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Instruments / Technical Area	Publication Details	Published
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
I15-1-X-ray Pair Distribution Function (XPDF)	Local structure and influence of Sb substitution on the structure–transport properties in AgBiSe2 Tim Bernges , Jan Peilstöcker , Moniak Dutta , Saneyuki Ohno , Kanishka Biswas , Wolfgang Zeier Inorganic Chemistry DOI: 10.1021/acs.inorgchem.9b00874 Diamond Proposal Number(s): [21273] Show Abstract ▼ Abstract: Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I–V–VI2 family, and especially AgBiSe2, have recently attracted interest as promising thermoelectric materials. However, further investigations are needed in order to develop a more fundamental understanding of the origin of the low thermal conductivity in AgBiSe2, to evaluate possible stereochemical activity of the 6s2 lone pair of Bi3+, and to further elaborate on chemical design approaches for influencing the occurring phase transitions. In this work, a combination of temperature-dependent X-ray diffraction, Rietveld refinements of laboratory X-ray diffraction data, and pair distribution function analyses of synchrotron X-ray diffraction data is used to tackle the influence of Sb substitution within AgBi1–xSbxSe2 (0 ⩽ x ⩽ 0.15) on the phase transitions, local distortions, and off-centering of the structure. This work shows that, similar to other lone-pair-containing materials, local off-centering and distortions can be found in AgBiSe2. Furthermore, electronic and thermal transport measurements, in combination with the modeling of point-defect scattering, highlight the importance of structural characterizations toward understanding changes induced by elemental substitutions. This work provides new insights into the structure–transport correlations of the thermoelectric AgBiSe2.	Jun 2019
I15-1-X-ray Pair Distribution Function (XPDF)	Origin of ultra-low thermal conductivity in n-type cubic bulk AgBiS2: Soft Ag vibrations and local structural distortion induced by Bi 6s2 lone pair Ekashmi Rathore , Rinkle Juneja , Sean P. Culver , Nicolὸ Minafra , Abhishek K. Singh , Wolfgang Zeier , Kanishka Biswas Chemistry Of Materials DOI: 10.1021/acs.chemmater.9b00001 Diamond Proposal Number(s): [17257] Show Abstract ▼ Abstract: Crystalline materials with ultralow thermal conductivity are essential for thermal barrier coating and thermoelectric energy conversion. Nontoxic n-type bulk cubic AgBiS2 exhibits exceptionally low lattice thermal conductivity (κlat) of 0.68-0.48 W/mK in the temperature range 298-820 K, which is near to the theoretical minimum (κmin). The low κlat is attributed to soft vibrations of predominantly Ag atoms and significant lattice anharmonicity due to local structural distortions along the [011] direction, arising due to the stereochemical activity of the 6s2 lone pair of Bi, as suggested by pair distribution function (PDF) analysis of the synchrotron X-ray scattering data. Low temperature heat capacity of AgBiS2 shows a broad hump due to the Ag-induced low energy Einstein modes as also suggested from phonon dispersion calculated by first principle density functional theory (DFT). Low energy optical phonons contributed by Ag and Bi strongly scatter heat carrying acoustic phonons, thereby decreasing the κlat to a low value. A maximum thermoelectric figure of merit of ~0.7 is attained at 820 K for bulk spark plasma sintered n-type AgBiS2.	Mar 2019

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