Abstract: In dibromomesitylene, only the methyl group denoted Me(2) and surrounded by two bromine atoms, is a quasi free rotor. In isotopic mixtures the Me(2) tunnelling gap is almost insensitive to variations in temperature and to isotopic dilution. The Me(2) hindering potential remains the same as in the pure material with a main component cos6 theta. Because there is no coupling between the methyl groups, dibromomesitylene provides an illustration of the 'Single Particle Model' in rotational tunnelling. An explanation is given why Density Functional Theory calculations fail to represent the characteristics of tunnelling produced by the Schrodinger equation. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: The out-of-plane motion of the pyridinium cation in the bis-thiourea pyridinium chloride inclusion compound has been studied in a wide temperature range using (1)H NMR, dielectric spectroscopy and quasielastic neutron scattering. The geometry of this motion is obtained from the Q-dependence of the elastic incoherent structure factor determined from the quasielastic neutron scattering measurements. We find that the pyridinium cation performs out-of-plane reorientations around the axis passing through two opposite atoms of the ring. The correlation times as a function of temperature were measured in the three known crystallographic phases, finding a good agreement between the three techniques employed. The activation energy for this motion changes from 5 +/- 1 kJ mol(-1) in the low-temperature phase to 1.2 +/- 0.2 kJ mol(-1) in the intermediate and high-temperature phases.
Abstract: Anisotropy effects can significantly control or modify the ground-state properties of magnetic systems. Yet the origin and the relative importance of the possible anisotropy terms are difficult to assess experimentally and often ambiguous. Here we propose a technique that allows a very direct distinction between single-ion and two-ion anisotropy effects. The method is based on high-resolution neutron spectroscopic investigations of magnetic cluster excitations. This is exemplified for manganese dimers and tetramers in the mixed compounds CsMn(x)Mg(1-x)Br(3) (0.05 <= x <= 0.40). Our experiments provide evidence for a pronounced anisotropy of the order of 3% of the dominant bilinear exchange interaction, and the anisotropy is dominated by the single-ion term. The detailed characterization of magnetic cluster excitations offers a convenient way to unravel anisotropy effects in any magnetic material.
Abstract: This tutorial introduction has been written for people who are not specialized in neutron scattering or in other scattering methods but who are interested and would like to get an impression and learn about the method of Quasielastic Neutron Scattering (QENS). The theoretical (scattering process) as well as the experimental basics (neutron sources, neutron scattering instruments, experimental periphery) are explained in a generally understandable way, with only the most essential formulas. QENS addresses the stochastic dynamics in condensed matter, and it is pointed out for which problems and for which systems in condensed matter research QENS is a powerful method. Thus sufficient information is provided to enable non-experts to think about their own QENS experiment and to understand related literature in this area of research.
Abstract: Biological membranes, consisting mainly of phospholipids and proteins, are organized in a bilayered structure which exhibits dynamical behaviour within time regimes ranging from 10 (12) s with the motion of alkyl chain defects and 1 s corresponding to collective excitations of the bilayer [Europhysics Letters 8 (1989), 201-206]. With the prominent role hydration plays on the structural phase behaviour of phospholipids membranes, it is essential for a better description of membranes to understand also the influence of hydration on the dynamics of membrane systems. In the present study we have performed neutron scattering investigations on highly oriented DMPC-d54 multilayers at two different relative humidity (rh) levels. Our results reveal the strong influence of hydration on the local membrane dynamics, i.e., head group dynamics.
Abstract: Dynamics of water sorbed in a reverse osmosis polyamide membrane (ROPM) as studied by quasielastic neutron scattering (QENS) is reported here. The trimesoylchloride-m-phenylene diamine based ROPM is synthesized by interfacial polymerization technique. QENS data indicates that translational motion of water confined in ROPM gets modified compared to bulk water whereas rotational motion remains unaltered. Translational motion of water in ROPM is found to follow random jump diffusion with lower diffusivity compared to bulk water. Translational diffusivity does not show the Arrhenius behaviour.
Abstract: We investigated molecular motions on a picosecond timescale of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) model membranes as a function of hydration by using elastic and quasielastic neutron scattering. Two different hydrations corresponding to approximately nine and twelve water molecules per lipid were studied, the latter being the fully hydrated state. In our study, we focused on head group motions by using chain deuterated lipids. Information on in-plane and out-of-plane motions could be extracted by using solid supported DMPC multilayers. Our studies confirm and complete former investigations by Konig et al. [J. Phys. II (France) 2, 1589 (1992)] and Rheinstadter et al. [Phys. Rev. Lett. 101, 248106 (2008)] who described the dynamics of lipid membranes, but did not explore the influence of hydration on the head group dynamics as presented here. From the elastic data, a clear shift of the main phase transition from the P(beta) ripple phase to the L(alpha) liquid phase was observed. Decreasing water content moves the transition temperature to higher temperatures. The quasielastic data permit a closer investigation of the different types of head group motion of the two samples. Two different models are needed to fit the elastic incoherent structure factor and corresponding radii were calculated. The presented data show the strong influence hydration has on the head group mobility of DMPC. (C) 2010 American Institute of Physics. [doi:10.1063/1.3495973]
Abstract: Single-crystal x-ray diffraction from clathrate-II Na(x)Si(136) (x = 24) prepared by a new technique reveals the exceptionally large Na@Si(28) atomic displacement parameter (U(eq)) is strongly temperature dependent, and can be attributed to low-energy rattling modes associated with the Na guest. Inelastic neutron scattering (INS) spectra collected from Na(x)Si(136) powder specimens (x = 3, 23) confirm the presence of low-energy guest-derived phonon modes for Na@Si(28) and Na@Si(20). The lower energy Na@Si(28) rattler mode falls in the frequency range of the silicon host acoustic phonons, indicating the possibility for interaction with these phonons. The presence of these low-energy modes combined with the ability to controllably vary the guest content presents a unique opportunity for exploring the influence of guest-framework interactions on the lattice dynamics in intermetallic clathrates.
Abstract: Hydrogen dynamics in crystalline calcium borohydride can be initiated by long-range diffusion or localized motion such as rotations, librations, and vibrations. Herein, the rotational and translational diffusion were studied by quasielastic neutron scattering (QENS) by using two instruments with different time scales in combination with density functional theory (DFT) calculations. Two thermally activated reorientational motions were observed, mound the 2-fold (C(2)) and 3-fold (C(3)) axes of the BH(4)(-) units, at temperature from 95 to 280K. The experimental energy barriers (Ea(C2) = 0.14 eV and Ea(C3) = 0.10 eV) and mean residence times are comparable with those obtained from DFT calculations. Long-range diffusion events, with an energy barrier Of E(aD) = 0.12 eV and an effective jump length of similar to 2.5 angstrom were observed at 224 and 260 K. Three vacancy-mediated diffusion events, H jumps between two neighboring BH(4)(-), and diffusion of BH(4)(-) and BH(3) groups were calculated and finally discarded because of their very high formation energies and diffusion barriers. Three interstitial diffusion processes (H, H(2), and H(2)O) were also calculated. The H interstitial was found to be highly unstable, whereas the H(2) interstitial has a low energy of formation (0.40 eV) and diffusion barrier (0.09 eV) with a jump length (2.1 angstrom) that corresponds well with the experimental values. H(2)O interstitial has an energy of formation of -0.05 eV, and two different diffusion pathways were found. The first gives a H jump distance of 2.45 angstrom with a diffusion barrier of 0.68 eV, the second one, more favorable, exhibits a H jump distance of 1.08 angstrom with a barrier of 0.40 eV. The correlation between the QENS and DFT calculations indicates that, most probably, it is the diffusion of interstitial H(2) that was observed. The origin of the interstitial H(2) might come from the synthesis of the compound or a side reaction with trapped synthesis residue leading to the partial oxidation of the compound and hydrogen release.
Abstract: Powder inelastic neutron scattering (INS) has been used to determine the guest atom "rattling" energy in thermoelectric clathrates Ba(8)Y(x)Ge(46-x) (Y(x)= Ni(6), Cu(6), Zn(8), Ga(16)) under different applied conditions. Chemical pressure was exerted by the atomic substitution, and a physical pressure of 9 kbars was applied using a clamp cell. The volume reduction induced by the physical pressure increases the energy of the guest atom rattling mode, but the local chemical environment in the cage also appears to have a similar effect. The guest atom energies were investigated as function of temperature, and softening of the guest atom modes was observed upon cooling the sample. Ba(8)Ga(16)Ge(30) with holes (p-type) and electrons (n-type) as charge carriers reveal similar temperature behavior, suggesting anharmonic potentials of similar shape for the Ba guest atom independent of the charge carrier type. For Sr(8)Ga(16)Ge(30) a much stronger anharmonic potential was observed compared with Ba(8)Ga(16)Ge(30). The guest atom energies for Ba(8)Y(x)Ge(46-x) (Y(x) = Ni(6), Cu(6), Zn(8)) extracted from powder INS were compared with Einstein energies obtained from atomic displacement parameters determined from multitemperature single crystal x-ray diffraction. Additionally, the Ba(8)Y(x)Ge(46-x) (Y(x)=Ni(6), Cu(6), Zn(8)) samples were characterized with respect to their thermoelectric properties. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3099589]
Abstract: We performed quasielastic neutron scattering experiments and atomistic molecular dynamics simulations on a poly(ethylene oxide) (PEO) homopolymer system above the melting point. The excellent agreement found between both sets of data, together with a successful comparison with literature diffraction results, validates the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field used to produce our dynamic runs and gives support to their further analysis. This provided direct information on magnitudes which are not accessible from experiments such as the radial probability distribution functions of specific atoms at different times and their moments. The results of our simulations on the H-motions and different experiments indicate that in the high-temperature range investigated the dynamics is Rouse-like for Q-values below approximate to 0.6 A(-1). We then addressed the single chain dynamic structure factor with the simulations. A mode analysis, not possible directly experimentally, reveals the limits of applicability of the Rouse model to PEO. We discuss the possible origins for the observed deviations.
Abstract: The thermally activated proton diffusion in BaZr(0.9)Y(0.1)O(3-delta) was studied with electrochemical impedance spectroscopy (IS) and quasi-elastic neutron scattering (QENS) in the temperature range 300-900 K. The diffusivities for the bulk material and the grain boundaries as obtained by IS obey an Arrhenius law with activation energies of 0.46 eV and 1.21 eV, respectively. The activation energies obtained by IS for the bulk are 0.26 eV above 700 K and 0.46 eV, below 700 K. The total diffusivity as obtained by IS is by one order of magnitude lower than the microscopic diffusivity as obtained by QENS. The activation energies obtained by QENS are 0.13 eV above 700 K and 0.04 eV, below 700 K. At about 700 K, the diffusion constants for IS and QENS have a remarkable crossover, suggesting two processes with different activation energies.
Abstract: Diffusion of water and solutes through compacted clays or claystones is important when assessing the barrier function of engineered or geological barriers in waste disposal. The shape and the connectivity of the pore network as well as electrostatic interactions between the diffusant and the charged clay surfaces or cations compensating negative surface charges affect the resistance of the porous medium to diffusion. Comparing diffusion measurements performed at different spatial or time scales allows identification and extraction of the different factors. We quantified the electrostatic constraint q for five different, highly compacted clays (rho(b) = 1.85 +/- 0.05 g/cm(3)) using quasielastic neutron scattering (DENS) data. We then compared the DENS data with macroscopic diffusion data for the same clays and could derive the true geometric tortuosities G of the samples. Knowing the geometric and electrostatic factors for the different clays is essential when trying to predict diffusion coefficients for other conditions. We furthermore compared the activation energies E(a) for diffusion at the two measurement scales. Because E(a) is mostly influenced by the local, pore scale surroundings of the water, we expected the results to be similar at bath scales. This was indeed the case for the nonswelling clays kaolinite and illite, which had E(a) values lower than that of bulk water, but not for montmorillonite, which had values lower than that in bulk water at the microscopic scale, but larger at the macroscopic scale. The differences could be connected to the strongly temperature dependent mobility of the cations in the clays, which may act as local barriers in the narrow pores at low temperatures.
Abstract: Non-negligible higher-order exchange terms are ubiquitous in the spin Hamiltonians of a large variety of magnetic compounds. Yet the origin of higher-order exchange has not been satisfactorily established. This question was addressed by performing inelastic neutron scattering experiments for the magnetically diluted compound KMn(0.1)Zn(0.9)F(3). The observed excitations can be associated with transitions between the low-lying electronic states of Mn(2+) multimers which are well described by a spin Hamiltonian including bilinear and biquadratic exchange interactions. The bilinear exchange parameter J derived from the dimer spectra exhibits a strong temperature dependence, whereas the biquadratic exchange parameter K is independent of temperature. By minimizing the total elastic and magnetic energy of the dimer, we can express the parameter K in terms of lattice, elastic, and magnetic properties. The good agreement between the calculated and observed values of K indicates that the mechanism of exchange striction is the likely origin of the biquadratic interaction in the title compound.
Abstract: Inelastic neutron scattering and high-field electron paramagnetic resonance data are presented for [Mn(bpia)(OAc)-(OCH3)](PF6), where bpia is bis(picolyl) (N-methylimidazole-2-yl)amine. Modeling of the data to the conventional fourth-order spin-Hamiltonian yielded the following parameters: D = 3.526(3) cm(-1), E = 0.588(6) cm(-1), B-4(0) = -0.00084(7) cm(-1), B-4(2) = -0.002(2) cm(-1), B-4(4) = -0.0082(5) cm(-1), g(x) = 1.98(1), g(y) = 1.952(6), and g(z) = 1.978(5). The complex is of particular interest given the biochemical activity and the unusual stereochemistry distinguished by a rare example of a tetragonally compressed octahedron and a pronounced angular distortion imposed by the tetradentate tripodal bpia ligand. Ligand field, density functional theory, and complete active space self-consistent field ab initio calculations are presented that aim to relate the spectroscopic data to the molecular geometry.
Abstract: Neutron radiation offers significant advantages for the study of biological molecular structure and dynamics. A broad and significant effort towards instrumental and methodological development to facilitate biology experiments at neutron sources worldwide is reviewed. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Adequate clay minerals considerably affect the macroscopic mechanical behavior of water even at concentrations of a few percent. Thus when 2 wt. % laponite clay mineral nanoparticles are added to water, the resulting colloidal suspension after some time takes on the semisolid characteristics of a jellylike material at room temperature. Cold neutron time-of-flight spectroscopy data are in agreement with the assumption that notwithstanding this macroscopic change, the mobility of the water molecules on intermolecular and intramolecular length scales remains largely unaffected. This observation is discussed in the context of the properties and the role of water in different more or less dilute ionic environments. The result contributes to the ongoing debate of the properties and role of water in living cells.
Abstract: The water diffusion in four different, highly compacted clays [montmorillonite in the Na- and Ca-forms, illite in the Na- and Ca-forms, kaolinite, and pyrophyllite (bulk dry density rho(b)=1.85 +/- 0.05 g/cm(3))] was studied at the atomic level by means of quasielastic neutron scattering. The experiments were performed on two time-of-flight spectrometers and at three different energy resolutions [FOCUS at SINQ, PSI (3.65 and 5.75 A), and TOFTOF at FRM II (10 A)] for reliable data analysis and at temperatures between 27 and 95 degrees C. Two different jump diffusion models were used to describe the translational motion. Both models describe the data equally well and give the following ranking of diffusion coefficients: Na-montmorillonite <= Ca-montmorillonite < Ca-illite < Na-illite < water <= pyrophyllite <= kaolinite. Uncharged clays had slightly larger diffusion coefficients than that of bulk water due to their hydrophobic surfaces. The time between jumps, tau(t), follows the sequence: Ca-montmorillonite >= Na-montmorillonite>Ca-illite>Na-illite >= kaolinite>pyrophyllite >= water, in both jump diffusion models. For clays with a permanent layer charge (montmorillonite and illite) a reduction in the water content by a factor of 2 resulted in a decrease in the self-diffusion coefficients and an increase in the time between jumps as compared to the full saturation. The uncharged clay kaolinite exhibited no change in the water mobility between the two hydration states. The rotational relaxation time of water was affected by the charged clay surfaces, especially in the case of montmorillonite; the uncharged clays presented a waterlike behavior. The activation energies for translational diffusion were calculated from the Arrhenius law, which adequately describes the systems in the studied temperature range. Na- and Ca-montmorillonite (similar to 11-12 kJ/mol), Na-illite (similar to 13 kJ/mol), kaolinite and pyrophyllite (similar to 14 kJ/mol), and Ca-illite (similar to 15 kJ/mol) all had lower activation energies than bulk water (similar to 17 kJ/mol in this study). This may originate from the reduced number and strength of the H-bonds between water and the clay surfaces, or ions, as compared to those in bulk water. Our comparative study suggests that the compensating cations in swelling clays have only a minor effect on the water diffusion rates at these high densities, whereas these cations influence the water motion in non-swelling clays.
Abstract: The freezing behavior of water confined in compacted charged and uncharged clays (montmorillonite in Na-and Ca-forms, illite in Na-and Ca-forms, kaolinite and pyrophyllite) was investigated by neutron scattering. Firstly, the amount of frozen (immobile) water was measured as a function of temperature at the IN16 backscattering spectrometer, Institute Laue-Langevin (ILL). Water in uncharged, partly hydrophobic (kaolinite) and fully hydrophobic (pyrophyllite) clays exhibited a similar freezing and melting behavior to that of bulk water. In contrast, water in charged clays which are hydrophilic could be significantly supercooled. To observe the water dynamics in these clays, further experiments were performed using quasielastic neutron scattering. At temperatures of 250, 260 and 270 K the diffusive motion of water could still be observed, but with a strong reduction in the water mobility as compared with the values obtained above 273 K. The diffusion coefficients followed a non-Arrhenius temperature dependence well described by the Vogel-Fulcher-Tammann and the fractional power relations. The fits revealed that Na-and Ca-montmorillonite and Ca-illite have similar Vogel-Fulcher-Tammann temperatures (T(VFT), often referred to as the glass transition temperature) of similar to 120 K and similar temperatures at which the water undergoes the 'strong-fragile' transition, T(s) similar to 210 K. On the other hand, Na-illite had significantly larger values of T(VFT) similar to 180 K and T(s) similar to 240 K. Surprisingly, Ca-illite has a similar freezing behavior of water to that of montmorillonites, even though it has a rather different structure. We attribute this to the stronger hydration of Ca ions as compared with the Na ions occurring in the illite clays.
Abstract: Engineering of materials with specific physical properties has recently focused on the effect of nano-sized 'guest domains' in a 'host matrix' that enable tuning of electrical, mechanical, photo-optical or thermal properties. A low thermal conductivity is a prerequisite for obtaining effective thermoelectric materials, and the challenge is to limit the conduction of heat by phonons, without simultaneously reducing the charge transport. This is named the 'phonon glass-electron crystal' concept and may be realized in host-guest systems. The guest entities are believed to have independent oscillations, so-called rattler modes, which scatter the acoustic phonons and reduce the thermal conductivity. We have investigated the phonon dispersion relation in the phonon glass-electron crystal material Ba8Ga16Ge30 using neutron triple-axis spectroscopy. The results disclose unambiguously the theoretically predicted avoided crossing of the rattler modes and the acoustic-phonon branches. The observed phonon lifetimes are longer than expected, and a new explanation for the low k(L) is provided.
Abstract: Diffusion in anisotropic samples is often studied by quasi-elastic neutron scattering at time-of-flight (t-o-f) instruments. During the data analysis the measured dynamic structure factor S(Q,E) is fitted by a model function, which - reflecting the anisotropy of the sample - depends on the sample orientation. The error introduced in this way will be discussed. Correct measurements can be performed for such cases at a triple axis spectrometer, where energy scan can be performed for a certain Q vector fixed to the sample orientation. Clays show 2D anisotropy in the structure, which is supposed to be visible also in the diffusion of water located between the clay sheets. A first experiment performed on TASP (SINQ) will be shown.
Abstract: Layered double hydroxides (LDH) are a class of ionic lamellar solids with positively charged layers of two kinds of metallic cations and exchangeable hydrated anions. Quasi-elastic neutron scattering (QENS) measurements are performed in this type of LDH structured hydrated hydrotalcite sample to study the dynamical behaviour of the water in geometric confinement within the layers. Dynamical parameters correspond to the confined water molecules revealed that depending on the amount of excess water present, behaves differently and approaches bulk values at high concentration. Both translational and rotational dynamical parameters showed that at very low concentration of excess water, water molecules are attached to the surfaces and show the confinement effect.
Abstract: The translational diffusion of water in compacted clays at a high hydration level has been investigated by quasielastic neutron scattering at a time-of-flight spectrometer FOCUS (SINQ). Four compacted clays with systematic structural differences have been studied: Na-montmorillonite, Na-illite, kaolinite and pyrophyllite. The QENS experiments were performed using two different incident wavelengths in order to access a larger Q range and verify the data analysis. The translational diffusion coefficient for water in Na-montmorillonite and Na-illite are lower than those for bulk water, whereas the preliminary results for kaolinite and pyrophyllite show larger diffusion coefficient.
Abstract: We report the first measurements of the dynamics of liquid germanium (l-Ge) by quasielastic neutron scattering on time-of-flight and triple-axis spectrometers. These results are compared with simulation data of the structure and dynamics of l-Ge which have been obtained with ab initio density functional theory methods. The simulations accurately reproduce previous results from elastic and inelastic scattering experiments, as well as the q dependence of the width of the quasielastic signal of the new experimental data. In order to understand some special features of the structure of the liquid we have also simulated amorphous Ge. Overall we find that the atomistic model represents accurately the average structure of real l-Ge as well as the time dependent structural fluctuations. The quasielastic neutron scattering data allows us to investigate to what extent simple theoretical models can be used to describe diffusion in l-Ge.
Abstract: Quasielastic cold neutron scattering experiments on molten Al1-xCux alloys (x = 0.10, 0.17 and 0.25) have been performed at three different temperatures on the FOCUS time-of-flight spectrometer at the Swiss neutron spallation source SINQ. As the main part of the scattering at small Q is incoherent and originating from Cu, self-diffusion coefficients D for the Cu ions could be determined from the widths of quasielastic peaks. The derived values are considerably larger than those found in capillary tube measurements. Even if the accuracy in the determination of D is better than about 4% it could not be concluded whether the temperature variation of D follows an Arrhenius law or a power law. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: This work reports neutron diffraction and incoherent neutron scattering experiments on N-methylacetamide (NMA), which can be considered the model building block for the peptide linkage of polypeptides and proteins. Using the neutron data, we have been able to associate the onset of a striking negative thermal expansion (NTE) along the a-axis with a dynamical transition around 230 K, consistent with our calorimetric experiments. Observation of the NTE raises the question of possible proton transfer in NMA, which, from our data alone, still cannot be settled. We can only speculate that intermolecular repulsive forces increase as the O center dot center dot center dot H distance decreases upon cooling, and that around 230 K the lattice relaxes without observation of an actual proton transfer. However, the existence of a nonharmonic potential, reflected by the behavior of the phonon vibrations together with the observation of NTE, could be justified by the "vibrational" polaron theory in which a dynamic localization of the vibrational energy is created by coupling an internal molecular mode to a lattice phonon. More generally, this work shows that neutron powder diffraction techniques can be very powerful for investigating structural deformations in small peptide systems.
Abstract: Interest in chiral versus racemic amino acid forms stems from their different solid-state properties associated to the packing arrangement that leads to starkly different physical behavior. Here we focus on the different dynamic properties of the chiral (L-form) and racemic (DL-form) crystals of serine, (C(3)H(7)NO(3)). Measuring incoherent inelastic neutron and Raman scattering over a wide temperature range we could assess a subtle dynamical transition in L-serine near 150 K. The difference in the dynamic properties of the crystals of L- and DL-serine evidenced from our spectroscopic studies is in a good agreement with the different C p (T ) dependences and different structural strain of the two forms on cooling. Despite a higher bulk density, the molecular fragments in the crystals of L-serine are more dynamic than in DL-serine, and this manifests itself in the incoherent inelastic neutron and Raman spectra, as well as in a higher heat capacity, a higher thermal expansion coefficient, or in the NMR-spectra.
Abstract: We report the observation by neutron diffraction of phase coherent Bragg reflections in a multi-k magnetic configuration with a spatial periodicity outside the conventional scattering cross-section. The peaks, which exist in the 3-k state of UAs0.8Se0.2, display long-range order with a wavevector dependence characteristic of a magnetic interaction. The results confirm the long-range order and temperature dependence reported in an earlier study of similar peaks in this material using X-ray resonant scattering by (a) the non-trivial extension to the technique of neutron diffraction, and (b) the observation of similar 3-k phase-coherent reflections in other samples by X-ray resonant scattering. The importance of the neutron diffraction results lies primarily in the fact that magnetic neutron diffraction is well established as a weak probe operating on thermodynamic time scales. This alleviates concern that the rapid (10(-15) - 10(-14) s), strong interaction, characteristic of the resonant X-ray technique, is imaging a transient or non-equilibrium configuration. Likewise, the extension of the X-ray resonant scattering results to other samples establishes the generality of this effect. The enigma of how to understand the observed diffraction, which appears to lie strictly outside both the conventional neutron and X-ray scattering cross-sections, remains.
Abstract: Inelastic neutron scattering (INS) spectra are presented for chromium(II) Tutton salts, as a function of the temperature and pressure. Transitions are observed between the levels of the (5)A(g) (C-i) ground term and the data modeled with a conventional S = 2 spin Hamiltonian. At 10 K and ambient pressure, the zero-field-splitting parameters of the ammonium salt, (ND4)(2)Cr(D2O)(6)(SO4)(2), are determined as D = - 2.431( 4) cm(-1) and E = 0.091( 4) cm(-1), evolving to D = - 2.517( 4) cm(-1) and E = 0.127(5) cm(-1) upon application of 7.5(1.0) kbar of quasi-hydrostatic pressure. By contrast, the change in the INS spectrum of the rubidium salt in this pressure range is comparitively minor. The results are interpreted using a (5)Exe vibronic-coupling Hamiltonian, in which low-symmetry strain, perturbing the adiabatic potential-energy surface, is pressure-dependent. It is argued that, for the ammonium salt, the change with pressure of the anisotropic strain impinging upon the [Cr(D2O)(6)](2+) cation is sufficient to cause a switch of the long and intermediate Cr - OD2 bonds, with respect to the crystallographic axes.
Abstract: The cubic high-temperature phase of silver orthophosphate is isomorphic with the rotor phase of sodium orthophosphate. Although both phases are fast cation conductors and have structural similarities, they exhibit different trends in their conductivity activation energy at the phase transition. In addition, the phase transition temperature of sodium orthophosphate is about 200 K lower than in the silver compound. Having successfully investigated the anion and cation dynamics of sodium orthophosphate, we now extend our view to the case of silver orthophosphate. Quasielastic neutron scattering spectra of Ag3PO4, taken at the time-of-flight spectrometer FOCUS, yield various details of the anion motion which turns out to be well described as a thermally activated isotropic rotational diffusion. The rotational diffusion constant varies between 0.36 ps(-1) (863 K) and 0.55 ps(-1) (1073 K), the activation energy being 0.17 eV In addition, we find that anion rotation is not only performed by phosphate ions alone, but also by anions who carry a cation along. The dynamic coupling of cation and anion motion detected in our data explains why the phosphate centers are slightly shifted away from the origin of the FCC lattice in silver orthophosphate: the center of mass of the coupled AgPO4 units is no longer identical with the position of the phosphorus atom. The effect is stronger in silver orthophosphate than in sodium orthophosphate due to the higher mass of the silver atom. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: We have studied the phonon density of states (PDOS) of thermoelectric type I clathrates using time-of-flight inelastic neutron scattering (INS). The open framework structure of clathrates contains rattling guest atoms, and this results in a remarkable low thermal conductivity. Powder samples of Sr8Ga16Ge30, n- and p-type Ba8Ga16Ge30, have been studied. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Neutron scattering studies on powder and single crystals have provided new evidence for unconventional magnetism in Cu2Te2O5Cl2. The compound is built from tetrahedral clusters of S=1/2 Cu2+ spins located on a tetragonal lattice. Magnetic ordering, emerging at T-N=18.2 K, leads to a very complex multi-domain, most likely degenerate, ground state, which is characterized by an incommensurate (ICM) wave vector k similar to[0.15,0.42,1/2]. The Cu2+ ions carry a magnetic moment of 0.67(1)mu(B)/Cu2+ at 1.5 K and form a four-helix spin arrangement with two canted pairs within the tetrahedra. A domain redistribution is observed when a magnetic field is applied in the tetragonal plane (H-c approximate to 0.5 T), but not for H parallel to c up to 4 T. The excitation spectrum is characterized by two well-defined modes, one completely dispersionless at 6 meV, the other strongly dispersing to a gap of 2 meV. The reason for such complex ground state and spin excitations may be geometrical frustration of the Cu2+ spins within the tetrahedra, intra- and inter-tetrahedral couplings having similar strengths and strong Dzyaloshinski-Moriya anisotropy. Candidates for the dominant intra- and inter-tetrahedral interactions are proposed. (C) 2005 American Institute of Physics.
Abstract: The dynamics of propylene glycol (PG) and its oligomers 7-PG and poly-propylene glycol (PPG), with M-w= 4000 (approximate to 70 monomers), confined in a Na-vermiculite clay have been investigated by quasielastic neutron scattering. The liquids are confined to single molecular layers between clay platelets, giving a true two-dimensional liquid. Data from three different spectrometers of different resolutions were Fourier transformed to S(Q, t) and combined to give an extended dynamical time range of 0.3-2000 ps. An attempt was made to distinguish the diffusive motion from the methyl group rotation and a fast local motion of hydrogen in the polymer backbone. The results show that the average relaxation time <tau(d)> of this diffusive process is, as expected, larger than the relaxation time <tau > averaged over all dynamical processes observed in the experimental time window. More interesting, it is evident that the severe confinement has a relatively small effect on <tau(d)> at T = 300 K, this holds particularly for the longest oligomer, PPG. The most significant difference is that the chain-length dependence of <tau(d)> is weaker for the confined liquids, although the slowing down in bulk PG due to the formation of a three-dimensional network of OH-bonded end groups reduces this difference. The estimated average relaxation time <tau > at Q=0.92 angstrom(-1) for all the observed processes is in excellent agreement with the previously reported dielectric alpha relaxation time in the studied temperature range of 260-380 K. The average relaxation time <tau > (as well as the dielectric alpha relaxation time) is also almost unaffected by the confinement to a single molecular layer, suggesting that the interaction with the clay surfaces is weak and that the reduced dimensionality has only a weak influence on the time scale of all the dynamical processes observed in this study. (c) 2005 American Institute of Physics.
Abstract: A new family of tetranuclear Ni complexes [Ni-4(ROH)(4)L-4] (H2L = salicylidene-2-ethanolamine; R = Me (1) or Et (2)) has been synthesized and studied. Complexes 1 and 2 possess a [Ni4O4] core comprising a distorted cubane arrangement. Magnetic susceptibility and inelastic neutron scattering studies indicate a combination of ferromagnetic and antiferromagnetic pairwise exchange interactions between the four Ni-II centers, resulting in an S = 4 spin ground state. Magnetization measurements reveal an easy-axis-type magnetic anisotropy with D approximate to -0.93 cm(-1) for both complexes. Despite the large magnetic anisotropy, no slow relaxation of the magnetization is observed down to 40 mK. To determine the origin of the low-temperature magnetic behavior, the magnetic anisotropy of complex 1 was probed in detail using inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The spectroscopic studies confirm the easy-axis-type anisotropy and indicate strong transverse interactions, These lead to rapid quantum tunneling of the magnetization, explaining the unexpected absence of slow magnetization relaxation for complex 1.
Abstract: The origin of higher-order exchange interactions in localized S-state systems has been the subject of intensive investigations in the past. In particular, it has been suggested that a biquadratic exchange term may arise from the magnetoelastic energy. Here we report on the pressure and temperature dependence of the excitation spectra of magnetic Mn2+ dimers in CsMn0.28Mg0.72Br3 probed by inelastic neutron scattering. Biquadratic exchange and a strong distance dependence of the bilinear exchange are observed. It is shown that the mechanism of local exchange striction may explain the occurrence of biquadratic exchange in accordance with the elastic properties of the compound.
Abstract: Elastic and inelastic neutron scattering has been employed to study the quadrupolar and dipolar ordering phenomena in DyPd3S4. At T-Q = 3.4 K the Dy quadrupole moments undergo a phase transition to an ordered lattice, reflected by a distinct change of the crystal-field level scheme observed by neutron spectroscopy. At T-N1 approximate to 1 K a phase transition of the Dy dipole moments to a long-range canted antiferromagnetic state is observed, followed by a second magnetic phase transition at T-N2 approximate to 0.75 K characterized by a change of the canting angle. (C) 2004 Elsevier B. V. All rights reserved.
Abstract: Variable-temperature spectroscopic and crystallographic studies on the chromium(II) Tutton's salts, (M-I)(2)Cr(X2O)(6)(SO4)(2), where M-I = ND4+, Rb+, or Cs+, and X = H or D, are reported. Inelastic neutron scattering (INS) and multifrequency EPR experiments facilitate a rigorous definition of the ground-state electronic structure from 1.5 up to 296 K, which is unprecedented for a high-spin d(4) complex. Modeling of the INS data using a conventional S = 2 spin Hamiltonian reveals a dramatic variation in the axial and rhombic zero-field-splitting parameters. For the ammonium salt, D and E are -2.454(3) and 0.087(3) cm(-1) at 10 K and -2.29(2) and 0.16(3) cm(-1) at 250 K, respectively. A temperature variation in the stereochemistry of the [Cr(D2O)(6)](2+) complex is also identified, with an apparent coalescence of the long and medium Cr-O bond lengths at temperatures above 150 K. The corresponding changes for the rubidium and cesium salts are notable, though less pronounced. The experimental quantities are interpreted using a (5)Ecircle timese Jahn-Teller Hamiltonian, perturbed by anisotropic strain. It is shown that good agreement can be obtained only by employing a model in which the anisotropic strain is itself temperature dependent. A new theoretical approach for calculating variable-temperature EPR spectra of high-spin d(4) complexes, developed within the (5)Ecircle timese coupling model, is described. Differences between spin-Hamiltonian parameters determined by INS and EPR are consistent with those of the different time scales of the two techniques.
Abstract: We investigated the self-diffusion of Ni in liquid Ni, Ni80P20, Pd40Ni40P20, and Pd43Ni10Cu27P20 at temperatures up to 1795 K with incoherent, quasielastic neutron scattering. Values of measured self-diffusion coefficients vary over the accessible temperature ranges as a function of composition only within 10%. Although mixing has a drastic effect on the liquidus temperature and the undercooling capabilities, a relation between these properties and the atomic diffusion in the liquid is not observed. Apparently, diffusive motion is governed by the packing fraction of the atoms, that is very similar in these dense liquids. (C) 2004 American Institute of Physics.
Abstract: We report the first quasielastic neutron scattering measurements on a room temperature ionic liquid: 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. Data were collected using a medium resolution spectrometer as a function of temperature in the range 250-320 K. The data unequivocally indicate the existence of two different relaxation processes: a fast, localized motion occurring in the subpicosecond range and a slower process spanning the subnanosecond regime. These results provide experimental support to recently published molecular dynamics simulations. Evidence for slower, unresolved dynamics (under the present experimental conditions) is also obtained. Both temperature and momentum transfer dependence of the intermediate incoherent dynamic structure factor were investigated, after Fourier transformation into the temporal domain. The fast process shows no appreciable Q- and T-dependence. On the other hand the slow process shows evidence of a complex, non-Debye and non-Arrhenius behavior. (C) 2003 American Institute of Physics.
Abstract: The crystal fields (CFs) of the binary rare-earth compounds PrAl3 and NdAl3 have been examined at ambient pressure by means of inelastic neutron scattering. The CF of the latter compound has also been measured under hydrostatic pressure (p = 0.84 GPa). The observed substantial changes of the CF under pressure are discussed within the framework of first-principles density functional theory calculations.
Abstract: Neutron spectroscopy has been applied in order to elucidate the magnetothermal properties of the lanthanide alloys ErxPr1-x (x=0.6,0.8). The observed crystal-field splittings are in accordance with the arithmetically weighted crystal fields of the two pure metals. A mean-field approach is presented, which couples the two effective internal fields originating from the Er and Pr ions. The model is successful in explaining many of the observed magnetic properties, including the ferrimagnetic arrangement of the Er and Pr moments, the values of the respective moments, the magnetocaloric effect for x=0.7 and the large increase of the magnetic entropy at low temperatures due to the addition of Pr. A large single-ion anisotropy is derived and it is predicted that an enhanced magnetocaloric effect should be observed in single-crystal samples for xsimilar to0.7 with the external magnetic field applied along the c axis. Additional magnetization measurements under pressure up to 0.72 GPa have shown no evidence for a pronounced sensitivity of the magnetic ordering temperature upon pressure in Er0.6Pr0.4.
Abstract: FOCUS is a cold neutron time-of-flight spectrometer at the swiss spallation neutron source SINQ (Paul Scherrer Institute). Various quasielastic and inelastic neutron scattering experiments have been performed so far using a pyrolithic graphite monochromator with incident wavelengths between 2 and 6 Angstrom. Recently, the instrument has been complementarily equipped with a new mica monochromator. This device is mechanically identical to the graphite monochromator but it covers the wavelength range from 6 to 15 Angstrom. With that new monochromator an energy resolution below 10 mueV (FWHM) is feasible on FOCUS which is a factor of 5 below the former value accessible with the graphite monochromator. The design as well as first experiences with the new monochromator will be discussed. One of the first measurements using the mica monochromator, which investigated the dynamics of Na4B(OH)(4)(3+) clusters confined in pores of a zeolite is presented. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: Inelastic neutron scattering experiments on (DyB2C2)-B-11 and (Dy0.07Y0.93B2C2)-B-11 have been performed. The results enable us to infer important properties of the crystal-field levels of Dy3+ in DyB2C2 which has fascinating structural and magnetic properties that are attributed to the electronic states of the Dy ion. While no clear excitation was observed in pure DyB2C2, three excitations from the ground state with the energies 1.4, 4.1 and 5.8 meV were observed in (Dy0.07Y0.93B2C2)-B-11. By comparing the present results with the previous crystal-field calculation, we infer that the ground state of Dy3+ in YB2C2 is Gamma(8) which is split into two Kramers doublets by the tetragonal potential. Additionally, the heat capacity measured on the diluted system is compared with a calculation using the low part of the energy level scheme obtained from the neutron experiments. Good agreement between theory and experiment is obtained.
Abstract: We investigated the atomic dynamics and the magnetic properties of the Cu100-xFex system. Samples with nominal Fe concentrations of 5, 17.5, 30 and 45 at. % were prepared in the same manner by mechanical alloying in a planetary ball mill. All samples are nanocrystalline supersaturated solid solutions, without crystalline bcc Fe phase or amorphous matrix. They have both ferromagnetic and antiferromagnetic order at 5 K, but the antiferromagnetic state transforms to a paramagnetic one well below room temperature. The inelastic neutron scattering measurements were performed at room temperature at FOCUS (PSI, SINQ). With increasing Fe concentration we additionally observed low-energy, most likely transverse modes in the dynamic structure factor.
Abstract: We have studied the influence of supersaturating Zr by Al on the atomic dynamics or the supersaturated solid solution using inelastic neutron scattering before and after the order-disorder transition. The samples (x = 5, 10 and 20 at.%) were characterized by X-ray diffraction and differential scanning calorimetry (DSC). From the neutron scattering data the generalized vibrational density of states (GVDOS), and the dynamic structure factors were determined in all cases. We find a small but distinct weakening of the lower energy (transverse) modes on supersaturation. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Solid solutions of Zr(100-x)Al(x) with x=10 and 15 were prepared by mechanical alloying in a planetary ball mill. The structure of the solid solutions was characterized by X-ray diffraction. The decomposition temperature of the samples investigated by DSC was found to be about 785K. Here results from preliminary inelastic neutron scattering experiments are reported which were performed in order to investigate the influence of the supersaturation on the phonon modes. The experiments were done at room temperature at the cold neutron time-of-flight spectrometers NEAT at the Berlin Research Reactor, and IN6 at the HFR of the Institute Laue-Langevin in Grenoble. From the measured time-of-flight spectra the generalized vibrational density of states (GVDOS) for both samples has been determined. The GVDOS consists of two bands, with the main band, centered at approximate to 17 meV, being dominant, and one at higher energies, centered at approximate to 33 meV, being rather weak.
Abstract: Infrared linear dichroism (IR-LD) measurements and ab initio and density functional (DFT) frequency and intensity calculations were performed on p-X-nitrobenzenes (X = F, Cl, Br, CH(3) and CHO). Simulation of the reduced IR-LD spectra using DFT (B3LYP/6-311G**) calculated band parameters led to excellent agreement between theory and experiment. (C) 1999 Elsevier Science B.V. All rights reserved.
