Large-cage occupation and quantum dynamics of hydrogen molecules in sII clathrate hydrates.

Hydrogen clathrate hydrates are ice-like crystalline substances in which hydrogen molecules are trapped inside polyhedral cages formed by the water molecules. Small cages can host only a single H2 molecule, while each large cage can be occupied by up to four H2 molecules. Here, we present a neutron scattering study on the structure of the sII hydrogen clathrate hydrate and on the low-temperature dynamics of the hydrogen molecules trapped in its large cages, as a function of the gas content in the samples. We observe spectral features at low energy transfer (between 1 and 3 meV), and we show that they can be successfully assigned to the rattling motion of a single hydrogen molecule occupying a large water cage. These inelastic bands remarkably lose their intensity with increasing the hydrogen filling, consistently with the fact that the probability of single occupation (as opposed to multiple occupation) increases as the hydrogen content in the sample gets lower. The spectral intensity of the H2 rattling bands is studied as a function of the momentum transfer for partially emptied samples and compared with three distinct quantum models for a single H2 molecule in a large cage: (i) the exact solution of the Schrödinger equation for a well-assessed semiempirical force field, (ii) a particle trapped in a rigid sphere, and (iii) an isotropic three-dimensional harmonic oscillator. The first model provides good agreement between calculations and experimental data, while the last two only reproduce their qualitative trend. Finally, the radial wavefunctions of the three aforementioned models, as well as their potential surfaces, are presented and discussed.

Chemical short-range order in liquid Ni-Cu.

Neutron diffraction in combination with isotopic substitution on the zero-scatterer62Ni4363Cu57shows indications for chemical short-range order in the stable liquid as evidenced by oscillations in the concentration-concentration structure factorSCC(q). This points towards a non-ideal solution behavior of Ni-Cu contrary to common believe but in agreement with measurements of free enthalpy of mixing. The temperature dependence ofSCCat small momentum transfer provides evidence of critical compositional fluctuations in Ni43Cu57melts.

Exsolution of Co-Fe Alloy Nanoparticles on the PrBaFeCoO5+δ Layered Perovskite Monitored by Neutron Powder Diffraction and Catalytic Effect on Dry Reforming of Methane.

Reversible exsolution and dissolution of metal nanoparticles (NPs) in complex oxides have been investigated as an efficient strategy to improve the performance and durability of the catalysts for thermal and electrochemical energy conversion. Here, in situ exsolution of Co-Fe alloy NPs from the layered perovskite PrBaFeCoO5+δ (PBFC) and their dissolution back into the oxide host have been monitored for the first time by in situ neutron powder diffraction and confirmed by X-ray diffraction and electron microscopy. Catalytic tests for dry reforming of methane showed stable operation over ∼100 h at 800 °C with negligible carbon deposition (<0.3 mg/gcat h). The CO2 and CH4 conversions are among the highest achieved by layered double perovskites. The cyclability of the PBFC catalyst and the potential to improve the catalytic activity by adjusting the composition, size, and the NP distribution would pave the way for highly efficient energy conversion applications.

Multi-Technique Experimental Benchmarking of the Local Magnetic Anisotropy of a Cobalt(II) Single-Ion Magnet.

A comprehensive understanding of the ligand field and its influence on the degeneracy and population of d-orbitals in a specific coordination environment are crucial for the rational design and enhancement of magnetic anisotropy of single-ion magnets (SIMs). Herein, we report the synthesis and comprehensive magnetic characterization of a highly anisotropic CoII SIM, [L2Co](TBA)2 (L is an N,N'-chelating oxanilido ligand), that is stable under ambient conditions. Dynamic magnetization measurements show that this SIM exhibits a large energy barrier to spin reversal U eff > 300 K and magnetic blocking up to 3.5 K, and the property is retained in a frozen solution. Low-temperature single-crystal synchrotron X-ray diffraction used to determine the experimental electron density gave access to Co d-orbital populations and a derived U eff, 261 cm-1, when the coupling between the d x 2 - y 2 and dxy orbitals is taken into account, in very good agreement with ab initio calculations and superconducting quantum interference device results. Powder and single-crystal polarized neutron diffraction (PNPD, PND) have been used to quantify the magnetic anisotropy via the atomic susceptibility tensor, revealing that the easy axis of magnetization is pointing along the N-Co-N' bisectors of the N,N'-chelating ligands (3.4° offset), close to the molecular axis, in good agreement with complete active space self-consistent field/N-electron valence perturbation theory to second order ab initio calculations. This study provides benchmarking for two methods, PNPD and single-crystal PND, on the same 3d SIM, and key benchmarking for current theoretical methods to determine local magnetic anisotropy parameters.

Revealing the Magnetic Structure and Properties of Mn(Co,Ge)2.

The atomic and magnetic structures of Mn(Co,Ge)2 are reported herein. The system crystallizes in the space group P63/mmc as a superstructure of the MgZn2-type structure. The system exhibits two magnetic transitions with associated magnetic structures, a ferromagnetic (FM) structure around room temperature, and an incommensurate structure at lower temperatures. The FM structure, occurring between 193 and 329 K, is found to be a member of the magnetic space group P63/mm'c'. The incommensurate structure found below 193 K is helical with propagation vector k = (0 0 0.0483). Crystallographic results are corroborated by magnetic measurements and ab initio calculations.

A double-walled sapphire single-crystal gas-pressure cell (type III) for in situ neutron diffraction.

In situ neutron diffraction is an important characterization technique for the investigation of many functional materials, e.g. for hydrogen uptake and release in hydrogen storage materials. A new sapphire single-crystal gas-pressure cell for elastic neutron scattering has been developed and evaluated; it allows conditions of 298 K and 9.5 MPa hydrogen pressure and 1110 K at ambient pressure. The pressure vessel consists of a sapphire single-crystal tube of 35 mm radius and a sapphire single-crystal crucible as sample holder. Heating is realized by two 100 W diode lasers. It is optimized for the D20 diffractometer, ILL, Grenoble, France, and requires the use of a radial oscillating collimator. Its advantages over earlier sapphire single-crystal gas-pressure cells are higher maximum temperatures and lower background at low and high diffraction angles. The deuterium uptake in palladium was followed in situ for validation, proving the potential of the type-III gas-pressure cell for in situ neutron diffraction on solid-gas reactions.

Design and use of a sapphire single-crystal gas-pressure cell for in situ neutron powder diffraction.

A sapphire single-crystal gas-pressure cell without external support allowing unobstructed optical access by neutrons has been developed and optimized for elastic in situ neutron powder diffraction using hydrogen (deuterium) gas at the high-intensity two-axis diffractometer D20 at the Institut Laue-Langevin (Grenoble, France). Given a proper orientation of the single-crystal sample holder with respect to the detector, parasitic reflections from the sample holder can be avoided and the background can be kept low. Hydrogen (deuterium) gas pressures of up to 16.0 MPa at 298 K and 8.0 MPa at 655 K were tested successfully for a wall thickness of 3 mm. Heating was achieved by a two-sided laser heating system. The typical time resolution of in situ investigations of the reaction pathway of hydrogen (deuterium) uptake or release is on the order of 1 min. Detailed descriptions of all parts of the sapphire single-crystal gas-pressure cell are given, including materials information, technical drawings and instructions for use.

A quantum liquid of magnetic octupoles on the pyrochlore lattice.

Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles1. Theories define such states using gauge fields and deconfined quasiparticle excitations that emerge from a local constraint governing the ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice2-4 in rare-earth pyrochlores. However, f-electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders5-6. Here we show that the correlated ground state of a Ce3+-based pyrochlore, Ce2Sn2O7, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with a fluid-like state where degrees of freedom have a more complex magnetization density than that of magnetic dipoles. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule7-8. Our work identifies Ce2Sn2O7 as a unique example of frustrated multipoles forming a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations.

First-time synthesis of a magnetoelectric core-shell composite via conventional solid-state reaction.

In recent years, multiferroics and magnetoelectrics have demonstrated their potential for a variety of applications. However, no magnetoelectric material has been translated to a real application yet. Here, we report for the first time that a magnetoelectric core-shell ceramic, is synthesized via a conventional solid-state reaction, where core-shell grains form during a single sintering step. The core consists of ferrimagnetic CoFe2O4, which is surrounded by a ferroelectric shell consisting of (BiFeO3)x-(Bi1/2K1/2TiO3)1-x. We establish the core-shell nature of these grains by transmission-electron microscopy (TEM) and find an epitaxial crystallographic relation between core and shell, with a lattice mismatch of 6 ± 0.7%. The core-shell grains exhibit exceptional magnetoelectric coupling effects that we attribute to the epitaxial connection between the magnetic and ferroelectric phase, which also leads to magnetic exchange coupling as demonstrated by neutron diffraction. Apparently, ferrimagnetic CoFe2O4 cores undergo a non-centrosymmetric distortion of the crystal structure upon epitaxial strain from the shell, which leads to simultaneous ferrimagnetism and piezoelectricity. We conclude that in situ core-shell ceramics offer a number of advantages over other magnetoelectric composites, such as lower leakage current, higher density and absence of substrate clamping effects. At the same time, the material is predestined for application, since its preparation is cost-effective and only requires a single sintering step. This discovery adds a promising new perspective for the application of magnetoelectric materials.

Cubic ice Ic without stacking defects obtained from ice XVII.

Amongst the more than 18 different forms of water ice, only the common hexagonal phase and the cubic phase are present in nature on Earth. Nonetheless, it is now widely recognized that all samples of 'cubic ice' discovered so far do not have a fully cubic crystal structure but instead are stacking-disordered forms of ice I (namely, ice Isd), which contain both hexagonal and cubic stacking sequences of hydrogen-bonded water molecules. Here, we describe a method to obtain large quantities of cubic ice Ic with high structural purity. Cubic ice Ic is formed by heating a powder of D2O ice XVII obtained from annealing of pristine C0 hydrate samples under dynamic vacuum. Neutron diffraction experiments performed on two different instruments and Raman spectroscopy measurements confirm the structural purity of the cubic ice, Ic. These findings contribute to a better understanding of ice I polymorphism and the existence of the two natural ice forms.

Idiosyncratic Ag7Pt2O7: An Electron Imprecise yet Diamagnetic Small Band Gap Oxide.

The seminal qualitative concepts of chemical bonding, as presented by Walter Kossel and Gilbert Newton Lewis back in 1916, have lasting general validity. These basic rules of chemical valence still serve as a touchstone for validating the plausibility of composition and constitution of a given chemical compound. We report on Ag7Pt2O7, with a composition that violates the basic rules of chemical valence and an exotic crystal structure. The first coordination sphere of platinum is characteristic of tetravalent platinum. Thus, the electron count corresponds to Ag7Pt2O7*e-, where excess electrons are associated with the silver substructure. Such conditions given, it is commonly assumed that the excess electrons are either itinerant or localized in Ag-Ag bonds. However, the material does not show metallic conductivity, nor does the structure feature Ag-Ag pairs. Instead, the excess electrons organize themselves in 2e-4c bonds within the silver substructure. This subvalent silver oxide reveals a new general facet pertinent to silver chemistry.

Transformation Pathway upon Heating of Metastable ß Titanium Alloy Ti-15Mo Investigated by Neutron Diffraction.

A transformation pathway during thermal treatment of metastable ß Ti-15Mo alloy was investigated by in situ neutron diffraction. The evolution of individual phases α , ß , and ω was investigated during linear heating with two heating rates of 1.9 ∘ C / min and 5 ∘ C / min and during aging at 450 ∘ C . The results showed that with a sufficient heating rate (5 ∘ C / min in this case), the ω phase dissolves before the α phase forms. On the other hand, for the slower heating rate of 1.9 ∘ C / min , a small temperature interval of the coexistence of the α and ω phases was detected. Volume fractions and lattice parameters of all phases were also determined.

Epsilon iron as a spin-smectic state.

Using X-ray emission spectroscopy, we find appreciable local magnetic moments until 30 GPa to 40 GPa in the high-pressure phase of iron; however, no magnetic order is detected with neutron powder diffraction down to 1.8 K, contrary to previous predictions. Our first-principles calculations reveal a "spin-smectic" state lower in energy than previous results. This state forms antiferromagnetic bilayers separated by null spin bilayers, which allows a complete relaxation of the inherent frustration of antiferromagnetism on a hexagonal close-packed lattice. The magnetic bilayers are likely orientationally disordered, owing to the soft interlayer excitations and the near-degeneracy with other smectic phases. This possible lack of long-range correlation agrees with the null results from neutron powder diffraction. An orientationally disordered, spin-smectic state resolves previously perceived contradictions in high-pressure iron and could be integral to explaining its puzzling superconductivity.

Ne- and O2-filled ice XVII: a neutron diffraction study.

Deuterated ice XVII, a metastable solid water polymorph, was filled with Ne and O2 at p ≈ 100 kPa and studied by in situ neutron diffraction (ILL, France). Powder patterns were collected in the ranges of 20-50 K (Ne) and 4.6-90 K (O2). Rietveld refinement and difference Fourier techniques showed that the gas molecules were located inside the hexagonal channels of the host ice. Both Ne atoms and O2 molecules are arranged in a spiral-like configuration off the channel axis, preserving the P6122 symmetry of the host in the case of Ne, but reducing it to P61 in O2. A larger Ne absorption compared to Ne-filled ice II is observed, which is consistent with longer host-guest contacts producing smaller hydrophobic repulsion. In O2-filled ice XVII, instead, short O-D distances (2.37 Å) have attractive character and stabilize the structure.

A hidden Markov model for describing turbostratic disorder applied to carbon blacks and graphene.

A mathematical framework is presented to represent turbostratic disorder in materials like carbon blacks, smectites and twisted n-layer graphene. In particular, the set of all possible disordered layers, including rotated, shifted and curved layers, forms a stochastic sequence governed by a hidden Markov model. The probability distribution over the set of layer types is treated as an element of a Hilbert space and, using the tools of Fourier analysis and functional analysis, expressions are developed for the scattering cross sections of a broad class of disordered materials.

Filling Ices with Helium and the Formation of Helium Clathrate Hydrate.

We have formed the long-sought He-clathrate. This was achieved by refilling helium into ice XVI, opening a new synthesis route for exotic forms of clathrate hydrates. The process was followed by neutron diffraction; structures and cage fillings were established. The stabilizing attractive van der Waals interactions are enhanced by multiple cage fillings with theoretically up to four helium atoms per large cage and up to one per small cage; He-clathrate hydrates can be considered as a solid-state equivalent of the clustering of small apolar entities dissolved in the liquid state of water. Unlike most other guests, helium easily enters and leaves the water cages at temperatures well below 100 K, hampering applications as a gas storage material. Despite the weak dispersive interactions, the inclusion of helium has a very significant effect on lattice constants; this is also established for helium inclusion in ice Ih and suggests that lattice parameters are arguably the most sensitive measure to gauge dispersive water-gas interactions.

Magnetic Structure of SmCo5 from 5 K to the Curie Temperature.

The crystal and magnetic structure of SmCo5 is determined by neutron powder diffraction between 5 K and the Curie temperature. In order to overcome the enormous neutron absorption of samarium, a 154Sm isotopically enriched sample was used. The ordered magnetic moments of both crystallographically distinct cobalt atoms are not significantly different over the whole temperature range. They decrease from 2.2 µB at 5 K to about 0.6 µB at 1029 K. Samarium's ordered magnetic moment decreases from 1.0 µB at 5 K, runs through a minimum of 0.2 µB around 650 K, and becomes larger than cobalt's ordered magnetic moment above 950 K. No sign or orientation change of the samarium and cobalt ordered magnetic moments is found between the Curie temperature and 5 K. SmCo5 is thus a ferromagnet and does not switch to a ferrimagnetic state as discussed in the literature.

Hydrogenation Properties of Laves Phases LnMg2 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb).

The hydrogenation properties of Laves phases LnMg2 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb) were investigated by thermal analysis, X-ray, synchrotron, and neutron powder diffraction. At 14.0 MPa hydrogen gas pressure and 393 K, PrMg2 and NdMg2 take up hydrogen and form the colorless, ternary hydrides PrMg2H7 (P41212, a = 632.386(6) pm, c = 945.722(11) pm) and NdMg2H7 (P41212, a = 630.354(9) pm, c = 943.018(16) pm). The crystal structures were refined by the Rietveld method from neutron powder diffraction data on the deuterides (PrMg2D7, P41212, a = 630.56(2) pm, c = 943.27(3) pm; NdMg2D7, P41212, a = 628.15(2) pm, c = 940.32(3) pm) and shown to be isotypic to LaMg2D7. The LaMg2D7 type of hydrides decompose at 695 K (La), 684 K (Ce), 684 K (Pr), 672 K (Nd), and 639 K (Sm) to lanthanide hydrides and magnesium. The Laves phase EuMg2 forms a hydride EuMg2Hx of black color. Its crystal structure (P212121, a = 664.887(4) pm, b = 1136.993(7) pm, c = 1069.887(7) pm) is closely related to the hexagonal Laves phase (MgZn2 type) of the hydrogen-free parent intermetallic. GdMg2 and TbMg2 form hydrides GdMg2Hx with orthorhombic unit cells (a = 1282.7(4) pm, b = 572.5(2) pm, c = 881.7(2) pm) and TbMg2Hx (a = 617.8(3) pm, b = 1045.8(8) pm, c = 997.1(5) pm), presumably also with a distorted MgZn2 type of structure. CeMg2H7 and NdMg2H7 are paramagnetic with effective magnetic moments of 2.49(1) µB and 3.62(1) µB, respectively, in good agreement with the calculated magnetic moments of the free trivalent rare-earth cations (µcalc(Ce3+) = 2.54 µB; µcalc(Nd3+) = 3.62 µB).

Large-moment antiferromagnetic order in overdoped high-Tc superconductor 154SmFeAsO1-x D x.

In iron-based superconductors, high critical temperature (Tc) superconductivity over 50 K has only been accomplished in electron-doped hREFeAsO (hRE is heavy rare earth (RE) element). Although hREFeAsO has the highest bulk Tc (58 K), progress in understanding its physical properties has been relatively slow due to difficulties in achieving high-concentration electron doping and carrying out neutron experiments. Here, we present a systematic neutron powder diffraction study of 154SmFeAsO1-x D x , and the discovery of a long-range antiferromagnetic ordering with x ≥ 0.56 (AFM2) accompanying a structural transition from tetragonal to orthorhombic. Surprisingly, the Fe magnetic moment in AFM2 reaches a magnitude of 2.73 µB/Fe, which is the largest in all nondoped iron pnictides and chalcogenides. Theoretical calculations suggest that the AFM2 phase originates in kinetic frustration of the Fe-3dxy orbital, in which the nearest-neighbor hopping parameter becomes zero. The unique phase diagram, i.e., highest-Tc superconducting phase adjacent to the strongly correlated phase in electron-overdoped regime, yields important clues to the unconventional origins of superconductivity.