Laboratoire de Cristallographie et Sciences des Matériaux

Abstract The search for single-ion magnetism (SIM) has been largely focused on hybrid metal-organic systems. Here, we experimentally demonstrate the spin-relaxation mechanism of SIM in a pure inorganic transition-metal oxide, Sr2Ca2Mn2CoO9, an Ising-chain magnet, employing neutron diffraction and inelastic neutron scattering (INS), complemented by SpinW simulations and a machine-learning framework. Interestingly, SIM mechanism persists even in the presence of long-range magnetic ordering, a phenomenon that is rarely observed. This pioneering investigation shows that bistability between the two quantum states Ms=±3/2 is maintained at zero magnetic field, driven by a dominant Orbach spin-relaxation mechanism with an effective energy barrier, U=4.4 meV. This behaviour arises from strong spin-phonon coupling in the presence of negative axial anisotropy (D approx -2.2meV) and the high-spin state S=3/2 of the Co(II) ion. Furthermore, we observe that magnon excitations persist up to room temperature, reflecting low-dimensional magnetic interactions and extended magnetic correlations within the oxide lattice. This interplay between single-ion magnetism and room-temperature spin excitations underscores the chemical tunability of magnetic anisotropy in oxide lattices.

Abstract The presence of a quantum-critical point (QCP) at which a nearby ordered phase is suppressed to zero temperature is often invoked to explain emergent quantum phases, e.g. superconductivity. Yet, identifying a QCP and establishing its correlation with superconductivity remains challenging. Materials featuring charge-density-wave (CDW) order and superconductivity offer a clear scenario as both states can be associated with electron-phonon coupling. Here, we uncover a CDW-QCP and demonstrate its interrelation with superconductivity in the prototypical transition-metal dichalcogenide 2 H -TaSe 2 . We determine the evolution of the CDW state up to and beyond its suppression at the critical pressure p c = 19.9(1) GPa by means of X-ray diffraction and inelastic X-ray scattering measurements providing a full crystallographic refinement of the commensurate CDW superstructure. The pressure-induced CDW-QCP in close vicinity to the maximum superconducting transition temperature. Ab-initio lattice dynamical calculations corroborate that 2 H -TaSe 2 features order-parameter fluctuation enhanced superconductivity and can serve as a paradigm to investigate superconductivity near a CDW-QCP.

Lead halide perovskites (LHPs) have disrupted the field of visible-range emitting colloidal semiconductor nanocrystals (NCs) owing to their exceptional emissivity as classical and quantum light sources. Their appeal is countered by the stability challenges arising from inherent lattice softness and labile surface ligand bonding. Recent ligand design efforts have drastically improved the structural integrity of LHP NCs by pursuing strongly and statically bound ligands, as found with zwitterionic head groups. Here we venture into a new class of cationic ligands featuring guanidinium head groups, which combine the compactness of primary ammoniums with the deprotonation resistance of quaternary ammoniums. We reasoned that strong yet dynamic ligand binding offers distinct benefits. A library of aliphatic guanidinium molecules was synthesized and surveyed as capping ligands for CsPbBr3, FAPbBr3, and CsPbI3 NCs, enabling up to 95 % photoluminescence quantum yields for bromide compositions. We then compare binding behavior of guanidinium ligands with those of other widely used ligands. Zwitterionic ligands phosphocholines and phosphoethanolamines bind more strongly than sulfobetaines and quaternary ammonium ligands, yet all these head groups impart relatively static binding with exchange rates below 5 s-1. Primary ammonium ligands champion in dynamicity but lag behind in ligand-coverage retention and, hence, long-term stability. Guanidinium ligands strike a favorable balance: they match the binding dynamics of primary ammonium ligands, while significantly enhancing binding strength, enabling rigorous purification and compatibility with relatively polar solvents such as tetrahydrofuran. We then showcase the benefits of dynamically bound ligands in photocatalytic C-C coupling reactions.

Transition-metal oxides with an $AB{\mathrm{O}}_{3}$ perovskite structure exhibit significant coupling between spin, orbital, and lattice degrees of freedom, highlighting the crucial role of lattice distortion in tuning the electronic and magnetic properties of these systems. In this study, we grow a series of antiferromagnetic ${\mathrm{GdVO}}_{3}$ thin films on different substrates introducing various strain values. The results demonstrate that the strain not only affects the material's structure and magnetic transition temperature, but also influences the growth direction of the orthorhombic unit cell. A correlation is observed between the orientation of the orthorhombic long axis direction and the material's N\'eel temperature. Density functional theory calculations highlighting the link between strain, lattice distortions, and magnetic characteristics confirm these conclusions. Our findings demonstrate that the development of novel features in $R{\mathrm{VO}}_{3}$ through material design requires control of growth orientation through strain engineering.

Mechanochemistry has been widely used to enable the formation of homogenous mixed phases, particularly in the field of fluoride ion conductors. In this work, we have investigated the effect of applying long duration time of high energy ball milling on the formation of disordered fluorite BaSnF 4 . We have systematically compared the properties of two samples, one prepared using a commonly reported protocol and one obtained after a longer duration. Microstructural analysis obtained by X-ray line refinement and Williamson–Hall diagrams showed that increasing the ball milling time resulted in a decrease in particle size from 13 to 6 nm and suppressed the occurrence of microstrains, which were identified as twinned regions by microscopy analysis. Prolongation of the ball milling treatment is also associated with a decrease in ionic conductivity with identical fluoride ion jump activation energy. 119 Sn Mössbauer spectroscopy revealed the presence of a new signature assigned to Sn(IV) in an oxide–fluoride environment. We hypothesized that this new environment originates from the formation of an anion-excess fluorite-type phase <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mi mathvariant="normal">BaSn</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>-</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi mathvariant="normal">II</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:msubsup> <mml:msubsup> <mml:mi mathvariant="normal">Sn</mml:mi> <mml:mrow> <mml:mi>x</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi mathvariant="normal">IV</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:msubsup> <mml:msub> <mml:mi mathvariant="normal">F</mml:mi> <mml:mn>4</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">O</mml:mi> <mml:mi>x</mml:mi> </mml:msub> </mml:mrow> </mml:math> induced by prolonged milling. Such a hypothesis is supported by the occurrence of fluoride ions located within octahedral sites, which are likely to be bound to Sn(IV) resulting to their lower mobility as shown by 19 F solid-state NMR. Overall, this work demonstrates that the interplay between the microstructure, composition and the transport properties of fluoride ion conductors is indeed complex, with many factors at play, including size, strain, defects or composition.

In this study, we investigate the magnetic and magnetocaloric properties of Cr-based spinel sulphides Fe1-xCuxCr2S4 (x = 0.0 and x = 0.5). The magnetic transition temperature of FeCr2S4 (176 K) increases to 330 K when 50 % of Fe in FeCr2S4 structure is substituted with Cu. Temperature-dependent Zero-Field-Cooled (ZFC) and Field-Cooled (FC) magnetic moment measurements show a paramagnetic (PM) to ferrimagnetic (FiM) phase transition in our samples. Magnetocaloric properties were investigated through magnetization isotherm measurements. The maximum magnetic entropy change values are −2.78 J/kg. K and −2.11 J/kg. K under 5 T magnetic field, for x = 0 and x = 0.5 in Fe1-xCuxCr2S4, respectively. Analysis of phenomenological universal curves and Arrott plots indicate a second-order magnetic transition for both compounds. Consequently, Fe0.5Cu0.5Cr2S4 emerges as a promising candidate for magnetic refrigeration applications due to its considerable high magnetic entropy change and its proximity to room temperature transition.

The fabrication of large silicon nitride ceramic components is intricate and demands expertise in gas pressure liquid phase sintering (GPS-LPS). The forefront technology of finite element (FEM) thermo-mechanical modeling plays a key role in sizing aerospace components that necessitate high precision and mechanical strength. A comprehensive sintering simulation should encompass not only densification and grain growth models but also consider the heating environment, accounting for gas convection, conduction, and surface-to-surface radiation. Moreover, silicon nitride sintering simulation addresses the intricate behavior, including the swelling phenomenon during the transitions from intermediate to final stage sintering. The challenge of significant slumping in hollow parts, resulting in costly rejection, is a primary concern for industrial companies when dealing with low-viscosity liquid phase sintering. Validation of the model is achieved through the computation of simple cylindrical cases, while a meticulous comparison between simulations and sintering experiments on complex hollow bodies and deflection bars facilitates precise adjustment of the shear viscosity theoretical parameter derived from the Skorohod-Olevsky continuum theory of sintering. The findings from these simple cases guide the simulation of a complex geometry representative of typical aerospace components, determining the ideal sintering configuration to mitigate slumping issues.