. This study reveals that a Jahn–Teller electronic instability leads to the formation of “molecular‐like” Cu57+ clusters and suppresses copper rattling vibrations due to the strengthening of direct copper–copper interactions. First principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes. These results provide insights on the interplay between phonon transport, electronic properties, and crystal structure in mixed‐valence compounds.	Feb 2020

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