Inelastic Neutron Scattering Spectra Research Articles

Origin of the boson peak in a network glassB2O3

We report coherent inelastic neutron-scattering spectra for the network glass former ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ measured at temperatures from 100 to 1300 K ${(T}_{g}\ensuremath{\sim}525 \mathrm{K}).$ At the first sharp diffraction peak, which stems from the van der Waals packing of layerlike units, one finds a correlated in-phase motion of these units at low frequency, and in addition a quasi-incoherent contribution for all temperatures. The contribution of the in-phase motion in the frequency range of the boson peak and the fast relaxations is significantly higher than the one expected for the sound waves.

Full Longitudinal Acoustic Mode (LAM) Spectrum of an N-Alkane: Comparison of Observed and Computed Incoherent Inelastic Neutron Scattering Spectrum of N-Octadecane

The incoherent inelastic neutron scattering spectrum of solid n-octadecane below 600 cm-1 is presented and compared with the results of a scaled HF ab initio calculation. Ten vibrations in the 300−550 cm-1 region, eight of which have not been previously observed, are easily assigned as longitudinal acoustic modes (LAMs). On the basis of the good agreement between calculated and observed modes in this region, the location of the lower frequency LAMs is postulated.

Translational/rotational coupling of the hindered CH3 quantum-rotor in lithium acetate dihydrate

The multiplet structure of the rotational-tunneling peaks in the inelastic neutron-scattering spectrum of lithium acetate is widely regarded as evidence of coupled pairs of CH3 quantum rotors. However, our molecular-mechanics study, which has no adjustable parameters, reveals that rotor/rotor coupling is less important than translational/rotational coupling. In order to remain at a potential-energy minimum during CH3 group reorientation the center-of-mass of this group follows an almost circular path around its time-average position. The energy levels from this dynamical model reproduce the observed tunneling-transitions reasonably well, and the predicted rectangular density distribution of the three methyl H-atoms is in good agreement with that measured by single-crystal neutron diffraction.

Phonons in nanocrystallineNi3Fe

Inelastic neutron-scattering spectra were measured to obtain the phonon density of states (DOS) of nanocrystalline fcc ${\mathrm{Ni}}_{3}\mathrm{Fe}.$ The materials were prepared by mechanical alloying, and were also subjected to heat treatments to alter their crystallite sizes and internal strains. In comparison to material with large crystallites, the nanocrystalline material shows two distinct differences in its phonon DOS. The nanocrystalline DOS was more than twice as large at energies below 15 meV. This increase was approximately proportional to the density of grain boundaries in the material. Second, features in the nanocrystalline DOS are broadened substantially. This broadening did not depend in a simple way on the crystallite size of the sample, suggesting that it has a different physical origin than the enhancement in phonon DOS at energies below 15 meV. A damped harmonic oscillator model for the phonons provides a quality factor ${Q}_{u},$ as low as 7 for phonons in the nanocrystalline material. The difference in vibrational entropy of the bulk and nanocrystalline ${\mathrm{Ni}}_{3}\mathrm{Fe}$ was small, owing to competing changes in the nanocrystalline phonon DOS at low and high energies.

Phonons in Nanocrystalline57Fe

Inelastic neutron-scattering spectra were measured to obtain the phonon density of states (DOS) of nanocrystalline fcc Ni3Fe. The materials were prepared by mechanical alloying, and were also subjected to heat treatments to alter their crystallite sizes and internal strains. In comparison to material with large crystallites, the nanocrystalline material shows two distinct differences in its phonon DOS. The nanocrystalline DOS was more than twice as large at energies below 15 meV. This increase was approximately proportional to the density of grain boundaries in the material. Second, features in the nanocrystalline DOS are broadened substantially. This broadening did not depend in a simple way on the crystallite size of the sample, suggesting that it has a different physical origin than the enhancement in phonon DOS at energies below 15 meV. A damped harmonic oscillator model for the phonons provides a quality factor Qu, as low as 7 for phonons in the nanocrystalline material. The difference in vibrational entropy of the bulk and nanocrystalline Ni3Fe was small, owing to competing changes in the nanocrystalline phonon DOS at low and high energies.

Moment reduction by the crystal field interaction in ErCu 4Al 8

A self-consistent mean field calculation of the ordered Er 3+ magnetic moment in tetragonal ErCu 4Al 8 as a function of temperature has been carried out assuming that the crystal field wave functions and splittings of the ground multiplet are known from the fitting of the inelastic neutron-scattering spectra. This calculation accounts well for the strong reduction of the moment along the c-axis, which has been observed in a recent neutron diffraction experiment for the determination of the magnetic structure of this compound. The effect of the crystal field on the Er 3+ moment is discussed.

Symmetrised quantum-mechanical force-fields and INS spectra: s-triazine, trichloro-s-triazine and pyrazine

A vibrational analysis of the Inelastic Neutron Scattering (INS) spectra of s-triazine, trichloro-a-triazine and pyrazine has been carried out starting from different ab initio calculations levels (HF/3-21G, HF/6-31G ∗ and MP2/6-31G ∗). Resulting force fields were transformed to the symmetry-coordinate system and refined to the observed INS spectral profile. Similar final force fields result regardless of the level of the ab initio calculation. The importance of INS intensities in determining the values for the interaction constants is demonstrated.

Vibrational entropy and microstructural effects on the thermodynamics of partially disordered and orderedNi3V

Samples of ${\mathrm{Ni}}_{3}$ V were prepared with two microstructures: (1) with equilibrium ${\mathrm{D}0}_{22}$ order, and (2) with partial disorder (having a large ${\mathrm{D}0}_{22}$ chemical order parameter, but without the tetragonality of the unit cell). For both materials, we measured the difference in their heat capacities from 60 to 325 K, inelastic neutron-scattering spectra at four values of Q at 11 and at 300 K, and Young's moduli and coefficients of thermal expansion. The difference in heat capacity at low temperatures was consistent with a harmonic model using the phonon density of states (DOS) curves determined from the inelastic neutron-scattering spectra. In contrast, at temperatures greater than 160 K the difference in heat capacity did not approach zero, as expected of harmonic behavior. The temperature dependence of the phonon DOS can be used to approximately account for the anharmonic contributions to the differential heat capacity. We also argue that some of the anharmonic behavior should originate with a microstructural contribution to the heat capacity involving anisotropic thermal contractions of the ${\mathrm{D}0}_{22}$ structure. We estimate the difference in vibrational entropy between partially disordered and ordered ${\mathrm{Ni}}_{3}$ V to be ${\mathrm{S}}^{\mathrm{pdis}}$ -${\mathrm{S}}^{\mathrm{ord}}$ =(+0.038\ifmmode\pm\else\textpm\fi{}0.015)${\mathrm{k}}_{\mathrm{B}}$ /atom at 300 K.

Vibrational analysis of the inelastic neutron scattering spectrum of s-triazine and trichloro-s-triazine

Abstract The inelastics neutron scattering (INS) spectra of s-triazine and trichloro-s-triazine at 5 K provide the first observation of the IR and Raman inactive modes, v 4 and v 5 . A full assingment is made by fitting a harmonic force field to the INS spectra profile. In the case of trichloro-s-triazine we demonstrate that this approach can be used successfully for a molecule without H atoms.

Magnetic susceptibility and specific heat of Pr2?xCexCuO4?y

We report here the measurements of magnetic susceptibility of Pr2−xCexCuO4−y (x=0, 0.05 and 0.15) from 2K to 300K and the specific heat of Pr1.85Ce0.15CuO4 between 6K and 60K. Fitting of the magnetic susceptibility data to a Curie-Weiss law gave effective magnetic moment (peff) values close to the free ion value of 3.58μB for Pr3+. peff was found to decrease with increase in cerium concentration. The site symmetry of Pr3+ in Pr2−xCexCuO4−y (PCCO) host is D4h and a crystal field (CF) Hamiltonian appropriate to tetragonal symmetry was used to analyze the magnetic susceptibility data. J-mixing of all the thirteen multiplets of Pr3+ and intermediate coupled wave functions have been included. A best set of CF parameters obtained not only explains the magnetic susceptibility but also the inelastic neutron scattering spectra (INS) reasonably well. The Schottky component together with Debye and Einstein contributions successfully accounts for the measured specific heat.

Magnetic susceptibility, specific heat, and crystal-field effects in neodymium cuprate superconductors.

We report here the results of magnetic susceptibility of both n- and p-type superconductors, ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ (x=0.14, 0.15, 0.16, and 0.18) and ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathit{z}}$${\mathrm{Zn}}_{\mathit{z}}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (z=0 and 0.02) in the temperature range 2 to 300 K and specific heat measurements on p-type superconductors in the temperature range 5\ensuremath{\le}T\ensuremath{\le}300 K. Crystal-field (CF) analysis of these results has been performed considering J mixing of the lowest eleven multiplets with intermediate-coupled wave functions. The CF analysis of the magnetic susceptibility data is consistent also with the reported inelastic neutron-scattering spectra. The specific heat data have been accounted for by considering Debye, Einstein, and Schottky contributions.

Proton transfer dynamics in polyglycine

Inelastic neutron-scattering spectra from 30 to 4000 cm −1 of partially deuterated polyglycine I and II of (—COCD 2-NH—) n were obtained at ∼ 25 K using the TFXA spectrometer. The dynamics of the (N)H proton are similar in the two structures, but totally different from those proposed previously. These dynamics are associated with a weakening of the N H bond due to the ionic character of the hydrogen bond (N δ−⋯ +⋯O δ−) and proton transfer.

Systematic analysis of the optical spectra of selected RE3+ ions in rare-earth oxyfluoride

The c.f. structure of the2S+1LJ levels of the Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, and Dy3+ ions in hexagonal rare-earth oxyfluoride (REOF) matrices was derived from an analysis of the optical absorption, luminescence, excitation, and inelastic neutron-scattering spectra. The energy-level schemes were simulated with a phenomenological model which included the parametrized electrostatic (the Racah parameters E0–3) and the two- and three-body configuration interactions (Tree's parameters α, β, and γ as well as Judd's parameters Tk) as well as spin-orbit coupling (the coupling constant ζ4f). The effect of the neighboring ions was taken into account by a crystal field of C3v symmetry. All parameter values were determined unambiguously by least-squares calculations by considering simultaneously both the free-ion and c.f. interactions. The standard deviations achieved in some simulations were low, considering the restricted degrees of freedom due to the limited basis set of experimentally observed levels. The B q k parameters were found to evolve smoothly across the RE3+ series. For the lighter RE3+ ions the shifts in the c.f. parameter values correlated well with the model emphasizing the effect of the increasing nuclear charge, whereas beyond Tb3+ additional effects may be needed to describe the evolution of the B q k values. Further work is hence needed for the heavier RE3+ ions in RE-oxyfluoride hosts in order to assess the importance of, e.g., the two-electron c.f. interactions and the correlation between the c.f. effect and the crystal structure.

Inelastic neutron-scattering study of the proton dynamics in coals

The inelastic neutron-scattering (INS) spectra from 30 to 4000 cm −1 of four coal samples of different rank (subbituminous A, low-volatile bituminous, semianthracite and anthracite) at 20 K are reported. For each coal, rather broad bands superimposed on a continuum with almost constant intensity over the whole frequency range are observed. This continuum is assigned to recoiling free protons located between the graphite-like planes. Band frequencies are compared with those observed in the infrared for the same samples in KBr matrices at room temperature, and consistent assignments in terms of chemical-grouping vibrations are proposed. The INS spectra of the coals are also compared with that of perylene (C 20H 12). The spectrum of anthracite is dominated by CH and skeletal modes of polyaromatic entities. Coals of lower rank show bands due to alkyl and vinyl CH 2 and CH 3 groups. A comparison of INS intensities in the CH stretching region for coals and perylene gives an estimate of the concentration of bound protons which ranges from ∼40% to ∼80%. A simple model is proposed to estimate the mean size of the graphite-like domains in the coals.

Inelastic neutron-scattering study of the proton transfer dynamics in polyglycine I at 20 K

Inelastic neutron-scattering (INS) spectra of three isotopic derivatives of polyglycine I (COCH 2NH) n , (COCD 2NH) n , and (COCH 2ND) n at 20 K are presented from 30 to 4000 cm −1. The band frequencies are compared to those observed in the infrared and Raman. Assignments in terms of group vibrations are proposed. These mostly resemble previous assignment schemes, except for the amide bands. The INS intensities reveal that the proton dynamics for the (N)H proton are totally different from those proposed previously. They are independent of the molecular frame and the valence bond approach is not consistent with observation. A phenomenological approach is proposed in terms of localized modes. The calculated intensities reveal that the (N)H stretching mode has two components at ∼ 1377 and 1553 cm −1. This is a dramatic change compared to all former assignments at ∼ 3280 cm −1 based on infrared and Raman data. These proton-dynamics are associated with a weakening of the NH bond due to the ionic character of the hydrogen bond (N δ−…H +…O δ′−) and proton transfer. The infrared and Raman spectra are re-examined and a new assignment scheme is proposed for the amide bands; the amide A and B bands are re-assigned to the overtones of the stretching modes. A symmetric double-minimum potential for the proton is consistent with all the observations.

Inelastic Neutron Scattering Spectrum and Normal Coordinate Analysis of the Insertion Compound H1.7MoO3

High-resolution, incoherent inelastic neutron scattering (IINS) spectra have been recorded for the hydrogen insertion compound H1.7MoO3 and the related oxide hydrate α-MoO3 · H2O. Using the techniques of normal coordinate analysis, the normal modes of vibration of H1.7MoO3 have been calculated for a structure in which hydrogen is present in the form of -OH2 groups. The results demonstrate that the measured IINS spectrum is consistent with this structural model and does not provide any support for alternative modes of attachment of hydrogen. The IINS spectrum and structural similarities of α-MoO3 · H2O are discussed in light of these results.

Characterization of the Interaction of Hydrogen with Iridium Clusters in Zeolites by Inelastic Neutron Scattering Spectroscopy

ABSTRACTIncoherent inelastic neutron scattering (IINS) measurements of hydrogen bound to Ir6 clusters in the nanoscale pores of zeolite NaY and similar clusters in KL zeolite are reported. On the basis of hydrogen uptake measurements and the observation of peaks in the IINS spectra, we infer that hydrogen is bound to the metal nanoclusters in the zeolite pores. The HINS data for both zeolites are similar and consistent with the presence of IrH2 species.

Inelastic neutron-scattering study of the proton dynamics in N-methylacetamide at 20 K

Inelastic neutron-scattering (INS) spectra of four isotopic derivatives of N-methylacetamide (CH 3CONHCH 3, CD 3CONHCH 3, CH 3CONHCD 3 and CD 3CONHCD 3) at 20 K are presented from 30 to 4000 cm −1. The band frequencies are compared with those observed in the infrared and Raman at low temperature. The quantitative simulation of the INS intensities, in the harmonic force field approximation, shows that the proton dynamics for the (N)H proton are totally different from those proposed previously. The valence-bond approach is not consistent with observation and the proton dynamics are independent of the molecular frame. A phenomenological approach is proposed in terms of localized modes. The calculated intensities reveal that the (N)H stretching mode is at ∼ 1575 cm −1. This is a dramatic change compared to all former assignments at ∼ 3250 cm −1 based on the infrared and Raman data. These unforeseen proton dynamics are associated with the weakening of the NH bond due to the ionic character of the hydrogen bond (N δ−…H +…O δ′−) and proton transfer. The infrared and Raman spectra are reconsidered and a new assignment scheme is proposed for the amide bands in terms of dynamicalproton exchange between the amidic (…OCNH…) and imidolic (…HOCN…) forms in infinite chains of hydrogen-bonded molecules.

Evidence for two kinds of hydrogen bond in ice

DESPITE its simplicity at the molecular level, water is a complex and poorly understood liquid1. The reasons for this centre around the existence of a dynamic hydrogen-bonded network throughout the liquid. Attempts to describe the structure of liquid water have tended to invoke either continuum models such as the 'distortedbond' model2, which assumes that the hydrogen-bonded structure relaxes on a timescale similar to that in other liquids, and mixture models, such as the 'flickering-cluster' model3, which postulate the coexistence of two or more long-lived structures in the liquid. Here we analyse inelastic neutron-scattering spectra for the ice I solid phase of water, which provide evidence for the existence of two different kinds of hydrogen-bond, of different strengths, in the solid. A model in which strong and weak hydrogen-bonds in the ratio of about 2:1 are randomly distributed throughout the network is able to reproduce the neutron spectra. If we can assume that the same kind of bimodal hydrogen-bonding exists in the liquid state, our model may be able to explain several of the anomalous properties of liquid water, such as the large specific heat and the unusual behaviour of water in thin films and clusters.

Inelastic neutron scattering study of γ-MnO 2: the dynamics of (H +) 4 entities in Mn 4+ vacancies

The inelastic neutron-scattering spectra of γ-MnO 2 are reported at 20, 100, 200 and 300 K. The difference spectra (100-20 K and 200-20 K) show bands at 40, 90, 185 and 345 cm −1 which are not observed at 20 or 300 K. These bands correspond to the J = 0, 1 2 and 3 levels of a freely rotating spherical top with a rotational constant B = 25 cm −1. This is assignment to (H +) 4 entities in the lattice vacancies. The frequency shift of the J=0 level is interpreted in terms of inversion, i.e., out-of-phase rotation and antitranslation of two H +…H + pairs inside the (H +) 4 entities. At 20 K the free rotation disppears. An equilibrium between two configurations for the four photons associated with the vacancy is proposed. At 300 K the spectrum indicates a gas of free particles.