[Show abstract][Hide abstract] ABSTRACT: A new hierarchical approach is presented for elucidating the structural disorder in Ce1-x Gd x O2-x/2 solid solutions on different scale lengths. The primary goal of this investigation is to shed light on the relations between the short-range and the average structure of these materials via an analysis of disorder on the mesocopic scale. Real-space (pair distribution function) and reciprocal-space (Rietveld refinement and microstructure probing) analysis of X-ray powder diffraction data and electron spin resonance (ESR) investigations were carried out following this approach. On the local scale, Gd- and Ce-rich droplets (i.e. small regions a few ångströms wide) form, exhibiting either a distorted fluorite (CeO2) or a C-type (Gd2O3) structure in the whole compositional range. These droplets can then form C-type nanodomains which, for Gd concentrations x Gd ≤ 0.25, are embedded in the fluorite matrix. At the site percolation threshold p C for a cubic lattice (x Gd = p C ≃ 0.311), C-type nanodomains percolate inside each crystallite and a structural phase transformation is observed. When this occurs, the peak-to-peak ESR line width ΔH pp shows a step-like behaviour, which can be associated with the increase in Gd-Gd dipolar interactions. A general crystallographic rationale is presented to explain the fluorite-to-C-type phase transformation. The approach shown here could be adopted more generally in the analysis of disorder in other highly doped materials.
[Show abstract][Hide abstract] ABSTRACT: The crystal structures of eight mono-methyl alkanes have been determined from single-crystal or high-resolution powder X-ray diffraction using synchrotron radiation. Mono-methyl alkanes can be found on the cuticles of insects and are believed to act as recognition pheromones in some social species, e.g. ants, wasps etc. The molecules were synthesized as pure S enantiomers and are (S)-9-methylpentacosane, C26H54; (S)-9-methylheptacosane and (S)-11-methylheptacosane, C28H58; (S)-7-methylnonacosane, (S)-9-methylnonacosane, (S)-11-methylnonacosane and (S)-13-methylnonacosane, C30H62; and (S)-9-methylhentriacontane, C32H66. All crystallize in space group P21. Depending on the position of the methyl group on the carbon chain, two packing schemes are observed, in which the molecules pack together hexagonally as linear rods with terminal and side methyl groups clustering to form distinct motifs. Carbon-chain torsion angles deviate by less than 10° from the fully extended conformation, but with one packing form showing greater curvature than the other near the position of the methyl side group. The crystal structures are optimized by dispersion-corrected DFT calculations, because of the difficulties in refining accurate structural parameters from powder diffraction data from relatively poorly crystalline materials.
[Show abstract][Hide abstract] ABSTRACT: The size of dopant-rich nanodomains was assessed in four samples of Ce1−μ
O2−μ/2 through systematic pair distribution function (PDF) refinements. Experimental G(r) curves were fitted by different structural models with the aim of finding a description which balanced precise structure parameterization and reasonable number of parameters. The most reliable model was a single Y2O3-like phase, which best accommodated to the close relationship between the fluorite (CeO2-like) and C-type (Y2O3-like) structures. In this model, a refined cation coordinate, x(M2), measured the relative occurrence in the G(r) of the chemical environment of Y and Ce at any value of r. The r-value at which x(M2) vanished, i.e. at which the refined C-type cell becomes a redundant, low-symmetry description of a fluorite cell, was assumed as the size of a C-type domain. Subtle features in G(r) could be attributed to the fluorite or C-type phase up to ~500 Å thanks to the narrow instrumental resolution function of the ID31 beamline (now ID22) at the ESRF, which allows us to get high resolution PDF data.
[Show abstract][Hide abstract] ABSTRACT: LiBH4 has gained much attention as a potential hydrogen storage material due to its high hydrogen storage capacity of 18.5 wt%. However, LiBH4 only releases its full hydrogen capacity at temperatures greater than 600 °C and requires hydrogen pressures of at least 350 bar to rehydrogenate the end products. The dehydrogenation temperature can be altered by thermodynamic tuning through the addition of a reactive agent resulting in a lower enthalpy of dehydrogenation. Most multicomponent hydride systems display dehydrogenation temperatures above 300 °C, making them less desirable for automotive applications. In this work we report the solid-state decomposition of LiBH4 in the 2LiBH4:CaNi5 system below the LiBH4 melting temperature of 270 °C. In situ neutron diffraction measurements confirmed the decomposition took place in the solid state at 200 °C, forming LiD, CaD2, Ni3B and Ni2B phases as end products. The solid-state decomposition was further supported by SEM and TEM measurements showing the presence of nano-crystalline particles.
International Journal of Hydrogen Energy 02/2015; 40(7). DOI:10.1016/j.ijhydene.2014.12.059 · 3.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Neutron diffraction studies performed on the solid solution of (BiFeO3)1−x(PbTiO3)x reveal a mixture of two nanoscale phases with different crystal structures: a rhombohedral BiFeO3-based phase and a tetragonal PbTiO3-based phase. The ratio of Fe3+ and Ti4+ ions in the two phases is practically constant; only the proportion of the phases changes. The magnetic moments in the BiFeO3-based phase, in contrast to BiFeO3, deviate from the basal plane. The temperature evolutions of the spin components along the hexagonal axis and within the perpendicular plane are different, leading to a spin re-orientation transition. The antiferromagnetic order in the PbTiO3-based phase corresponds to a simple structure with the propagation vector (1/2, 1/2, 1/2). The temperature dependence of the antiferromagnetic moment in the tetragonal phase at x = 0.5 indicates a canted antiferromagnetic order and a net ferromagnetic moment. A strong magnetic coupling between the two constituting phases due to the nanoscale character of the phases and well-developed interface between nanoparticles has been observed. The system of (BiFeO3)1−x(PbTiO3)x demonstrates an interesting scenario, where the proximity effects in the unstable system play a crucial role in the appearance of the unusual magnetic properties.
[Show abstract][Hide abstract] ABSTRACT: The crystallographic structure of Tb- and Pr-doped ceria is investigated through X-ray and neutron powder diffraction, combining pair distribution function and UV–vis spectroscopy. Compared to trivalent dopants, Tb and Pr show peculiar optical and crystallographic properties: whilst Tb have a mixed +3/+4 valence state, Pr is stable mostly in the +4 valence state up to ~50% dopant concentration. For larger Pr amounts, doping promotes a fluorite to monoclinic phase transformation. A straightforward method for extracting the dopant oxidation state by the cell parameter dependence on the ionic radius is also proposed.
Solid State Ionics 12/2014; 268. DOI:10.1016/j.ssi.2014.10.020 · 2.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Inelastic neutron scattering experiments and ab-initio calculations have been used to investigate the vibrational modes, in a wide energy region between 0 and 200 meV, of hydrogenated graphene produced by chemical method. The results show the presence of atomic hydrogen chemisorbed at the graphene surface. At 10 K, the measured high energy density of states is remarkably similar to that of hydrogenated ball-milled graphite, in which hydrogen is most likely bonded to C atoms at the edges. In fact, in both hydrogenated graphene and hydrogenated ball-milled graphite, the high frequency modes (100-200 meV) show strong similarities with the C-H bending modes of the coronene molecule, in which hydrogen is bonded at the edges. This hypothesis has been supported by ab-initio calculations.
Journal of Physics Conference Series 11/2014; 554(1). DOI:10.1088/1742-6596/554/1/012009
[Show abstract][Hide abstract] ABSTRACT: We report the structural analysis of the highly-doped lithium fulleride Li12C60, performed using low-temperature neutron powder diffraction. Although the main reflections could be initially indexed with a fcc cell, Monte Carlo Simulated Annealing suggests an unusual monoclinic arrangement for the fullerene molecules. In this structure, C-60 units with the same orientation are alternatively stacked to form layers, thus maximising their sterical crowding. Rietveld refinement allowed to localise Li ions, which are organised in clusters belonging to the pseudo-tetrahedral voids with a non-complete charge transfer as confirmed by Raman spectroscopy.
Chemical Physics Letters 08/2014; 609:155–160. DOI:10.1016/j.cplett.2014.06.036 · 1.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An exhaustive structural investigation of a Y-doped ceria (Ce1–xYxO2–x/2) system over different length scales was performed by combining Rietveld and Pair Distribution Function analyses of X-ray and neutron powder diffraction data. For low doping amounts, which are the most interesting for application, the local structure of Y-doped ceria can be envisaged as a set of distorted CeO2- and Y2O3-like droplets. By considering interatomic distances on a larger scale, the above droplets average out into domains resembling the crystallographic structure of Y2O3. The increasing spread and amount of the domains with doping forces them to interact with each other, leading to the formation of antiphase boundaries. Single phase systems are observed at the average ensemble level.
Chemistry of Materials 10/2013; 25(21):4278–4289. DOI:10.1021/cm402359d · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: LiH is a highly stable light metal hydride with a hydrogen capacity of 12.5 wt%. However, having a dehydrogenation enthalpy, ΔHdehy, of 181.2 kJ mol(-1)(H2) and a resultant T(1 bar) of 944 °C, it is not a practical hydride for most hydrogen storage applications. In the work presented here, germanium has been found to dramatically reduce the dehydrogenation temperature for LiH down to just 270 °C. The enthalpy of dehydrogenation was reduced through the formation of lithium germanides. The reaction pathway was identified in this study using in situ powder neutron diffraction, showing the successive formation of more Li-rich germanides, following the series: LiGe, Li4Ge2H, Li9Ge4, and Li7Ge2. The enthalpy of formation for these germanides provides the thermodynamic tuning to reduce the ΔHdehy for the system. The 3LiH-Ge system investigated is found to be reversible with a maximum capacity of 3.0 ± 0.1 wt%.
[Show abstract][Hide abstract] ABSTRACT: MOF zipper: Thermal removal of axial pyridine ligands from the non-covalently pillared metal-organic framework [Zn (camph) (py) ]⋅2EtOH prompts migration of alternate camphorate ligands to zip Zn centers from adjacent layers into continuous chains within the nonporous material [Zn (camph) ]. Unzipping to generate new pillared, layered MOFs occurs on exposure to new axial ligands.
Chemistry - A European Journal 03/2013; 19(11):3552. DOI:10.1002/chem.201204492 · 5.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Doped ceria materials are widely studied for their application in solid oxide fuel cell devices. In this work we report on the average and local structure evolution of La-doped ceria (Ce(1-x)La(x)O(2-x/2), x = 0.25) under fuel cells' operating conditions. The effect of doping on the average structure is investigated using conventional Rietveld analysis of neutron powder diffraction data. The extent of disorder as well as the local structure evolution at high temperature are investigated by employing very hard X-rays under both air and reducing atmosphere.
[Show abstract][Hide abstract] ABSTRACT: The current contribution aims to investigate the stability of commonly occurring motifs present in certain amino acid structures after introducing additional molecules to form multicomponent crystals. The crystal structures of the amino acid cis-4-aminocyclohexanecarboxylic acid hemi-hydrate I and dehydrate II forms and that of its oxalate salt cocrystallized with oxalic acid III, were investigated employing a combination of techniques. Both single-crystal and powder X-ray diffraction were used to solve the structures, while temperature-control powder X-ray diffraction was used to follow the dehydration of I. Regardless of the added molecules that induce modifications of the intermolecular interactions within the crystals, some recurring supramolecular structures were identified: double head to tail dimers, graph symbol R 2 2 (16), and ribbons, graph symbol R 2 2 (16)R 3 4 (10). Stabilities of these supramolecular motifs were investigated using theoretical modeling with DFT/B3LYP/6-31++G (d,p) and PM6-D2H calculations. The theoretical calculations reproduced the experimental findings, confirming the extraordinary stability of these motifs. The molecular recognition of amino acid pairs to form double head to tail-dimers is undoubtedly the initial driving force for the crystal formation in all the three crystals investigated.
[Show abstract][Hide abstract] ABSTRACT: The inclusion properties of a calixarene-based porous material have been studied to investigate the adsorption and the desorption of carbon tetrachloride, chloroform, and water in the zeolite-like structure. Uptake and release processes have been studied both by time-resolved powder X-ray diffraction and by thermogravimetric analysis to obtain structural and kinetic information. The selected guests are able to enter the structure with an increase in the host cell volume and with time-dependent diffusivity coefficients. Chloroform molecules act as a permanent porosity switch promoting a phase transition to non-porous triclinic form.
[Show abstract][Hide abstract] ABSTRACT: Rare Earth doped ceria materials (Ce1�xRExO2�x/2) are widely studied for their application
in solid oxide fuel cell devices. In this work, RE(Yb, Y, Nd, La)-doped ceria samples at constant (x ¼ 0.25) doping rate were subjected to a combined synchrotron radiation and neutron powder diffraction study. The dopants were chosen in order to cover a wide range of dopant-ionic radii.
The effect of doping on the average structure is investigated using conventional Rietveld analysis, while the
Pair Distribution Function technique is used to explore the spatial extent of disorder as well as the local structure.
Two models for mapping the local structure, in terms of oxygen relaxation and nano-phase separation, are presented.
Zeitschrift für Kristallographie 05/2012; 227(5):272. DOI:10.1524/zkri.2012.1493 · 1.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work the first Pair Distribution Function (PDF) study on
Ce1‑xGdxO2‑x/2 (CGO) electrolytes for solid oxide fuel cells is presented, aiming to unveil
the complex positional disorder induced by gadolinium doping and oxygen vacancies
formation in these materials. The whole range of Gd concentration xGd (0 ≤ xGd ≤ 1) of
the CGO solid solutions was investigated through high resolution synchrotron radiation
powder diffraction. The reciprocal space Rietveld analysis revealed in all the solid solutions
the presence of positional disorder, which has been explicitly mapped into the real space.
The average structural models, as obtained by the Rietveld method, fit well the
experimental PDF data only for a spatial range r > ∼10 Å. The same models applied at
lower r values fails to reproduce the experimental curves. A clear improvement of the fit
quality in the 1.5 < r < ∼6 Å range was obtained for all the CGO samples applying a
biphasic model encompassing both a fluorite CeO2-like and a C-type Gd2O3-like phases.
This provides evidence that extended defects at local scale exist in the CGO system. Gdrich
and Ce-rich droplets coexist in the subnanometric range.
Chemistry of Materials 04/2012; 24:1338. DOI:10.1021/cm203819u · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Yttrium doped ceria materials (Ce1−xYxO2−x/2) are widely studied for their application in Solid Oxide Fuel Cells devices. An anomalous decrease in the isothermal ionic conductivity at increasing Y3+ concentration above a critical value has been observed and attributed to the formation of defect clusters / domains at the nanometric scale, the crystallographic structure of which is still under debate. In this context we present a combined Synchrotron Radiation and Neutron Powder Diffraction study. In particular, neutrons allow to determine accurately oxygen related parameters, the contribution of which in terms of X-ray scattering power is almost negligible when compared to that of cations. The effect of doping on the average structure is investigated using conventional Rietveld analysis, while the Pair Distribution Function (PDF) technique is used to explore structural distortions and the spatial extent of disorder as well. The local structure observed in the real space is not consistent with the mean crystallographic one and is better modeled considering a biphasic model.
Journal of Physics Conference Series 02/2012; 340(1). DOI:10.1088/1742-6596/340/1/012056
[Show abstract][Hide abstract] ABSTRACT: Transient (non-equilibrium) microstructures in crystals may arise in an order-disorder phase transition that generates lattice strain. A two-phase field can develop if fluctuations of the order parameter lead to nucleation of an ordered phase in a disordered matrix, as we describe here for columbite. Synchrotron X-ray diffraction and transmission electron microscopy show that ordering in columbite involves two discrete phases with different degree of order but the same composition. A highly unusual distribution of ordered rhombic-shaped domains within a disordered matrix establishes on a nanometer scale and remains relatively stable over a prolonged period of annealing. Progressive ordering takes place within the ordered domains and the disordered matrix but the domains maintain more or less constant shape and distribution. We speculate that a new family of such microstructures could develop in other oxide phases with cation ordering transitions that are strongly first order in character. Long-term stability of such microstructures and their dependence on strain could open up the possibility of engineering the properties of crystals containing a percolating disordered matrix with ordered nanodomains of controlled dimensions.
American Mineralogist 02/2012; 96(2-3):374. DOI:10.2138/am.2011.3571 · 1.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The adsorption properties of a new nanoporous organic zeolite with respect to acetylene and Ar were studied by volumetric adsorption analysis, microcalorimetric experiments, and synchrotron high-resolution X-ray powder diffraction. This allowed us to locate the guest molecules inside the host channels and characterize the host-guest interactions.