I15-1-X-ray Pair Distribution Function (XPDF)
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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
Diamond Proposal Number(s):
[30162]
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.
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Oct 2022
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John
Irvine
,
Jennifer
Rupp
,
Gang
Liu
,
Xiaoxiang
Xu
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Sossina M
Haile
,
Xin
Qian
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Alem
Snyder
,
Robert
Freer
,
Dursun
Ekren
,
Stephen
Skinner
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Ozden
Celikbilek
,
Shigang
Chen
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Shanwen
Tao
,
Tae Ho
Shin
,
Ryan
O'Hayre
,
Jake
Huang
,
Chuancheng
Duan
,
Meagan
Papac
,
Shuangbin
Li
,
Andrea
Russell
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Veronica
Celorrio
,
Brian
Hayden
,
Hugo
Nolan
,
Xiubing
Huang
,
Ge
Wang
,
Ian
Metcalfe
,
Dragos
Neagu
,
Susana Garcia
Martin
Open Access
Abstract: Inorganic perovskites exhibit many important physical properties such as ferroelectricity, magnetoresistance and superconductivity as well their importance as Energy Materials. Many of the most important energy materials are inorganic perovskites and find application in batteries, fuel cells, photocatalysts, catalysis, thermoelectrics and solar thermal. In all these applications, perovskite oxides, or their derivatives offer highly competitive performance, often state of the art and so tend to dominate research into energy material. In the following sections, we review these functionalities in turn seeking to facilitate the interchange of ideas between domains. The potential for improvement is explored and we highlight the importance of both detailed modelling and in situ and operando studies in taking these materials forward.
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May 2021
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I11-High Resolution Powder Diffraction
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Abstract: Ga2O3(ZnO)m (m = integer) homologous compounds are naturally occurring nanostructured materials. Their intrinsically low thermal conductivity makes them attractive for thermoelectric applications. High density Ga2O3(ZnO)m (m = 9, 11, 13, and 15) single phase ceramics were prepared by solid-state reaction. Nano-sized, twin-like V-shaped boundaries parallel to b-axis (apex angle ∼ 60°) were observed for all compositions. Atomic resolution Z-contrast imaging and EDS analysis for m = 15 showed segregation of Ga ions at the interface of V-shaped twin boundaries. Thermal and charge transport properties depend on the value of m. Compositions with m = 9 exhibited very low lattice thermal conductivity of 2 to 1.5 W/m.K at 300 K to 900 K; compositions with m=15 showed improved power factor of 140 µW/m. K2 at 900 K leading to a thermoelectric figure of merit (ZT value) of 0.055. This study explores the structural variants and routes to improve the thermoelectric properties of these materials.
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Jul 2020
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I11-High Resolution Powder Diffraction
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Open Access
Abstract: Tungsten bronze (TB) structured materials have attracted attention as possible thermoelectrics because of their complex crystal structure. In this work, a new thermoelectric ceramic with a tetragonal tungsten bronze (TB) structure, Ba6Ti2Nb8O30 (BTN), was prepared by the conventional mixed oxide route with some samples processed by Spark Plasma Sintering (SPS). The addition of MnO enabled the fabrication of high density BTN ceramics at a low sintering temperature of 1580 K in air and by SPS. All samples were annealed in a reducing atmosphere after sintering. X-ray diffraction showed that Ba6Ti2Nb8O30 crystallizes with tetragonal symmetry (P4bm space group). High angle annular dark field-electron energy loss spectroscopy analysis confirmed the proposed crystal structure and provided exact elemental distributions in the lattice, showing higher concentrations of Ti in the 2b lattice sites compared to the 8d lattice sites. XPS showed the presence of two spin-orbit double peaks at 207.7 eV in the reduced BTN samples, confirming the presence of Nb4+ ions. By the use of a sintering aid and optimization of the processing parameters, the ceramics achieved a high power factor of 280 μW/m K2 at 873 K. The BTN ceramics showed phonon-glass-type thermal conduction behavior with a low thermal conductivity of 1.7–1.65 W/m K at 300–873 K. A thermoelectric figure of merit (ZT) of 0.14 was achieved at 873 K. This ZT value is comparable with results for many TB thermoelectrics. However, BTN has the advantage of much easier processing.
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Sep 2019
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I11-High Resolution Powder Diffraction
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Abstract: Donor-doped SrTiO3 ceramics are very promising n-type oxide thermoelectrics. We show that significant improvements in the thermoelectric power factor can be achieved by control of the nanostructure and microstructure. Using additions of B2O3 and ZrO2, high density, high quality Sr0.9Nd0.1TiO3 ceramics were synthesised by the mixed oxide route; samples were heat treated in a single step under reducing atmosphere at 1673 K. Synchrotron and electron diffraction studies revealed an I4/mcm tetragonal symmetry for all specimens. Microstructure development depended on the ZrO2 content; low level additions of ZrO2 (up to 0.3 wt%) led to a uniform grain size with transformation-induced sub-grain boundaries. HRTEM studies showed a high density of dislocations within the grains; the dislocations comprised (100) and (110) edge dislocations with Burger vectors of d(100) and d(110) respectively. Zr doping promoted atomic level homogenization and a uniform distribution of Nd and Sr in the lattice, inducing greatly enhanced carrier mobility. Transport property measurements showed a significant increase in the power factor, mainly resulting from the enhanced electrical conductivity while the Seebeck coefficients were unchanged. In optimised samples a power factor of 2.0 × 10−3 W m−1 K−2 was obtained at 500 K. This is an ∼30% improvement compared to the highest values reported for SrTiO3-based ceramics. The highest ZT value for Sr0.9Nd0.1TiO3 was 0.37 at 1015 K. This paper demonstrates the critical importance of controlling the structure at the atomic level and the effectiveness of minor dopants in enhancing the thermoelectric response.
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Nov 2018
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I11-High Resolution Powder Diffraction
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Open Access
Abstract: The Ga2O3(ZnO)m family of homologous compounds have been identified as potential thermoelectric materials, but properties are often limited due to low densification. By use of B2O3 as an effective liquid phase sintering aid, high density, high quality ceramic samples of Ga2O3(ZnO)9 have been synthesized. The atomic structure and local chemical composition of Ga2O3(ZnO)9 have been determined by means of high resolution X-ray diffraction and atomic resolution STEM-HAADF, EDS and EELS measurements. X-ray analysis showed that the compound crystalizes in the Cmcm orthorhombic symmetry. Atomically resolved HAADF-STEM images unambiguously showed the presence of nano-sized, wedge-shaped twin boundaries, parallel to the b-axis. These nano-scale structural features were chemically investigated, for the first time, revealing the exact distributions of Zn and Ga; it was found that Ga ions occupy sites at the junction of twin boundaries and inversion boundaries. HAADF-EDS analysis showed that the calcination step has a significant impact on crystal structure homogeneity. By use of a sintering aid and optimization of processing parameters the ceramics achieved a low thermal conductivity of 1.5–2.2 W/m.K (for the temperature range 300–900 K), a power factor of 40–90 μW/K.m2, leading to a ZT of 0.06 at 900 K. The work shows a route to exploit nanoscale interface features to reduce the thermal conductivity and thereby enhance the thermoelectric figure of merit in complex thermoelectric materials.
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May 2018
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I11-High Resolution Powder Diffraction
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Abstract: [Bi0.87SrO2]2[CoO2]1.82 (BSCO) is one of the best p-type thermoelectric oxides but its structural and electronic properties are still poorly understood. BSCO is a misfit-layered compound consisting of an incommensurate stacking of hexagonal CoO2 and double rock-salt BiSrO2 layers. Here we combine experimental and computational approaches to investigate its crystallographic and electronic structure as well as thermoelectric transport properties. Considering different approximations for the subsystems stacking, we present a structural model that agrees well with both bulk and atomic-scale experimental data. This model, which suggests a level of Bi deficiency in the rock-salt layers, is then used to discuss the material’s electronic, magnetic, and transport properties. We show that Bi deficiency leads to a band gap opening and increases p-type electronic conductivity due to the formation of Co4+ species that serve as itinerant holes within the predominantly Co3+ framework of the CoO2 layer. We validate these predictions using electron energy loss spectroscopy in the scanning transmission electron microscope. The relationship between the hole-doping mechanism and the changes of the local structure (in particular the level of Bi deficiency) is evaluated. The reliability of the simulations is supported by the calculated temperature dependence of the Seebeck coefficient, in good agreement with experimental measurements.
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Oct 2016
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I11-High Resolution Powder Diffraction
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Feridoon
Azough
,
Robert
Cernik
,
Bernhard
Schaffer
,
Demie
Kepaptsoglou
,
Quentin Mathieu
Ramasse
,
Marco
Bigatti
,
Amir
Ali
,
Ian
Maclaren
,
Juri
Barthel
,
Marco
Molinari
,
Jakub Dominik
Baran
,
Stephen Charles
Parker
,
Robert
Freer
Abstract: We investigated the structure of the tungsten bronze barium neodymium titanates Ba6–3nNd8+2nTi18O54, which are exploited as microwave dielectric ceramics. They form a complex nanostructure, which resembles a nanofilm with stacking layers of ∼12 Å thickness. The synthesized samples of Ba6–3nNd8+2nTi18O54 (n = 0, 0.3, 0.4, 0.5) are characterized by pentagonal and tetragonal columns, where the A cations are distributed in three symmetrically inequivalent sites. Synchrotron X-ray diffraction and electron energy loss spectroscopy allowed for quantitative analysis of the site occupancy, which determines the defect distribution. This is corroborated by density functional theory calculations. Pentagonal columns are dominated by Ba, and tetragonal columns are dominated by Nd, although specific Nd sites exhibit significant concentrations of Ba. The data indicated significant elongation of the Ba columns in the pentagonal positions and of the Nd columns in tetragonal positions involving a zigzag arrangement of atoms along the b lattice direction. We found that the preferred Ba substitution occurs at Nd[3]/[4] followed by Nd[2] and Nd[1]/[5] sites, which is significantly different to that proposed in earlier studies. Our results on the Ba6–3nNd8+2nTi18O54 “perovskite” superstructure and its defect distribution are particularly valuable in those applications where the optimization of material properties of oxides is imperative; these include not only microwave ceramics but also thermoelectric materials, where the nanostructure and the distribution of the dopants will reduce the thermal conductivity.
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Mar 2016
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I11-High Resolution Powder Diffraction
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Abstract: A-site deficient Nd2/3TiO3 ceramics stabilized with CaTiO3, with an overall composition of 0.9 Nd2/3TiO3–0.1 CaTiO3, were synthesized by the mixed oxide route. Synchrotron X-ray diffraction was used to identify the basic perovskite structure and revealed cross-type superlattice reflections. An incommensurate superlattice structure with dimensions of a ≈ b ≈ 20ap and c = 2ap (where ap is the cell parameter for the parent perovskite phase) was identified, giving rise to contrast features resembling a nanochessboard pattern in electron microscopy images. The superlattice was further characterized by aberration-corrected scanning transmission electron microscopy (STEM): atomically resolved lattice images were obtained along ⟨100⟩ orientations to visualize the A-site (Ca, Nd, and vacancies) and B-site (Ti) cation column intensities, in correlation with observations of the nanochessboard superlattice. Electron energy loss spectroscopy (EELS) was used to precisely determine the distribution of Nd and Ca across the structure, confirming the absence of long-range elemental segregation or phase separation across the nanochessboard superstructure. Closer inspection of the chemical maps in two orthogonal directions, however, suggests the presence of localized ordering of cations and vacancies. The chessboard pattern superlattice is thus likely to be caused by periodic octahedral tilt distortions of the O sublattice, possibly induced by these short-range chemical variations, as a result of a complex interplay between cation and vacancy ordering in three dimensions.
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Jan 2015
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I11-High Resolution Powder Diffraction
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Abstract: Very high resolution powder diffraction structural studies of the potentially room temperature multiferroic Ga2-xFexO3 solid solution series (x = 0.7-1.3, 0.1 steps) have been undertaken using a fixed angle of incidence geometry. The applied absorption correction was seen to improve the goodness of fit ([chi]2) of the Rietveld refinements from an average of 1.41 to 1.06. The correction also resulted in an increased mean isotropic displacement parameter from 0.5 to 0.65. The mean difference in the fractional coordinates of the atoms between the refined models from the corrected and uncorrected data was 0.0007 Å, compared with the mean fractional coordinate error of 0.0003 Å. It is concluded that the final crystal structures refined from the corrected and uncorrected data are not significantly different. The number of reflections in each data set was over 2700, and the average peak half-width was 0.018° with the data binned in 4 millidegree steps. The data quality allowed bond length and angle determinations of sufficiently high accuracy to measure significant metal site distortions to an average precision of ±0.007 Å. A lattice parameter nonlinearity was observed on either side of the x = 1 composition; this was attributed to local distortions, primarily of the Fe1 and Ga2 sites.
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Apr 2012
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