I19-Small Molecule Single Crystal Diffraction
|
Marta
Mon
,
Rosaria
Bruno
,
Estefanía
Tiburcio
,
Aida
Grau-atienza
,
Antonio
Sepulveda-escribano
,
Enrique V
Ramos-fernandez
,
Alessio
Fuoco
,
Elisa
Esposito
,
Marcello
Monteleone
,
Johannes Carolus
Jansen
,
Joan
Cano
,
Jesus
Ferrando-soria
,
Donatella
Armentano
,
Emilio
Pardo
Diamond Proposal Number(s):
[18768]
Abstract: Understand/visualize the established interactions between gases and adsorbents is mandatory to implement better per-formant materials in adsorption/separation processes. Here we report the unique behavior of a rare example of a hemila-bile chiral three-dimensional metal-organic framework (MOF) with an unprecedented qtz-e-type topology, with formula CuII2(S,S)-hismox . 5H2O (1) (hismox = bis[(S)-histidine]oxalyl diamide). 1 exhibits a continuous and reversible breath-ing behavior, based on the hemilability of carboxylate groups from L-histidine. In-situ powder (PXRD) and single crystal X-ray diffraction (SCXRD) using synchrotron radiation allowed to unveil the crystal structures of 4 different host-guest adsorbates (Ar, N2, CO2 and C3H6@1), the rationalization of the breathing motion and unravel the mechanisms govern-ing the adsorption of these gases. Then, this information has been transferred to implement efficient separations of mix-tures of industrial and environmental relevance –CO2/N2, CO2/CH4 and C3H8/C3H6 using 1 in packed columns as the sta-tionary phase and dispersed in a mixed matrix membrane.
|
Jul 2019
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[19602]
Abstract: The strive for increased energy density in Li-ion batteries is particularly dependent on the cathode materials, that so far have been limiting for the overall battery performance. A new class of materials, Li-rich disordered rock-salts, has recently been brought forward as promising candidates for next-generation cathodes due to their ability to reversibly cycle more than one Li-ion per transition metal. Several variants of these Li-rich cathode materials have been developed recently and show promising initial capacities, but challenges concerning capacity fade and voltage decay during cycling need yet to be overcome. Mechanisms behind the significant capacity fade of some materials must be understood to allow for the design of new materials in which detrimental reactions can be mitigated. In this study the origin of the capacity fade in the Li-rich material Li2VO2F is investigated and it is shown to begin with a degradation of the particle surface that spreads inwards with continued cycling.
|
Jun 2019
|
|
I11-High Resolution Powder Diffraction
|
Diamond Proposal Number(s):
[16297]
Abstract: There has been significant recent interest in exploiting the large dimension changes that can occur in molecular materials as a function of temperature, stress or under optical illumination. Here we report the remarkable thermal expansion properties of chloranilic acid pyrazine (CA-Pyz) co-crystals. We show that the compound shows uniaxial negative thermal expansion over a wide temperature range with a linear contraction coefficient as low as (−)1500×10−6 K−1 at 250 K. The corresponding 10% contraction between 200 and 300 K is an order of magnitude larger than in the so-called colossal contraction materials. We adopt a symmetry-inspired approach to describe both the structural changes that occur (using rotational-symmetry modes) and the thermal expansion (using strain modes). This allows an extremely compact description of the phase transition and detailed understanding of its atomic origins. We show how the coupling of primary and secondary strain modes in materials showing extreme expansion and contraction can lead to unusual reversals in the temperature dependence of cell parameters.
|
May 2019
|
|
I15-1-X-ray Pair Distribution Function (XPDF)
|
Diamond Proposal Number(s):
[17315]
Abstract: The recent introduction of organometal halide perovskites to solar cells has significantly enhanced the power conversion efficiency of alternative photovoltaic devices, revolutionizing the development of photovoltaic technologies. To produce perovskite thin films with high device performances, various fabrication methodologies have been developed leading to thin films with different surface structures and crystal morphologies. Tremendous efforts have been devoted to characterizing macro- and microscopic structures within these films to better understand the processing-property-performance relationship. However, their atomic structure and its influence on device performance remains poorly understood. To this end, we employed pair distribution function analysis of X-ray total scattering data to obtain crystallographic and compositional information of methylammonium-lead-iodide (MAPbI3) thin films. This analysis revealed the ubiquitous presence of two near-amorphous intermediate phases with local structures that share subtle but significant correlations with the PbI2 precursor and the desired perovskite phase. The structure transformation from these intermediates to the perovskite deviates from the intuitive belief where the molecular cations get inserted between the sheets of layered PbI2 upon the crystallization of perovskite. This knowledge offers critical insight into the perovskite formation thermodynamics and reveals an important link between the short-range structure of the thin films and their corresponding device performance.
|
Apr 2019
|
|
B18-Core EXAFS
I09-Surface and Interface Structural Analysis
|
Robert A.
House
,
Urmimala
Maitra
,
Liyu
Jin
,
Juan G.
Lozano
,
James W.
Somerville
,
Nicholas H.
Rees
,
Andrew J.
Naylor
,
Laurent C.
Duda
,
Felix
Massel
,
Alan V.
Chadwick
,
Silvia
Ramos
,
David M.
Pickup
,
Daniel E.
Mcnally
,
Xingye
Lu
,
Thorsten
Schmitt
,
Matthew R.
Roberts
,
Peter G.
Bruce
Diamond Proposal Number(s):
[14239, 20870]
Abstract: It is possible to increase the charge capacity of transition metal oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explore what affects oxygen loss in oxygen redox materials, we have compared two analogous Na-ion cathodes, P2-Na0.67Mg0.28Mn0.72O2 and P2-Na0.78Li0.25Mn0.75O2. On charging to 4.5 V, >0.4 e- are removed from the oxide ions of these materials, but neither compound exhibits oxygen loss. Li is retained in P2-Na0.78Li0.25Mn0.75O2 but displaced from the transition metal to the alkali metal layers, showing that vacancies in the transition metal layers, which also occur in other oxygen redox compounds that exhibit oxygen loss such as Li[Li0.2Ni0.2Mn0.6]O2, is not a trigger for oxygen loss. On charging at 5 V, P2-Na0.78Li0.25Mn0.75O2 exhibits oxygen loss whereas P2-Na0.67Mg0.28Mn0.72O2 does not. Under these conditions both Na+ and Li+ are removed from P2-Na0.78Li0.25Mn0.75O2 resulting in underbonded oxygen (fewer than 3 cations coordinating oxygen) and surface localised O loss. In contrast, for P2-Na0.67Mg0.28Mn0.72O2, oxygen remains coordinated by at least 2 Mn4+ and 1 Mg2+ ions, stabilising the oxygen and avoiding oxygen loss.
|
Apr 2019
|
|
I15-1-X-ray Pair Distribution Function (XPDF)
|
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.
|
Mar 2019
|
|
B18-Core EXAFS
I15-1-X-ray Pair Distribution Function (XPDF)
|
Diamond Proposal Number(s):
[16168, 19240, 16460]
Abstract: K0.5Bi0.5TiO3 (KBT) – one of the few perovskite-like ferroelectric compounds with room-temperature tetragonal symmetry – differs from other members of this family (BaTiO3 and PbTiO3) by the presence of a disordered mixture of K and Bi on cuboctahedral sites. This disorder is expected to affect local atomic displacements and their response to an applied electric field. We have derived nanoscale atomistic models of KBT by refining atomic coordinates to simultaneously fit neutron/X-ray total-scattering and extended X-ray absorption fine-structure data. Both Bi and Ti ions were found to be offset relative to their respective oxygen cages in the high-temperature cubic phase; in contrast, the coordination environment of K remained relatively undistorted. In the cubic structure, Bi displacements prefer the <100> directions and the probability-density distribution of Bi features six well-separated sites; a similar preference exists for the much smaller Ti displacements although the split sites for Ti could not be resolved. The cation displacements are correlated, yielding polar nanoregions, while on average the structure appears as cubic. The cubic<->tetragonal phase transition involves both order-disorder and displacive mechanisms. A qualitative change in the form of the Bi probability-density distribution occurs in the tetragonal phase on cooling to room temperature because Bi displacements “branch off” to <111> directions. This change, which preserves the average symmetry, is accompanied by the development of nanoscale polar heterogeneities that exhibit significant deviations of their polarization vectors from the average polar axis.
|
Mar 2019
|
|
|
|
Abstract: Modification of TiO2 to increase its visible light activity and promote higher performance photocatalytic ability has become a key research goal for materials scientists in the past two decades. One of the most popular approaches proposed this as “passivated codoping”, whereby an equal number of donor and acceptor dopants are introduced into the lattice, producing a charge neutral system with a reduced band gap. Using the archetypal codoping pairs of [Nb+N] and [Ta+N] doped anatase, we demonstrate using hybrid density functional theory that compensated codoping is not achievable in TiO2. Our results indicate that the natural defect chemistry of the host system (in this case n-type anatase TiO2) is dominant, and so concentration parity of dopant types is not achievable under any thermodynamic growth conditions. The implications of compensated codoping for band gap manipulation in general are discussed.
|
Mar 2019
|
|
I11-High Resolution Powder Diffraction
|
Diamond Proposal Number(s):
[14809]
Abstract: Prussian blue analogues (PBAs) have recently shown outstanding electrochemical properties ascribable to their unique open-framework crystal structure that allows the reversible insertion of alkali ions with negligible perturbation to the framework itself. Many hexacyanoferrate materials have shown excellent properties and are some of the most promising sodium- and potassium-ion cathode materials in both aqueous and organic electrolytes. However, there is a distinct lack of candidate PBA materials that operate at low potentials, as their characteristic crystalline framework shows instability. In this article, we characterize the structure and electrochemical behavior of manganese hexacyanochromate, which exhibits reversible sodium insertion at −0.86 V vs standard hydrogen electrode (1.84 V vs Na+/Na) while maintaining the characteristic PBA cubic structure. This is the lowest redox potential of reported PBA materials and shows fast kinetics in a high-voltage water-in-salt electrolyte. Further reduction in potential in an organic electrolyte shows decomposition of the crystalline structure.
|
Mar 2019
|
|
|
|
Abstract: Halide perovskite semiconductors such as methylammonium lead iodide (CH3NH3PbI3) have achieved great success in photovoltaic devices; however, concerns surrounding toxicity of lead and material stability have motivated the field to pursue alternative perovskite compositions and structures. Vacancy-ordered double perovskites are a defect-ordered variant of the perovskite structure characterized by an antifluorite arrangement of isolated octahedral units bridged by A-site cations. In this perspective, we focus upon the structure-dynamics-property relationships in vacancy-ordered double perovskite semiconductors as they pertain to applications in photovoltaics, and propose avenues of future study within the context of the broader perovskite halide literature. We describe the compositional and structural motifs that dictate the optical gaps and charge transport behavior and discuss the implications of charge ordering, lattice dynamics, and organic-inorganic coupling upon the properties of these materials. The design principles we elucidate here represent an important step towards extending our understanding of perovskite functionality to defect-ordered perovskites.
|
Jan 2019
|
|
