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The diffusional behaviour of H2 adsorbed in a novel mesoporous N-doped carbon material and on exfoliated graphite has been studied with quasi-elastic neutron scattering. While the activation energies for diffusion obtained for exfoliated graphite was in line with prior results, H2 in the mesoporous carbon showed an exceptionally low diffusion barrier and behaved qualitatively like liquid hydrogen. Complementary results from neutron and X-ray diffraction as well as from isothermal nitrogen sorption studies provide a full characterisation of this novel mesoporous carbon.

International audience; Solid electrolytes are crucial for next-generation solid-state batteries, and Na 3 PS 4 is one of the most promising Na + conductors for such applications, despite outstanding questions regarding its structural polymorphs. In this contribution, we present a detailed investigation of the evolution in structure and dynamics of Na 3 PS 4 over a wide temperature range 30 < T < 600°C through combined experimental−computational analysis. Although Bragg diffraction experiments indicate a second-order phase transition from the tetragonal ground state (α, P4̅ 2 1 c) to the cubic polymorph (β, I4̅ 3m) above ∼250°C, pair distribution function analysis in real space and Raman spectroscopy indicate remnants of a tetragonal character in the range 250 < T < 500°C, which we attribute to dynamic local tetragonal distortions. The first-order phase transition to the mesophasic high-temperature polymorph (γ, Fddd) is associated with a sharp volume increase and the onset of liquid-like dynamics for sodium-cations (translational) and thiophosphate-polyanions (rotational) evident by inelastic neutron and Raman spectroscopies, as well as pair-distribution function and molecular dynamics analyses. These results shed light on the rich polymorphism of Na 3 PS 4 and are relevant for a range host of high-performance materials deriving from the Na 3 PS 4 structural archetype.

The processability and optoelectronic properties of organic semiconductors can be tuned and manipulated via chemical design. The substitution of the popular alkyl side chains by oligoethers has recently been successful for applications such as bioelectronic sensors and photocatalytic hydrogen evolution. Beyond the differences in polarity, the carbon–oxygen bond in oligoethers is likely to render the system softer and more prone to dynamical disorder that can be detrimental to charge transport for example. In this context, we use neutron spectroscopy as a master method of probe, in addition to characterisation techniques such as X-ray diffraction, differential scanning calorimetry and polarized optical microscopy to study the effect of the substitution of n-hexyl (Hex) chains by triethylene glycol (TEG) chains on the structural dynamics of two organic semiconducting materials: a phenylene–bithiophene–phenylene (PTTP) small molecule and a fluorene-co-dibenzothiophene (FS) polymer. Counterintuitively, inelastic neutron scattering (INS) reveals a general softening of the modes of PTTP and FS materials with Hex chains, pointing towards an increased dynamical disorder in the Hex-based systems. However, temperature-dependent X-ray and neutron diffraction as well as INS and differential scanning calorimetry evidence an extra reversible transition close to room temperature for PTTP with TEG chains. The observed extra structural transition, which is not accompanied by a change in birefringence, can also be observed by quasi-elastic neutron scattering (QENS). A fastening of the TEG chains dynamics is observed in the case of PTTP and not FS. We therefore assign this transition to the melt of the TEG chains. Overall the TEG chains are promoting dynamical order at room temperature, but if crystallising, may introduce an extra reversible structural transition above room temperature leading to thermal instabilities. Ultimately, a deeper understanding of chain polarity and structural dynamics can help guide new materials design and navigate the intricate balance between electronic charge transport and aqueous swelling that is being sought for a number of emerging organic electronic and bioelectronic applications.

Hydrogen fuel can contribute as a masterpiece in conceiving a robust carbon-free economic puzzle if cleaner methods to produce hydrogen become technically efficient and economically viable. Organic photocatalytic materials such as conjugated microporous materials (CMPs) are potential attractive candidates for water splitting as their energy levels and optical band gap as well as porosity are tunable through chemical synthesis. The performances of CMPs depend also on the mass transfer of reactants, intermediates, and products. Here, we study the mass transfer of water (H2O and D2O) and of triethylamine, which is used as a hole scavenger for hydrogen evolution, by means of neutron spectroscopy. We find that the stiffness of the nodes of the CMPs is correlated with an increase in trapped water, reflected by motions too slow to be quantified by quasi-elastic neutron scattering (QENS). Our study highlights that the addition of the polar sulfone group results in additional interactions between water and the CMP, as evidenced by inelastic neutron scattering (INS), leading to changes in the translational diffusion of water, as determined from the QENS measurements. No changes in triethylamine motions could be observed within the CMPs from the present investigations.

The quest for efficient and economically accessible cleaner methods to develop sustainable carbon-free energy sources induced a keen interest in the production of hydrogen fuel. This can be achieved via the water-splitting process and by exploiting solar energy. However, the use of adequate photocatalysts is required to reach this goal. Covalent triazine-based frameworks (CTFs) are potential target photocatalysts for water splitting. Both electronic and structural characteristics of CTFs, particularly energy levels, optical band gaps, and porosities are directly relevant to water splitting and can be engineered through chemical design. Porosity can, in principle, be beneficial to water splitting by providing a larger surface area for the catalytic reactions to take place. However, porosity can also affect both charge transport within the photocatalyst and mass transfer of both reactants and products, thus impacting the overall kinetics of the reaction. Here, we focus on the link between chemical design and water (reactant) mass transfer, which plays a key role in the water uptake process and the subsequent hydrogen generation in practice. We use neutron spectroscopy to study the mass transfer of water in two porous CTFs, CTF-CN and CTF-2, that differ in the polarity of their struts. Quasi-elastic neutron scattering is used to quantify the amount of bound water and the translational diffusion of water. Inelastic neutron scattering measurements complement the quasi-elastic neutron scattering study and provide insights into the softness of the CTF structures and the changes in librational degrees of freedom of water in the porous CTFs. We show that two different types of interaction between water and CTFs take place in CTF-CN and CTF-2. CTF-CN exhibits a smaller surface area and lower water uptake due to its softer structure than CTF-2. However, the polar cyano group interacts locally with water leading to a large amount of bound water and a strong rearrangement of the water hydration monolayer, while water diffusion in CTF-2 is principally impacted by microporosity. The current study leads to new insights into the structure-dynamics-property relationship of CTF photocatalysts that pave the road for a better understanding of the guest-host interaction on the basis of water-splitting applications.

International audience; The functionalization of nanoparticles is nowadays a standard procedure for many applications. Carbon nanotubes are no exception, and their functionalization has become an important part of the research effort these past decades. Nevertheless, there is still a lack of knowledge concerning the efficiency of the grafting processes, and more specifically concerning the question of the competition between covalent and non-covalent grafting when functionalized for example with aromatic fluorescent molecules. In a previous article, we investigated the grafting of fluorescein isothiocyanate onto double-walled carbon nanotubes. In order to explore the influence of the geometry of the fluorophore on the efficiency of the grafting processes, this work investigated the grafting of a streptocyanine onto the same carbon nanotubes using a combination of X-ray photoelectron spectroscopy, inelastic neutron scattering spectroscopy and computational simulations. The selected streptocyanine presents a very different geometry compared to FITC. Nevertheless, we show that, as in the case of FITC, both covalent and non-covalent grafting occurred simultaneously and that a comparable fraction of the fluorophore remained simply adsorbed onto the DWNTs despite thorough washings, indicating that the geometry of the streptocyanine did not exhibit the expected influence on the grafting process efficiency.

We present a combined experimental and theoretical study of the self-diffusion of ammonia on exfoliated graphite. Using neutron time-of-flight spectroscopy we are able to resolve the ultrafast diffusion process of adsorbed ammonia, NH3, on graphite. Together with van der Waals corrected density functional theory calculations we show that the diffusion of NH3 follows a hopping motion on a weakly corrugated potential energy surface with an activation energy of about 4 meV which is particularly low for this type of diffusive motion. The hopping motion includes further a significant number of long jumps and the diffusion constant of ammonia adsorbed on graphite is determined with D = 3.9 ⋅ 10 − 8 m 2 / s at 94 K.

Magnesium phosphate ceramics are a class of acid–base cements for bioengineering and civil engineering applications. We report on quasielastic neutron scattering results focusing on the evolution of the state of water in the system during the setting reaction, to shed light on the reaction mechanisms and the nature of the products. In the first few minutes, a consistent fraction of water molecules appears as immobile, and after a transient time, they start to be progressively bound into a reaction product. The kinetics of this last process has been described with an equation combining an Avrami model and a first-order reaction model with apparent activation energies of 18 and 6 kJ/mol, respectively. The results indicate that during the reaction the water molecules experience confinement effects inside a restricted space. The size of the confining volume decreases as the reaction progresses. It is proposed that an amorphous precursor with high surface area, bonding a relevant fraction of water, but also ho...

β-Eucryptite (LiAlSiO4) is known to show super-ionic conductivity above 700 K. We performed inelastic neutron scattering measurements in β-eucryptite over 300-900 K and calculated the phonon spectrum using classical molecular dynamics (MD) simulations. The MD simulations were used to interpret the inelastic neutron spectra at high temperatures. The calculated diffusion coefficient for Li showed superionic conduction above 1200 K in the perfect crystal. The presence of defects was found to enhance diffusion and lower the temperature for Li diffusion. The calculated trajectory of Li atoms at higher temperatures shows that preferential movement of the Li atom is along the hexagonal c-axis, which is further confirmed by the ab initio calculated activation energy profile for cooperative lithium ion displacements. The inter- and intra-channel correlated motion of Li along the hexagonal c-axis gives the minimum energy pathway for Li ion conduction in LiAlSiO4.

The structural dynamics of a quasi-one-dimensional (1D) mixed-metal cyanide, Cu1/3Ag1/3Au1/3CN, with intriguing thermal properties is explored. All the current known related compounds with straight-chain structures, such as group 11 cyanides CuCN, AgCN, AuCN and bimetallic cyanides MxM’1-xCN (M, M’ = Cu, Ag, Au), exhibit 1D negative thermal expansion (NTE) along the chains and positive thermal expansion (PTE) perpendicular to them. Cu1/3Ag1/3Au1/3CN exhibits similar PTE perpendicular to the chains, however PTE, rather than NTE, is also observed along the chains. In order to understand the origin of this unexpected behavior, inelastic neutron scattering (INS) measurements were carried out, underpinned by lattice-dynamical density-functional-theory (DFT) calculations. Synchrotron-based pair-distribution-function (PDF) analysis and 13C solid-state nuclear-magnetic-resonance (SSNMR) measurements were also performed to build an input structural model for the lattice dynamical study. The results indicate that transverse motions of the metal ions are responsible for the PTE perpendicular to the chains, as is the case for the related group 11 cyanides. However NTE along the chain due to the tension effect of these transverse motions is not observed. As there are different metal-to-cyanide bond lengths in Cu1/3Ag1/3Au1/3CN, the metals in neighboring chains cannot all be truly co-planar in a straight-chain model. For this system, DFT-based phonon calculations predict small PTE along the chain due to low-energy chain-slipping modes induced by a bond-rotation effect on the weak metallophilic bonds. However the observed PTE is greater than that predicted with the straight-chain model. Small bends in the chain to accommodate truly co-planar metals provide an alternative explanation for thermal behavior. These would mitigate the tension effect induced by the transverse motions of the metals and, as temperature increases and the chains move further apart, a straightening could occur resulting in the observed PTE. This hypothesis is further supported by unusual evolution in the phonon spectra, which suggest small changes in local symmetry with temperature.

This study explores the relationship between phonon dynamics and negative thermal expansion (NTE) in CuxNi2-x¬(CN)4. The partial replacement of nickel (II) by copper (II) in Ni(CN)2 leads to a line phase, CuNi(CN)4 (x = 1), and a solid solution, CuxNi2-x¬(CN)4 (0 ≤ x ≤ 0.5). CuNi(CN)4 adopts a layered structure related to that of Ni(CN)2¬ (x = 0), and interestingly exhibits 2D NTE which is ~ 1.5 times larger. Inelastic neutron scattering (INS) measurements combined with first principles lattice dynamical calculations provide insights into the effect of Cu2+ on the underlying mechanisms behind the anomalous thermal behavior in all the CuxNi2-x¬(CN)4 compounds. The solid solutions are presently reported to also show 2D NTE. The INS results highlight that as the Cu2+ content increases in CuxNi2-x(CN)4, large shifts to lower energies are observed in modes consisting of localized in- and out-of-plane librational motions of the CN ligand, which contribute to the NTE in CuNi(CN)4. Mode Gruneisen parameters calculated for CuNi(CN)4 show that acoustic and low-energy optic modes contribute the most to the NTE, as previously shown in Ni(CN)2. However, mode eigenvectors reveal a large deformation of the [CuN4] units compared to the [NiC4] units, resulting in phonon modes not found in Ni(CN)2, whose NTE-driving phonons consist predominately of rigid-unit modes. The deformations in CuNi(CN)4 arise because the d9 square-planar center is easier to deform than the d8 one, resulting in a greater range of out-of-plane motions for the adjoining ligands.

ZnNi(CN)4 is a 3-D framework material consisting of two interpenetrating PtS-type networks in which tetrahedral [ZnN4] units are linked by square-planar [NiC4] units. Both the parent compounds, cubic Zn(CN)2 and layered Ni(CN)2, are known to exhibit 3-D and 2-D negative thermal expansion (NTE), respectively. Temperature-dependent inelastic neutron scattering (INS) measurements were performed on a powdered sample of ZnNi(CN)4 to probe phonon dynamics. The measurements were underpinned by ab-initio lattice dynamical calculations. Good agreement was found between the measured and calculated generalized phonon density-of-states, validating our theoretical model and indicating that it is a good representation of the dynamics of the structural units. The calculated linear thermal expansion coefficients are αa = -21.2 × 10-6 K-1 and αc = +14.6 × 10-6 K-1, leading to an overall volume expansion coefficient, αV of -26.95 × 10-6 K-1, pointing towards pronounced NTE behaviour. Analysis of the derived mode-Gruneisen parameters shows that the optic modes around 12 and 40 meV make a significant contribution to the NTE. These modes involve localised rotational motions of the [NiC4] and/or [ZnN4] rigid units, echoing what has previously been observed in Zn(CN)2 and Ni(CN)2. However, in ZnNi(CN)4, modes below 10 meV have the most negative Gruneisen parameters. Analysis of their eigenvectors reveals that a large transverse motion of the Ni atom in the direction perpendicular to its square-planar environment induces a distortion of the units. This mode is a consequence of the Ni atom being constrained only in two dimensions within a 3-D framework. Hence, although rigid-unit modes account for some of the NTE-driving phonons, the added degree of freedom compared with Zn(CN)2 results in modes with twisting motions, capable of inducing greater NTE.

The molecular dynamics of organic semiconductor blend layers are likely to affect the optoelectronic properties and the performance of devices such as solar cells. We study the dynamics (5-50 ps) of the poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) blend by time-of-flight quasi-elastic neutron scattering, at temperatures in the range 250-360 K, thus spanning the glass transition temperature region of the polymer and the operation temperature of an OPV device. The behavior of the QENS signal provides evidence for the vitrification of P3HT upon blending, especially above the glass transition temperature, and the plasticization of PCBM by P3HT, both dynamics occurring on the picosecond time scale.

We report powder neutron diffraction studies in $\mathrm{L}{\mathrm{a}}_{0.95}\mathrm{P}{\mathrm{O}}_{4}:\mathrm{E}{\mathrm{u}}_{0.05}\phantom{\rule{4pt}{0ex}}(\mathrm{LaP}{\mathrm{O}}_{4}:\mathrm{Eu})$ at various temperatures up to 1163 K. From Rietveld refinement of the diffraction data, we identify the presence of water up to 567 K and an irreversible phase transition from hexagonal to monoclinic structure around 900 K involving the coexistence of phases over 700 to 1050 K. We observe from the neutron experiments that the thermal expansion in the hexagonal phase is much smaller than that in the monoclinic phase. We have performed first-principles lattice dynamics calculations in both the phases to identify the anharmonic phonons and thereby understand the thermal expansion behavior. The low-energy translational and librational modes of $\mathrm{P}{\mathrm{O}}_{4}$ tetrahedral units in the hexagonal phase show negative thermal expansion behavior, which is responsible for the overall smaller positive thermal expansion coefficient in the hexagonal phase in comparison to that in the monoclinic phase.

We have performed temperature dependent inelastic neutron scattering measurements to study the anharmonicity of phonon spectra of AgC4N3. The analysis and interpretation of the experimental spectra is done using ab-initio lattice dynamics calculations. The calculated phonon spectrum over the entire Brillouin zone is used to derive linear thermal expansion coefficients. The effect of van der Waals interaction on structure stability has been investigated using advanced density functional methods. The calculated isothermal equation of states implies a negative linear compressibility along the c-axis of the crystal, which also leads to a negative thermal expansion along this direction. The role of elastic properties inducing the observed anomalous lattice behavior is discussed.

Layered copper-nickel cyanide, CuNi(CN)4, a 2-D negative thermal expansion material, is one of a series of copper(ii)-containing cyanides derived from Ni(CN)2. In CuNi(CN)4, unlike in Ni(CN)2, the cyanide groups are ordered generating square-planar Ni(CN)4 and Cu(NC)4 units. The adoption of square-planar geometry by Cu(ii) in an extended solid is very unusual.

We have experimentally identified a different magnetic ground state for the kagome lattice, in the perfectly hexagonal ${\mathrm{Fe}}^{2+}$ ($3{d}^{6},\phantom{\rule{0.16em}{0ex}}S=2$) compound ${\mathrm{Fe}}_{4}{\mathrm{Si}}_{2}{\mathrm{Sn}}_{7}{\mathrm{O}}_{16}$. A representational symmetry analysis of neutron diffraction data shows that below ${T}_{N}=3.5$ K, the spins on $\frac{2}{3}$ of the magnetic ions order into canted antiferromagnetic chains, separated by the remaining $\frac{1}{3}$ which are geometrically frustrated and show no long-range order down to at least $T=0.1$ K. M\"ossbauer spectroscopy confirms that there is no static order on the latter $\frac{1}{3}$ of the magnetic ions---i.e., they are in a liquidlike rather than a frozen state---down to at least 1.65 K. A heavily Mn-doped sample ${\mathrm{Fe}}_{1.45}{\mathrm{Mn}}_{2.55}{\mathrm{Si}}_{2}{\mathrm{Sn}}_{7}{\mathrm{O}}_{16}$ has the same magnetic structure. Although the propagation vector $q=\left(0,\frac{1}{2},\frac{1}{2}\right)$ breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. We discuss the relationship to partially frustrated magnetic order on the pyrochlore lattice of ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, and to theoretical models that predict symmetry breaking ground states for perfect kagome lattices.

We report inelastic neutron scattering measurements of phonon spectra in the parent superconductor iron-tuned chalcogenide $\mathrm{F}{\mathrm{e}}_{1+x}\mathrm{Te}$ for two different $x$ contents $(x\ensuremath{\le}0.11)$ using neutron time-of-flight technique. Thermal neutron spectroscopy allowed the collection of the low-temperature Stokes spectra over an extended $Q$ range at 2, 40, and 120 K, hence covering both the magnetic monoclinic and the paramagnetic tetragonal phases, whereas cold neutrons allowed the measurement of high-resolution anti-Stokes spectra at 140, 220, and 300 K, thus covering the tetragonal phase. Our results evidence a spin-phonon coupling behavior towards the observed noticeable temperature-dependent change of the Stokes spectra across the transition temperatures. On the other hand, the anti-Stokes spectra reveal a pronounced hardening of the low-energy, acoustic region of the phonon spectrum upon heating, indicating a strong anharmonicity and a subtle dependence of phonons on structural evolution within the tetragonal phase. Experimental results are accompanied by ab initio calculations of phonon spectra of the tetragonal stoichiometric phase for a comparison with the high-resolution anti-Stokes spectra. Calculations included different density functional methods. Spin polarization and van der Waals interaction were either considered or neglected, individually or concomitantly, in order to study their respective effect on lattice dynamics description. Our results suggest that including van der Waals interaction has only a slight effect on phonon dynamics; however, phonon spectra are better described when spin polarization is included in a cooperative way with van der Waals interactions.

The optoelectronic properties of blends of conjugated polymers and small molecules are likely to be affected by the molecular dynamics of the active layer components. We study the dynamics of regioregular poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular dynamics (MD) simulation on time scales up to 50 ns and in a temperature range of 250–360 K. First, we compare the MD results with quasi-elastic neutron-scattering (QENS) measurements. Experiment and simulation give evidence of the vitrification of P3HT upon blending and the plasticization of PCBM by P3HT. Second, we reconstruct the QENS signal based on the independent simulations of the three phases constituting the complex microstructure of such blends. Finally, we found that P3HT chains tend to wrap around PCBM molecules in the amorphous mixture of P3HT and PCBM; this molecular interaction between P3HT and PCBM is likely to be responsible for the observed frustration of P3HT, the plasticization of PCBM, and the partial miscibility of P3HT and PCBM.

We provide experimental evidence for confinement of water molecules in the pores of hexagonal structure of YPO4 at elevated temperatures upto 600 K using powder neutron diffraction. In order to avoid the large incoherent scattering from the hydrogen, deuterated samples of doped YPO4:Ce-Eu were used for diffraction measurements. The presence of water molecules in the triangular and hexagonal pores in the hexagonal structure was established by detailed simulation of the diffraction pattern and Rietveld refinement of the experimental data. It was observed that the presence of water leads specifically to suppression of the intensity of a peak around Q = 1.04 {\AA}-1while the intensity of peaks around Q=1.83{\AA}-1 is enhanced in the neutron diffraction pattern. We estimate the number of water molecules as 2.36 (6) per formula units at 300 K and the sizes of the hexagonal and triangular pores as7.2 (1) {\AA} and 4.5 (1) {\AA}, respectively. With increase in temperature, the water content in both the pores decreases above 450 K and vanishes around 600 K. Analysis of the powder diffraction data reveals that the hexagonal structure with the pores persist up to 1273 K, and transforms to another structure at 1323 K. The high temperature phase is not found to have the zircon or the monazite type structure, but a monoclinic structure (space group P2/m) with lattice parameters am= 6.826 (4) {\AA}, bm= 6.645 (4) {\AA}, cm= 10.435 (9){\AA}, and \b{eta}= 107.21 (6){\deg}. The monoclinic structure has about 14 % smaller volume than the hexagonal structure which essentially reflects the collapse of the pores. The phase transition and the change in the volume are also confirmed by x-ray diffraction measurements. The hexagonal to the monoclinic phase transition is found to be irreversible on cooling to room temperature., Comment: 16 Pages, 8 Figures

Quasielastic neutron scattering (QENS), underpinned by ab initio molecular dynamics (AIMD) simulations, has been used to directly observe oxide ion dynamics in solid electrolyte La2Mo2O9 on the nanosecond timescale, the longest timescale probed in oxide ion conductors by neutron scattering to date. QENS gives the activation energy of 0.61(5) eV for this process, while AIMD simulations reveal that the exchange processes, which ultimately lead to long-range oxide ion diffusion in La2Mo2O9, rely on the flexibility of the coordination environment around Mo6+, with oxide ions jumps occurring between vacant sites both within and between Mo coordination spheres. Simulations also differentiate between the crystallographic sites which participate in the oxide ion exchange processes, offering the first atomic-level understanding of the oxide ion dynamics in La2Mo2O9, which is consistent with the macroscopic experimental observations on this material.

The functionalization of carbon nanotubes with fluorescent molecules is a standard procedure in many toxicity studies aiming at knowing their distribution within cells or whole organisms. Nevertheless, there is a lack of knowledge concerning the efficiency of the grafting processes, and more specifically concerning the question of the competition between covalent and non-covalent grafting. In this work, we investigated the grafting process of the fluorescein isothiocyanate (FITC) onto double-walled carbon nanotubes (DWNTs) using X-ray photoelectron spectroscopy, inelastic neutron scattering spectroscopy and computational simulations.We demonstrated that both covalent and non-covalent grafting occurred during the functionalization with the FITC. Moreover, we showed that a significant fraction of the fluorophore remained simply adsorbed onto the DWNTs despite thorough washing steps, which raises concerning questions about the use of this fluorophore in some toxicity studies and its possible ability to mislead their conclusions., The authors would like to acknowledge the grants from the Université Paul Sabatier (UPS) and the Institut Laue-Langevin (ILL) (number ILL-1389.1).

${\mathrm{Ba}}_{3}{\mathrm{BiRu}}_{2}{\mathrm{O}}_{9}$ is a 6H-type perovskite compound containing face-sharing octahedral ${M}_{2}{\mathrm{O}}_{9}$ ($M=\mathrm{Ir}$, Ru) dimers, which are magnetically frustrated at low temperatures. On cooling through ${T}^{*}=176$ K, it undergoes a pronounced magnetostructural transition, which is not accompanied by any change in space group symmetry, long-range magnetic ordering, or charge ordering. Here, we report the first direct evidence from inelastic neutron scattering that this transition is due to an opening of a gap in the excitation spectra of dimers of low-spin ${\mathrm{Ru}}^{4+}$ ($S=1$) ions. X-ray absorption spectroscopy reveals a change in Ru-Ru orbital overlap at ${T}^{*}$, linking the emergence of this spin-gap excitation to the magnetostructural transition. Ab initio calculations point to a geometrically frustrated magnetic ground state due to antiferromagnetic interdimer exchange on a triangular ${\mathrm{Ru}}_{2}{\mathrm{O}}_{9}$ dimer lattice. X-ray total-scattering data rule out long-range magnetic ordering at low temperatures, consistent with this geometrically frustrated model.

Organic conducting materials exhibit promising functionalities, inducing hence a keen interest due to their potential use as a next generation photoconverters. However, unlike the more expensive inorganic analogues, the underlying properties that give rise to these advantages also cause organic materials to be inherently inefficient as photovoltaics. Understanding their properties at the microscopic level is a major step towards an efficient and targeted design. We probed the morphological and dynamical aspects of a model organic discotic liquid crystal material hexakis(n-hexyloxy)triphenylene (HAT6) by using neutron-based diffraction and quasielastic scattering techniques to gain deeper insights into structure and dynamics. The neutron measurements are accompanied, in a synergistic way, by molecular dynamics simulations for the sake of the analysis and interpretation of the observations.

The hexagonal perovskites Ba3BiIr2O9, Ba3BiRu2O9, and Ba4BiIr3O12 all undergo pressure-induced 1% volume collapses above 5 GPa. These first-order transitions have been ascribed to internal transfer of valence electrons between bismuth and iridium/ruthenium, which is driven by external applied pressure because the reduction in volume achieved by emptying the 6s shell of bismuth upon oxidation to Bi(5+) is greater in magnitude than the increase in volume by reducing iridium or ruthenium. Here, we report direct observation of these valence transfers for the first time, using high-pressure X-ray absorption near-edge spectroscopy (XANES) measurements. Our data also support the highly unusual "4+" nominal oxidation state of bismuth in these compounds, although the possibility of local disproportionation into Bi(3+)/Bi(5+) cannot be definitively ruled out. Ab initio calculations reproduce the transition, support its interpretation as a valence electron transfer from Bi to Ir/Ru, and suggest that the high-pressure phase may show metallic behavior (in contrast to the insulating ambient-pressure phase).

Nickel cyanide is a layered material showing markedly anisotropic behaviour. High-pressure neutron diffraction measurements show that at pressures up to 20.1 kbar, compressibility is much higher in the direction perpendicular to the layers, c, than in the plane of the strongly chemically bonded metal-cyanide sheets. Detailed examination of the behaviour of the tetragonal lattice parameters, a and c, as a function of pressure reveal regions in which large changes in slope occur, for example, in c(P) at 1 kbar. The experimental pressure dependence of the volume data is fitted to a bulk modulus, B0, of 1050 (20) kbar over the pressure range 0-1 kbar, and to 124 (2) kbar over the range 1-20.1 kbar. Raman spectroscopy measurements yield additional information on how the structure and bonding in the Ni(CN)2 layers change with pressure and show that a phase change occurs at about 1 kbar. The new high-pressure phase, (Phase PII), has ordered cyanide groups with sheets of D4h symmetry containing Ni(CN)4 and Ni(NC)4 groups. The Raman spectrum of phase PII closely resembles that of the related layered compound, Cu1/2Ni1/2(CN)2, which has previously been shown to contain ordered C≡N groups. The phase change, PI to PII, is also observed in inelastic neutron scattering studies which show significant changes occurring in the phonon spectra as the pressure is raised from 0.3 to 1.5 kbar. These changes reflect the large reduction in the interlayer spacing which occurs as Phase PI transforms to Phase PII and the consequent increase in difficulty for out-of-plane atomic motions. Unlike other cyanide materials e.g. Zn(CN)2 and Ag3Co(CN)6, which show an amorphization and/or a decomposition at much lower pressures (~100 kbar), Ni(CN)2 can be recovered after pressurising to 200 kbar, albeit in a more ordered form.

The dynamics of conjugated polymers are known to influence the performance of optoelectronic devices. Polyalkylthiophenes are a widely studied class of conjugated polymers, which exhibit a glass transition around room temperature and consequently are sensitive to temperature variations. We studied the dynamics of two polyalkylthiophenes of different side chain lengths (hexyl and octyl) as a function of temperature, by comparing their quasi-elastic neutron scattering (QENS) with molecular dynamics simulations (MD). We found a good agreement between the simulated and experimental data within the explored time window (of ∼4 ns), demonstrating that the force fields used in MD simulations are appropriate and that the QENS technique can be used as a validation of such force fields. Using MD allows us to identify and to assign contributions to the QENS signal from different parts of the polymers and to determine the activation energies of the different motions.

β-Eucryptite (LiAlSiO4) shows anisotropic thermal expansion as well as one-dimensional super-ionic conductivity. We have performed the lattice dynamical calculations using ab-initio density functional theory along with inelastic neutron scattering measurements. The anisotropic stress dependence of the phonon spectrum is calculated to obtain the thermal expansion behavior along various axes. The calculations show that the Gruneisen parameters of the low-energy phonon modes around 10 meV have large negative values and govern the negative thermal expansion behavior at low temperatures along both the “a”- and “c”-axes. On the other hand, anisotropic elasticity along with anisotropic positive values of the Gruneisen parameters of the high-energy modes in the range 30–70 meV are responsible for the thermal expansion at high temperatures, which is positive in the a-b plane and negative along the c-axis. The analysis of the polarization vectors of the phonon modes sheds light on the mechanism of the anomalous the...

International audience; Nonstoichiometric Nd2NiO4+δ shows oxygen ion mobility already at roomtemperature and as such is a promising material in the field of solid state ionic conductors.The present work aims to understand the impact of oxygen overstoichiometry on the lattice dynamics of the Nd2NiO4 framework and to correlate dynamic instabilities to the anionic mobility at room temperature. We performed neutron time-of-flight spectroscopy experiments coupled with DFT-based molecular dynamics calculations on the phases Nd2NiO4.0, Nd2NiO4.10, and Nd2NiO4.25. Specific signatures in the phonon density of states gave evidence that excess oxygen on interstitial lattice sites activates large displacements of the apical oxygen atoms along the [110] direction, thus favoring their diffusion toward interstitial sites within the rock salt-type layer already at ambient temperature. We implemented a position recurrence method to analyze apical oxygen displacements during the 40 ps time scale of molecular dynamics simulations. The resultshighlighted that dynamical delocalization of apical oxygen atoms along [110] is necessary to observe diffusion at room temperature, and thus ionic mobility at room temperature is activated by lattice dynamics

Correction for ‘Spin-phonon coupling and high-temperature phase transition in multiferroic material YMnO3’ by Mayanak Kumar Gupta et al., J. Mater. Chem. C, 2015, 3, 11717–11728.

We are interested in the phonon response in the frustrated magnets ${\text{SrCr}}_{9x}{\text{Ga}}_{12\ensuremath{-}9x}{\text{O}}_{19}$ (SCGO) and ${\text{Ba}}_{2}{\text{Sn}}_{2}{\text{ZnCr}}_{7x}{\text{Ga}}_{10\ensuremath{-}7x}{\text{O}}_{22}$ (BSZCGO). The motivation of the study is the recently discovered, phonon-driven, magnetic relaxation in the SCGO compound [Mutka et al., Phys. Rev. Lett. 97, 047203 (2006)] pointing out the importance of a low-energy $(\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\sim}7\text{ }\text{meV})$ phonon mode. In neutron-scattering experiments on these compounds, the phonon signal is partly masked by the magnetic signal from the Cr moments and we have therefore examined in detail the nonmagnetic isostructural counterparts ${\text{SrGa}}_{12}{\text{O}}_{19}$ (SGO) and ${\text{Ba}}_{2}{\text{Sn}}_{2}{\text{ZnGa}}_{10}{\text{O}}_{22}$ (BSZGO). Our ab initio lattice-dynamics calculations on SGO reveal a peak in the vibrational density of states matching with the neutron observations on SGO and SCGO. A strong contribution in the vibrational density of states comes from the partial contribution of the Ga atoms on the $2b$ and $12k$ sites, involving modes at the M point of the hexagonal system. These modes comprise dynamics of the Kagom\'e planes of the pyrochlore slab magnetic sublattice, $12k$ sites, and therefore can drive magnetic relaxation via spin-phonon coupling. Both BSZCGO and BSZGO show a similar low-energy Raman peak but no corresponding peak in the neutron-determined density of states of BSZGO is seen. However, a strong non-Debye enhancement of low-energy phonon response is observed. We attribute this particular feature to the Zn/Ga disorder on the $2d$ site, already evoked earlier to affect the magnetic properties of BSZCGO. We propose that this disorder-induced phonon response explains the absence of a characteristic energy scale and the much faster magnetic relaxation observed in BSZCGO.