Faits marquants scientifiques 2021

Virginie Vergnat, Benoît Heinrich, Michel Rawiso, René Muller, Geneviève Pourroy and Patrick Masson

Embedding nanoparticles (NPs) with organic shells is a way to control their aggregation behavior. Using polymers allows reaching relatively high shell thicknesses but suffers from the difficulty of obtaining regular hybrid objects at gram scale. Here, we describe a three-step synthesis in which multi-gram NP batches are first obtained by thermal decomposition, prior to their covalent grafting by an atom transfer radical polymerization (ATRP) initiator and to the controlled growing of the polymer shell. Specifically, non-aggregated iron oxide NPs with a core principally composed of γ-Fe₂O₃ (maghemite) and either polystyrene (PS) or polymethyl methacrylate (PMMA) shell were elaborated. The oxide cores of about 13 nm diameter were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). After the polymerization, the overall diameter reached 60 nm, as shown by small-angle neutron scattering (SANS). The behavior in solution as well as rheological properties in the molten state of the polymeric shell resemble those of star polymers. Strategies to further improve the screening of NP cores with the polymer shells are discussed.

https://doi.org/10.3390/nano11092453

J. Wolanin, L. Michel, D. Tabacchioni, J. M. Zanotti, J. Peters, I. Imaz, B. Coasne, M. Plazanet, and C. Picard

With their strong confining porosity and versatile surface chemistry, zeolitic imidazolate frameworks—including the prototypical ZIF-8—display exceptional properties for various applications. In particular, the forced intrusion of water at high pressure (∼25 MPa) into ZIF-8 nanopores is of interest for energy storage. Such a system reveals also ideal to study experimentally water dynamics and thermodynamics in an ultrahydrophobic confinement. Here, we report on neutron scattering experiments to probe the molecular dynamics of water within ZIF-8 nanopores under high pressure up to 38 MPa. In addition to an overall confinement-induced slowing down, we provide evidence for strong dynamical heterogeneities with different underlying molecular dynamics. Using complementary molecular simulations, these heterogeneities are found to correspond to different microscopic mechanisms inherent to vicinal molecules located in strongly adsorbing sites (ligands) and other molecules nanoconfined in the cavity center. These findings unveil a complex microscopic dynamics, which results from the combination of surface residence times and exchanges between the cavity surface and center.

https://doi.org/10.1021/acs.jpcb.1c06791

T. Fiuza, M. Sarkar, J. C. Riedl, A. Cebers, F. Cousin, G. Demouchy, J. Depeyrot, E. Dubois, F. Gelebart, G. Meriguet, R. Perzynski and V. Peyre

Ferrofluids based on maghemite nanoparticles (NPs), typically 10 nm in diameter, are dispersed in an ionic liquid (1-ethyl 3-methylimidazolium bistriflimide – EMIM-TFSI). The average interparticle interaction is found to be repulsive by small angle scattering of X-rays and of neutrons, with a second virial coefficient A2 = 7.3. A moderately concentrated sample at F = 5.95 vol% is probed by forced Rayleigh scattering under an applied magnetic field (up to H = 100 kA m1 ) from room temperature up to T = 460 K. Irrespective of the values of H and T, the NPs in this study are always found to migrate towards the cold region. The in-field anisotropy of the mass diffusion coefficient Dm and that of the (always positive) Soret coefficient ST are well described by the presented model in the whole range of H and T. The main origin of anisotropy is the spatial inhomogeneities of concentration in the ferrofluid along the direction of the applied field. Since this effect originates from the magnetic dipolar interparticle interaction, the anisotropy of thermodiffusion progressively vanishes when temperature and thermal motion increase.

https://doi.org/10.1039/d0sm02190c

Sebastian Jaksch, Alexis Chennevière, Sylvain Désert, Tadeusz Kozielewski, Herbert Feilbach, Pascal Lavie, Romuald Hanslik, Achim Gussen, Stephan Butterweck, Ralf Engels, Henrich Frielinghaus, Stephan Förster and Peter Müller-Buschbaum

Small-K Advanced DIffractometer (SKADI is a Small-Angle Neutron Scattering (SANS) instrument to be constructed at the European Spallation Source (ESS). SANS instruments allow investigations of the structure of materials in the size regime between Angstroms up to micrometers. As very versatile instruments, they usually cater to the scientific needs of communities, such as chemists, biologists, and physicists, ranging from material and food sciences to archeology. They can offer analysis of the micro- and mesoscopic structure of the samples, as well as an analysis of the spin states in the samples, for example, for magnetic samples. SKADI, as a broad range instrument, thus offers features, such as an extremely flexible space for the sample environment, to accommodate a wide range of experiments, high-flux, and optimized detector-collimation system to allow for an excellent resolution of the sample structure, short measurement times to be able to record the internal kinetics during a transition in the sample, as well as polarized neutron scattering. In this manuscript, we describe the final design for the construction of SKADI. All of the features and capabilities presented here are projected to be included into the final instrument when going into operation phase.

Christopher D. O’Neill, Gino Abdul-Jabbar, Didier Wermeille, Philippe Bourges, Frank Krüger, and Andrew D. Huxley

Quantum order by disorder revealed 
 
Ferromagnets are ubiquitous in everyday life, present in household items ranging from fridge-magnets on a fridge door to the sensors and motors inside, while dozens can be found in an average car. As temperature is raised ferromagnetic order disappears, but more rarely the formation of more fascinating states can occur in which the uniform magnetic order is replaced by a static magnetic wave. One mechanism for this is known as quantum order by disorder; the wave (the order) forms because it has more low energy excited states (the disorder) available to it than the uniform state and this lowers its energy, offsetting the energy cost of producing the wave.  In a recent paper appearing in Physical Review Letters such a static oscillation was achieved for the first time by applying a magnetic field at right angles to the easy axis moment in a ferromagnet, allowing the resulting increase in magnetic excitations to be clearly seen with neutron scattering.  In the long term the creation of such magnetic oscillations with variable pitch could be useful, for example, to make magnetic diffraction gratings for spintronics.
 

https://doi.org/10.1103/PhysRevLett.126.197203

S. Biesenkamp, D. Gorkov, D. Brüning, A. Bertin, T. Fröhlich, X. Fabrèges, A. Gukasov ,M. Meven, P. Becker, L. Bohaty, T. Lorenz and M. Braden, Phys. Rev. B 103 (2021) 134412.

The crystal and magnetic structures of multiferroic LiFe(WO₄)₂ were investigated by temperature and magnetic-field-dependent specific heat, susceptibility, and neutron diffraction experiments on single crystals. Considering only the two nearest-neighbor magnetic interactions, the system forms a J₁, J₂ magnetic chain, but more extended interactions are sizable. Two different magnetic phases exhibiting long-range incommensurate order evolve at T_N1 ≈ 22.2 K and T_N2 ≈ 19 K. First, a spin-density wave develops with moments lying in the ac plane. In its multiferroic phase below T_N2, LiFe(WO₄)₂ exhibits a spiral arrangement with an additional spin component along b. Therefore, the inverse Dzyaloshinskii-Moriya mechanism fully explains the multiferroic behavior in this material. A partially unbalanced multiferroic domain distribution was observed even in the absence of an applied electric field. For both phases only a slight temperature dependence of the incommensurability was observed, and there is no commensurate phase emerging at low temperature or at finite magnetic fields up to 6  T. LiFe(WO₄)₂ thus exhibits a simple phase diagram with the typical sequence of transitions for a type-II multiferroic material.

DOI : http://doi.org/10.1103/PhysRevB.103.134412

Dalila Bounoua, Lucile Mangin-Thro, Jaehong Jeong, Romuald Saint-Martin, Loreynne Pinsard-Gaudart, Yvan Sidis & Philippe Bourges, Communications Physics 3, (2020) 123.

New phases with broken discrete Ising symmetries are uncovered in quantum materials with strong electronic correlations. The two-leg ladder cuprate Sr_14−xCa_xCu₂₄O₄₁ hosts a very rich phase diagram where, upon hole doping, the system exhibits a spin liquid state ending to an intriguing ordered magnetic state at larger Ca content.

Using polarized neutron diffraction, we report here the existence of short range magnetism in this material for two Ca contents, whose origin cannot be ascribed to Cu spins. This magnetism develops exclusively within the two-leg ladders with a diffraction pattern at forbidden Bragg scattering, which is the hallmark of loop current-like magnetism breaking both time-reversal and parity symmetries.

Our discovery shows local discrete symmetry breaking in a one dimensional spin liquid system as theoretically predicted. It further suggests that a loop current-like phase could trigger the long range magnetic order reported at larger doping in two-leg ladder cuprates.

https://doi.org/10.1038/s42005-020-0388-1.

Julie Wolanin, Jérôme Giraud, Claude Payre, Marianne Benoit, Claire Antonelli, Damien Quemener, Iliass Tahiri, Matthieu Vandamme, Jean-Marc Zanotti, and Marie Plazanet, Review of Scientific Instruments 92 (2021) 024106

In comparison to condensed matter, soft matter is subject to several interplaying effects (surface heterogeneities and swelling effect) that influence transport at the nanoscale. In consequence, transport in soft and compliant materials is coupled to adsorption and deformation phenomena. The permeance of the material, i.e., the response of the material to a pressure gradient, is dependent on the temperature, the chemical potential, and the external constraint.

Therefore, the characterization of water dynamics in soft porous materials, which we address here, becomes much more complex. In this paper, the development of an original setup for scattering measurements of a radiation in the transmitted geometry in oedometric conditions is described. A specially designed cell enables a uniaxial compression of the investigated material, PIM-1 (Polymers of Intrinsic Microporosity), in the direction perpendicular to the applied hydraulic pressure gradient (up to 120 bars). High pressure boosting of the circulating water is performed with a commercially available high-pressure pump Karcher. This particular setup is adapted to the quasi-elastic neutron scattering technique, which enables us to probe diffusion and relaxation phenomena with characteristic times of 10−9 s–10−12 s. Moreover, it can easily be modified for other scattering techniques.

https://doi.org/10.1063/5.0030297

Laëtitia Mayen, Nicholai D. Jensen, Maximilien Desbord, Danielle Laurencin, Christel Gervais, Christian Bonhomme, Mark E. Smith, Florence Porcher, Erik Elkaim, Cédric Charvillat, Pierre Gras, Christian Rey, Jérémy Soulié and Christèle Combes, CrystEngComm, 22 (2020) 3130-3143.

α-Canaphite (CaNa₂P₂O₇·4H₂O) is a layered calcium disodium pyrophosphate tetrahydrate phase of significant geological and potential biological interest. This study overcomes the lack of a reliable protocol to synthesize pure α-canaphite by using a novel simple and reproducible approach of double decomposition in solution at room temperature.

The pure α-canaphite is then characterized from the atomic to the macroscopic level using a multitool and multiscale advanced characterization strategy, providing for the first time full resolution of the α-canaphite monoclinic structure, including the hydrogen bonding network. Synchrotron X-ray diffraction and neutron diffraction are combined with multinuclear solid state NMR experimental data and computational modeling via DFT/GIPAW calculations. Among the main characteristics of the α-canaphite structure are some strong hydrogen bonds and one of the four water molecules showing a different coordination scheme. This peculiar water molecule could be the last to leave the collapsed structure on heating, leading eventually to anhydrous α-CaNa₂P₂O₇ and could also be involved in the internal hydrolysis of pyrophosphate ions as it is the closest water molecule to the pyrophosphate ions.

Relating such detailed structural data on α-canaphite to its physico-chemical properties is of major interest considering the possible roles of canaphite for biomedical applications. The vibrational spectra of α-canaphite (deuterated or not) are analyzed and Raman spectroscopy appears to be a promising tool for the identification/diagnosis of such microcrystals in vitro, in vivo or ex vivo

https://doi.org/10.1039/D0CE00132E.

Jaehong Jeong, Benjamin Lenz, Arsen Gukasov, Xavier Fabrèges, Andrew Sazonov, Vladimir Hutanu, Alex Louat, Dalila Bounoua, Cyril Martins, Silke Biermann, Véronique Brouet, Yvan Sidis, and Philippe Bourges, Phys. Rev. Lett. 125 (2020) 097202.

5d iridium oxides are of huge interest due to the potential for new quantum states driven by strong spin-orbit coupling. The strontium iridate Sr₂IrO₄ is particularly in the spotlight because of the so-called j_eff = 1/2 state consisting of a quantum superposition of the three local t_2g orbitals with, in its simplest version, nearly equal populations, which stabilizes an unconventional Mott insulating state.

Here, we report an anisotropic and aspherical magnetization density distribution measured by polarized neutron diffraction in a magnetic field up to 5 T at 4 K, which strongly deviates from a local j_eff = 1/2 picture even when distortion-induced deviations from the equal weights of the orbital populations are taken into account. Once reconstructed by the maximum entropy method and multipole expansion model refinement, the magnetization density shows four cross-shaped positive lobes along the crystallographic tetragonal axes with a large spatial extent, showing that the xy orbital contribution is dominant. The analogy to the superconducting copper oxide systems might then be weaker than commonly thought.

https://doi.org/10.1103/PhysRevLett.125.097202.