I11-High Resolution Powder Diffraction
I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[26379]
Abstract: Na+closo-hydroborates are a heavily researched solid electrolyte class for applications in all-solid-state Na batteries. The structural characterization of these materials is notoriously challenging due to the elements involved and the fast rotational motion of hydroborate cages. The average structures obtained by Bragg diffraction have numerous atomic positions with low occupancies, complicating the determination of actual atom–atom distances. Total average scattering and derived pair distribution functions display atom–atom distances in real space, providing additional structure information to the average crystal structure. In this work, we present the pair distribution functions of the five different Na+closo-hydroborates: Na2B10H10, Na2B12H12, NaCB11H12, and the mixtures of 1:1 Na2B12H12/Na2B10H10 and 2:1 NaCB11H12/Na2B12H12. All pair distribution functions show a fast decay of peak height with increasing atom–atom distance on the local scale, suggesting a low correlation of atom motions between hydroborate cages, as observed in various other molecular crystals. The combination of Bragg diffraction, showing the average ordering of closo-hydroborate cages, and pair distribution function analysis, providing local atom–atom distances, is a useful tool to develop a deeper understanding of the closo-hydroborates and also of other plastic crystals.
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Jan 2023
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I15-Extreme Conditions
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Anna
Vogel
,
Alfred
Rabenbauer
,
Philipp
Deng
,
Ruben
Steib
,
Thorben
Böger
,
Wolfgang G.
Zeier
,
Renée
Siegel
,
Jürgen
Senker
,
Dominik
Daisenberger
,
Katharina
Nisi
,
Alexander W.
Holleitner
,
Janio
Venturini
,
Tom
Nilges
Diamond Proposal Number(s):
[30094]
Abstract: A diode or transistor requires the combination of p- and n-type semiconductors or at least the defined formation of such areas within a given compound. This is a prerequisite for any IT application, energy conversion technology, and electronic semiconductor devices. Since 2009, when the first pnp-switchable compound Ag10Te4Br3 was described, it is in principle possible to fabricate a diode from a single material without adjusting the semiconduction type by a defined doping level. After this discovery, a handful of other materials that are capable of reversibly switching between these two semiconducting stages was reported. In all cases, a structural phase transition accompanied by a dynamic change of charge carriers or a charge density wave (CDW) within certain substructures are responsible for this effect. Unfortunately, a certain feature hinders the application of this phenomenon in convenient devices, namely the pnp-switching temperature, which generally occurs well above room temperature, between 364 and 580 K. This effect is far removed from a suitable operation temperature at ambient conditions. Here, we report on Ag18Cu3Te11Cl3, a room temperature pnp-switching material, and the realization of the first single-material position-independent diode. The title compound shows the highest ever reported Seebeck coefficient drop that takes place within a few Kelvin at room temperature. Combined with its reasonably low thermal conductivity, this material offers great application potential within an easily accessible and applicable temperature window. Ag18Cu3Te11Cl3 and pnp-switching materials have the potential for applications and processes where diodes, transistors, or any defined charge separation with junction formation are utilized.
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Oct 2022
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[24332]
Abstract: Highly conductive solid electrolytes are fundamental for all solid-state batteries with low inner cell resistance. Such fast solid electrolytes are often found by systematic substitution experiments in which one atom is exchanged for another, and corresponding changes in ionic transport are monitored. With this strategy, compositions with the most promising transport properties can be identified fast and reliably. However, the substitution of one element does not only influence the crystal structure and diffusion channel size (static) but also the underlying bonding interactions and with it the vibrational properties of the lattice (dynamic). Since both static and dynamic properties influence the diffusion process, simple one-dimensional substitution series only provide limited insights to the importance of changes in the structure and lattice dynamics for the transport properties. To overcome these limitations, we make use of a two-dimensional substitution approach, investigating and comparing the four single-substitution series Na3P1–xSbxS4, Na3P1–xSbxSe4, Na3PS4–ySey, and Na3SbS4–ySey. Specifically, we find that the diffusion channel size represented by the distance between S/Se ions cannot explain the observed changes of activation barriers throughout the whole substitution system. Melting temperatures and the herein newly defined anharmonic bulk modulus─as descriptors for bonding interactions and corresponding lattice dynamics─correlate well with the activation barriers, highlighting the relevance of lattice softness for the ion transport in this class of fast ion conductors.
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Feb 2022
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[13560]
Abstract: Non-graphitic carbons (NGCs) represent the most abundant class of \(sp^2\)-hybridized carbon materials (coal, char coal, activated carbon, etc.). These carbons consist of small graphene layer stacks possessing significant structural disorder both in the single graphene sheets and the stacking. In this study an advanced evaluation approach for the wide-angle neutron scattering (WANS) was developed, based on the method introduced by Ruland and Smarsly (2002). In particular, we elucidated if and how the enhanced WANS data quality and larger values of the modulus of the scattering vector \(s\)-range affect the accuracy and the values of the size and disorder parameters - being fitting parameters by themselves - in comparison to wide-angle x-ray scattering (WAXS), which is usually performed by laboratory equipment. We find a reasonable agreement for the parameters \(L_\mathrm{a}\) and \(L_\mathrm{c}\), i.e. the lateral dimension and stack height, within the error bars, while for the disorder parameters different results for WAXS and WANS were found, the origin of which is discussed. Thus, this study addresses the general issue of how reliably microstructural parameters can be determined from WAXS/WANS, by fitting simulated WAXS and WANS curves, which are quality-impaired by added Gaussion noise at different levels and cut-off at different \(s\)-values. From this analysis we estimated the minimal data quality required for a reliable NGC microstructural analysis based on WAXS/WANS. As an important finding, these simulations show that typical, standard WAXS laboratory setups are sufficient to provide reliable values for the most relevant structural parameters. Furthermore, pair-distribution function (PDF) analyses were performed on WAXS data obtained from a Synchrotron facility. Comparing PDF and WAXS/WANS fitting analysis thus suggests the presence of small highly ordered oligoaromatic domains embedded in the larger graphene sheets, questioning the classical view on the NGC microstructure.
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Jul 2019
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[21273]
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.
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Jun 2019
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[17257]
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.
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Mar 2019
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[13560]
Abstract: Advanced lithium-ion batteries are of great interest for consumer electronics and electric vehicle applications, however they still suffer from drawbacks stemming from cathode active material limitations (e.g. insufficient capacities and capacity fading). One approach for alleviating such limitations and stabilizing the active material structure may be anion doping. In this work, fluorine and nitrogen are investigated as potential dopants in Li1.02(Ni0.8Co0.1Mn0.1)0.98O2 (NCM-811) as a prototypical nickel-rich cathode active material. Nitrogen doping is achieved by ammonia treatment of NCM in the presence of oxygen, which serves as an unconventional and new approach. The crystal structure was investigated by means of Rietveld and pair distribution function analysis of X-ray diffraction data, which provide very precise information regarding both the average and local structure, respectively. Meanwhile, time-of-flight secondary ion mass spectroscopy was used to assess the efficacy of dopant incorporation within the NCM structure. Moreover, scanning electron microscopy and scanning transmission electron microscopy were conducted to thoroughly investigate the dopant influences on the NCM morphology. Finally, the electrochemical performance was tested via galvanostatic cycling of half- and full-cells between 0.1 and 2 C. Ultimately, a dopant-dependent modulation of the NCM structure was found to enable the enhancement of the electrochemical performance, thereby opening a route to cathode active material optimization.
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Dec 2018
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[17257]
Abstract: Recent work on superionic conductors has demonstrated the influence of lattice dynamics and the softness of the lattice on ionic transport. When examining either the changes in the acoustic phonon spectrum or the whole phonon density of states, both a decreasing activation barrier of migration and a decreasing entropy of migration have been observed, highlighting that the paradigm of “the softer the lattice, the better” does not always hold true. However, both approaches to monitor the changing lattice dynamics probe different frequency ranges of the phonon spectrum and thus, it is unclear if they are complementary. In this work, we investigate the lattice dynamics of the superionic conductor Na3PS4-xSex by probing the optical phonon modes and the acoustic phonon modes, as well as the phonon density of states via inelastic neutron scattering. Notably, Raman spectroscopy shows the evolution of multiple local symmetry reduced polyhedral species, which likely affect the local diffusion pathways. Meanwhile, density functional theory and the ionic transport data are used to compare the different approaches for assessing the lattice dynamics. This work shows that while acoustic and inelastic methods may be used to experimentally assess the overall changing lattice stiffness, calculations of the average vibrational energies between the mobile ions and the anion framework are important to assess and computationally screen for ionic conductors.
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Oct 2018
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B18-Core EXAFS
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Diamond Proposal Number(s):
[13861]
Abstract: Inspired by reports of redox active interphases in all-solid-state batteries employing fast conducting lithium thiophosphate solid-state electrolytes, we investigated the compositional depolymerization of interconnected PS4 tetrahedra in (LiS)x(P2S5)100−x glasses (50 < x < 80) by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Based on the observed energy shifts with composition, we present a structural model of the three different bonding types describing the structures of either crystalline or amorphous thiophosphates. This model and reference data characterizes amorphous thiophosphates based on their inter-tetrahedral connectivity and helps to distinguish malign decomposition reactions from reversible redox reactions at the cathode active material/solid-state electrolyte interface. This work highlights the importance of a combined analytical approach and appropriate reference compounds to elucidate the interface reactions in all-solid-state battery systems.
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Jul 2018
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[13560]
Abstract: Lithium titanate Li4Ti5O12 (LTO) is regarded as a promising alternative to carbon-based anodes in lithium-ion batteries. Despite its stable structural framework, LTO exhibits disadvantages such as the sluggish lithium-ion diffusion and poor electronic conductivity. In order to modify the performance of LTO as an anode material, nanosizing constitutes a promising approach, and the impact is studied here by systematical experimental approach. Phase-pure polycrystalline LTO nanoparticles (NPs) with high crystallinity and crystallite sizes ranging from 4 to 12 nm are prepared by an optimized solvothermal protocol and characterized by several state-of-the-art technologies including HRTEM, XRD, PDF (pair distribution function) analysis, Raman spectroscopy and XPS. Through a wide array of electrochemical analyses, including charge/discharge profiles, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), a crystallite size of approx. 7 nm is identified as optimum particle size. Such NPs exhibit as good reversible capacity as the ones with larger crystallite sizes, but a more pronounced interfacial charge storage. By decreasing the crystallite size to about 4 nm the interfacial charge storage increases remarkably, however resulting in a loss of reversible capacity. An in-depth structural characterization using the PDF obtained from synchrotron XRD data indicates an enrichment in Ti for NPs with the small crystallite sizes, and this Ti-rich structure enables a higher Li storage. The electrochemical characterization confirms this result and furthermore points to a plausible reason why a higher Li-storage in very small nanoparticles (4 nm) results in a loss in the reversible capacity.
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Jun 2018
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