25 research outputs found
Effect of pressure on octahedral distortions in RCrO3 (R = Lu, Tb, Gd, Eu, Sm): The role of R-ion size and its implications
The effect of rare-earth ion size on the octahedral distortions in rare-earth
chromites (RCrO3, R = Lu, Tb, Gd, Eu, Sm) crystallizing in the orthorhombic
structure has been studied using Raman scattering and synchrotron powder x-ray
diffraction up to 20 GPa. From our studies on RCrO3 we found that the
octahedral tilts (distortions) increase with pressure. This is contrary to the
earlier report which suggests that in LaCrO3, the distortions decrease with
pressure leading to a more ordered phase at high pressure. Here we observe that
the rate of increase in distortion decreases with the increase in R-ion radii.
This occurs due to the reduction in the compression of RO12 polyhedra with a
corresponding increase in the compression of the CrO6 octahedra with increasing
R-ion radii. From the Raman studies, we predict a critical R-ion radii, above
which we expect the distortions in RCrO3 to reduce with increasing pressure
leading to what is observed in the case of LaCrO3. These Raman results are
consistent with our pressure dependent structural studies on RCrO3 (R = Gd, Eu,
Sm). Also, our results suggest that the pressure dependence of N\'eel
temperature, TNCr, (where the Cr3+ spin orders) in RCrO3 is mostly affected by
the compressions of Cr-O bonds rather than the alteration of octahedral tilts.Comment: 17 pages, 8 figures This manuscript has been published in Material
Research Expres
Self-Assembly and Gelation Study of Dipeptide Isomers with Norvaline and Phenylalanine
Dipeptides have emerged as attractive building blocks for supramolecular materials thanks to their low-cost, inherent biocompatibility, ease of preparation, and environmental friendliness as they do not persist in the environment. In particular, hydrophobic amino acids are ideal candidates for self-assembly in polar and green solvents, as a certain level of hydrophobicity is required to favor their aggregation and reduce the peptide solubility. In this work, we analyzed the ability to self-assemble and the gel of dipeptides based on the amino acids norvaline (Nva) and phenylalanine (Phe), studying all their combinations and not yielding to enantiomers, which display the same physicochemical properties, and hence the same self-assembly behavior in achiral environments as those studied herein. A single-crystal X-ray diffraction of all the compounds revealed fine details over their molecular packing and non-covalent interactions
Synchrotron soft X-ray imaging and fluorescence microscopy reveal novel features of asbestos body morphology and composition in human lung tissues
Background: Occupational or environmental exposure to asbestos fibres is associated with pleural and
parenchymal lung diseases. A histopathologic hallmark of exposure to asbestos is the presence in lung
parenchyma of the so-called asbestos bodies. They are the final product of biomineralization processes resulting in
deposition of endogenous iron and organic matter (mainly proteins) around the inhaled asbestos fibres. For
shedding light on the formation mechanisms of asbestos bodies it is of fundamental importance to characterize at
the same length scales not only their structural morphology and chemical composition but also to correlate them
to the possible alterations in the local composition of the surrounding tissues. Here we report the first correlative
morphological and chemical characterization of untreated paraffinated histological lung tissue samples with
asbestos bodies by means of soft X-ray imaging and X-Ray Fluorescence (XRF) microscopy, which reveals new
features in the elemental lateral distribution.
Results: The X-ray absorption and phase contrast images and the simultaneously monitored XRF maps of tissue
samples have revealed the location, distribution and elemental composition of asbestos bodies and associated
nanometric structures. The observed specific morphology and differences in the local Si, Fe, O and Mg content
provide distinct fingerprints characteristic for the core asbestos fibre and the ferruginous body. The highest Si
content is found in the asbestos fibre, while the shell and ferruginous bodies are characterized by strongly
increased content of Mg, Fe and O compared to the adjacent tissue. The XRF and SEM-EDX analyses of the
extracted asbestos bodies confirmed an enhanced Mg deposition in the organic asbestos coating.
Conclusions: The present report demonstrates the potential of the advanced synchrotron-based X-ray imaging
and microspectroscopy techniques for studying the response of the lung tissue to the presence of asbestos fibres.
The new results obtained by simultaneous structural and chemical analysis of tissue specimen have provided clear
evidence that Mg, in addition to Fe, is also involved in the formation mechanisms of asbestos bodies. This is the
first important step to further thorough investigations that will shed light on the physiopathological role of Mg in
tissue response to the asbestos toxicity
Self-Assembly of Homo- and Hetero-Chiral Cyclodipeptides into Supramolecular Polymers towards Antimicrobial Gels
There is an increasing interest towards the development of new antimicrobial coatings, especially in light of the emergence of antimicrobial resistance (AMR) towards common antibiotics. Cyclodipeptides (CDPs) or diketopiperazines (DKPs) are attractive candidates for their ability to self-assemble into supramolecular polymers and yield gel coatings that do not persist in the environment. In this work, we compare the antimicrobial cyclo(Leu-Phe) with its heterochiral analogs cyclo(D-Leu-L-Phe) and cyclo(L-Leu-D-Phe), as well as cyclo(L-Phe-D-Phe), for their ability to gel. The compounds were synthesized, purified by HPLC, and characterized by 1H-NMR, 13C-NMR, and ESI-MS. Single-crystal X-ray diffraction (XRD) revealed details of the intermolecular interactions within the supramolecular polymers. The DKPs were then tested for their cytocompatibility on fibroblast cells and for their antimicrobial activity on S. aureus. Overall, DKPs displayed good cytocompatibility and very mild antimicrobial activity, which requires improvement towards applications
Networks of superconducting nano-puddles in 1/8 doped YBa2Cu3O6.5+y controlled by thermal manipulation
While it is known that the nature and the arrangement of defects in complex
oxides have an impact on the material functionalities little is known on
control of superconductivity by oxygen interstitial organization in cuprates.
Here we report direct compelling evidence for the control of Tc, by
manipulation of the superconducting granular networks of nanoscale puddles,
made of ordered oxygen stripes, in a single crystal of YBa2Cu3O6.5+y with
average formal hole doping p close to 1/8. Upon thermal treatments we were able
to switch from a first network of oxygen defects striped puddles with OVIII
modulation (qOVIII(a*)=(h+3/8,k,0) and qOVIII(a*)=(h+5/8,k,0)), to second
network characterized by OXVI modulation (qOXVI(a*)=(h+7/16,k,0) and
qOXVI(a*)=(h+9/16,k,0)), and finally to a third network with puddles of OV
periodicity (qOV(a*)=(4/10,1,0) and qOV(a*)=(6/10,1,0)). We map the microscopic
spatial evolution of the out of plane OVIII, OXVI and OV puddles nano-size
distribution via scanning micro-diffraction measurements. In particular, we
calculated the number of oxygen chains (n) and the charge density (holes
concentration p) inside each puddle, analyzing areas of 160x80 {\mu}m2, and
recording 12800 diffraction patterns to reconstruct each spatial map. The high
spatial inhomogeneity shown by all the reconstructed spatial maps reflects the
intrinsic granular structure that characterizes cuprates and
iron-chalcogenides, disclosing the presence of several complex networks of
coexisting superconducting domains with different lattice modulations, charge
density and different gaps like in the proposed multi-gaps scenario called
superstripes.Comment: 5 figure
The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: the case of NdOFeAs
We report the temperature dependent x-ray powder diffraction of the
quaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 K
and 95 K. We have detected the structural phase transition from the tetragonal
phase, with P4/nmm space group, to the orthorhombic or monoclinic phase, with
Cmma or P112/a1 (or P2/c) space group, over a broad temperature range from 150
K to 120 K, centered at T0 ~137 K. Therefore the temperature of this structural
phase transition is strongly reduced, by about ~30K, by increasing the internal
chemical pressure going from LaOFeAs to NdOFeAs. In contrast the
superconducting critical temperature increases from 27 K to 51 K going from
LaOFeAs to NdOFeAs doped samples. This result shows that the normal striped
orthorhombic Cmma phase competes with the superconducting tetragonal phase.
Therefore by controlling the internal chemical pressure in new materials it
should be possible to push toward zero the critical temperature T0 of the
structural phase transition, giving the striped phase, in order to get
superconductors with higher Tc.Comment: 9 pages, 3 figure
Self-Assembly and Gelation Study of Dipeptide Isomers with Norvaline and Phenylalanine
Dipeptides have emerged as attractive building blocks for supramolecular materials thanks to their low-cost, inherent biocompatibility, ease of preparation, and environmental friendliness as they do not persist in the environment. In particular, hydrophobic amino acids are ideal candidates for self-assembly in polar and green solvents, as a certain level of hydrophobicity is required to favor their aggregation and reduce the peptide solubility. In this work, we analyzed the ability to self-assemble and the gel of dipeptides based on the amino acids norvaline (Nva) and phenylalanine (Phe), studying all their combinations and not yielding to enantiomers, which display the same physicochemical properties, and hence the same self-assembly behavior in achiral environments as those studied herein. A single-crystal X-ray diffraction of all the compounds revealed fine details over their molecular packing and non-covalent interactions
Tunable "In-Chain" and "At the End of the Branches" Methyl Acrylate Incorporation in the Polyolefin Skeleton through Pd(II) Catalysis
The synthesis of functionalized polyolefins through coordination-insertion polymerization is a highly challenging reaction. The ideal catalyst, in addition to showing a high productivity, has to be able to control the copolymer microstructure and, in particular, the way of the polar vinyl monomer incorporation. In this contribution, we modified the typical Brookhart's catalyst by introducing in the fourth coordination site of palladium a hemilabile, potentially bidentate ligand, such as a thiophenimine (N-S). The obtained cationic Pd(II) complexes, [Pd(Me)(N-N)(N-S)][PF6], generated active catalysts for the ethylene/methyl acrylate (MA) copolymerization leading to the desired copolymer with a different incorporation of the polar monomer depending on both the reaction medium and the N-S ligand. Surprisingly enough, the produced copolymers have the inserted acrylate both at the end of the branches (T(MA)) and in the main chain (M(MA)) in a ratio M(MA)/T(MA) that goes from 9:91 to 45:55 moving from dichloromethane to trifluoroethanol (TFE) as a solvent for the catalysis and varying the N-S ligand. The catalytic behavior of the new complexes was compared to that of the parent compound [Pd(Me)(N-N)(MeCN)][PF6], highlighting the fact that when the copolymerization is carried out in trifluoroethanol, this complex is also able to produce the E/MA copolymer with MA inserted both in the main chain and at the end of the branches. Accurate NMR studies on the reactivity of the precatalyst [Pd(Me)(N-N)(MeCN)][PF6] with the two comonomers allowed us to discover that in the fluorinated solvent, the catalyst resting state is an open-chain intermediate having both the organic fragment, originated from the migratory insertion of MA into the Pd-Me bond, and the acetonitrile coordinated to palladium and not the six-membered palladacycle typically observed for the Pd-alpha-diimine catalysts. This discovery is also supported by both DFT calculations and in situ NMR studies carried out on [Pd(Me)(N-N)(N-S)][PF6] complexes that point out that N-S remains in the palladium coordination sphere during catalysis. The open-chain intermediate is responsible for the growth of the copolymer chain with the polar monomer inserted into the main chain
Temperature-Dependent Structural Phase Transition in Rubrene Single Crystals: The Missing Piece from the Charge Mobility Puzzle?
International audienceAccurate structural models for rubrene, the benchmark organic semiconductor, derived from synchrotron X-ray data in the temperature range of 100–300 K, show that its cofacially stacked tetracene backbone units remain blocked with respect to each other upon cooling to 200 K and start to slip below that temperature. The release of the blocked slippage occurs at approximately the same temperature as the hole mobility crossover. The blocking between 200 and 300 K is caused by a negative correlation between the relatively small thermal expansion along the crystallographic b-axis and the relatively large widening of the angle between herringbone-stacked tetracene units. DFT calculations reveal that this blocked slippage is accompanied by a discontinuity in the variation with temperature of the electronic couplings associated with hole transport between cofacially stacked tetracene backbone