R. Mukhopadhyay

Diffusion of water in porous alumina: Neutron scattering study

Dynamics of water molecules contained in the pores of alumina gel have been studied by the neutron scattering technique. Neutron incoherent quasi elastic scattering data from the confined water are analysed following the procedure described by Volino and Dianoux [15]. We have obtained the radius of confinement (or the localisation radius), the translational diffusion coefficient and the residence time for translational diffusion at room temperature. It was found that the bulk water diffusion phenomena are very much different from the diffusion of water molecules in a confined system.

Phonon density of states in Tl2CaBa2Cu2O8

Abstract The neutron-weighted phonon density of states in the high-temperature superconductor Tl 2 CaBa 2 Cu 2 O 8 ( T c =107 K) is obtained from coherent inelastic neutron scattering measurements at the Dhruva reactor at 300 K. The phonon spectrum is qualitatively similar to that in the 90 K superconductor YBa 2 Cu 3 O 7 , and compares well with an independent lattice dynamical calculation.

Low temperature magnetic phase of KMnFeF6 : A neutron depolarisation and diffraction study

Abstract Neutron diffraction and zero-field-cooled, field-cooled neutron depolarisation measurements have been carried out for KMnFeF6 system. Our study does not indicate any feature of spin-glass like transition near 108 K as reported in the literature. On the other hand, results show the presence of long range ferrimagnetic correlation in this geometrically frustrated system. The observed features of a.c. susceptibility, low-field magnetisation and neutron depolarisation measurements have been explained on the basis of thermally activated domain wall movement of ferrimagnetic arrangement of canted spins.

Methyl group dynamics in glassy polymers by neutron scattering: from classical to quantum motions

Abstract Recent experimental neutron scattering results have stimulated a new revival of the interest on methyl group dynamics in glasses and polymer systems. The existence of quantum rotational tunnelling of methyl groups in polymers was expected for a long time. Only very recently, these processes have been directly observed by high-resolution neutron scattering techniques (IN16, ILL Grenoble). The goal of this paper is to revise and summarise the work done in these topics over the last few years by means of neutron scattering. It will be shown that the results obtained, for both quantum and classical regimes, in different polymers can be consistently described in terms of a model which assumes a pure single-particle threefold potential for methyl group dynamics together with a distribution of potential barriers. The microscopic origin of such distributions, which should be associated to the disorder present in any structural glass, can be investigated by molecular dynamics simulations in realistic polymers. It is concluded that the width of the distribution is mainly controlled by the non-bonded interactions.

The distribution of tunnelling frequencies for methyl group rotation in poly(vinyl acetate)

Abstract The methyl group rotational tunnelling in poly(vinyl acetate) (PVAc) has been analysed by means of neutron scattering covering a range of tunnelling frequencies from 0.5 μeV to the free rotor line at 0.65 meV. The measured spectra show an apparently quasielastic broadening that has been interpreted in terms of a distribution of tunnelling frequencies. The results obtained in present experiments support those obtained previously from the analysis of the tunnelling spectrum in a smaller width window in combination with measurements in the classical hopping regime.

Rotational excitations of NH4+ ions in metal alkali halide lattices—Are they strain field mediated?

Abstract We show that the rotational excitations of NH4+ ions in mixed ammonium metal alkali halides at low temperatures—observed in inelastic neutron scattering experiments—can be satisfactorily explained if the random strain fields in the system can be modeled in terms of Gaussian distributed barrier heights to their rotations.

Real time deuterium loading investigation in palladium using neutron diffraction

Abstract This paper reports the first real time observation of electrochemical loading of deuterium in palladium as observed through neutron diffraction. The development of the β-phase from the parent α-phase of PdD x is studied as a function of loading time. The value of x is seen to saturate at 0.55 (±0.05). Estimated value of the diffusion constant for deuterium is 5(±2) × 10 −7 cm 2 s −1 .

Dynamics of water contained in porous alumina

Abstract We report dynamics of water molecules contained in the pores of alumina gel as studied by neutron scattering technique. Neutron incoherent quasi-elastic scattering data from confined water are analysed. Radius of confinement (or the localisation radius), the translational diffusion coefficient and the residence time for translational diffusion at room temperature are obtained. It is found that the diffusion of water molecules contained in the pores of alumina gel differ from the bulk water diffusion phenomena.

Rotational excitations of NH+4 ion in the dipolar glass (NH4)xK1−xI

Abstract It has been demonstrated that inelastic neutron scattering profiles which qualitatively agree with the observed ones from the dipolar glass, (NH 4 ) x K 1− x I, can be generated within the framework of the nearest neighbour binomial model.

Quantum molecular tunneling of NH4 + ions in mixed ammonium-alkali halide systems

The rotational excitations of NH4+ ions in dilute solution in metal alkali halide lattices have been studied at 4K by inelastic neutron scattering technique. It is found that the ions exist in matrix isolation at very low ammonium ion concentration,c (c → 0), and it transforms to orientational glass phase on increase ofc. It is shown that the onset of the orientational glass phase is moderated through the strain field generated by the substituted impurity in the lattice. The variation of the intensity profiles with increase ofc have been successfully explained by a phenomenological model based on gaussian distributed heights.