Anderson Is

Neutron scattering studies of the structure and dynamics of rare-earth hydrides and deuterides

Abstract The optic-vibrational density of states and the structural arrangement of hydrogen and/or deuterium dissolved in or compounded with the rare-earth metals (RH(D) x where 0 x ≤3) have been measured by neutron vibrational spectroscopy (NVS) and neutron powder diffraction (NPD) for both the yttrium– and lanthanum–hydrogen systems. The spectra reflect the diversity in the phases encountered, from the α -phase solid solutions at low hydrogen concentrations to the superstoichiometric β -phase dihydrides above x =2 as well as the γ -phase trihydrides. The structure and dynamics of these systems are examined in detail for YH x (0 x ≤3), where all three phases occur within the full concentration range, and LaH x (2≤ x ≤3), where the fcc metal lattice remains the stable configuration up to the trihydride stoichiometry. In particular for LaH x , NVS measurements of both pure and isotopically diluted samples indicate that the vibrational dynamics of hydrogen located in the octahedral ( o ) and tetrahedral ( t ) interstices are dramatically altered by the presence of significant x -dependent H–H interactions. NPD measurements are used for mapping out the x -dependent lattice symmetry due to the occurrence of long-range ordering of hydrogen in the o sublattice and as an aid in the interpretation of the vibrational spectra. In La hydrides prepared with H/D mixtures, both NVS and NPD confirm an isotopic enrichment of hydrogen in the o sublattice at low temperature.

Neutron spectroscopic comparison of β-phase rare earth hydrides

Abstract The β-phase hydrides β-RH 2 + x of a variety of rare earth metals (R ≡ Tb, Y, La, Ce and Dy) were investigated by incoherent inelastic neutron scattering spectroscopy. The broad, complex, vibrational density of states (DOS) of the tetrahedrally coordinated (t-site) hydrogen (H t ) typically found for these systems is a manifestation of the dynamic coupling within the H t sublattice due to significant H t -H t interactions. A breakdown of this dynamic coupling via isotopic dilution of the H t atoms with larger mass deuterium atoms is observed spectroscopically by a collapse of the complex DOS into a single sharp feature. For x > 0, the extra hydrogen atoms reside in the octahedral (o-site) interstices. Typically, the H o DOS is found to be sensitive to the temperature- and concentration-dependent arrangement of the H o atoms in the o-site sublattice. For small x , the H o atoms are isolated in a local cubic environment and possess a sharp DOS. As x increases, the transformation to short-range then long-range order occurs as shown by the development of a broadened complex DOS. The presence of H o atoms also perturbs the H t DOS. Typically, H o -H t interactions cause the affected H t vibrations to stiffen as illustrated by the formation of a higher energy feature split from the main H t DOS. In short, hydrogen-hydrogen interactions in the rare earth β-phase hydrides lead to interesting structural and dynamic behavior which can be probed nicely by neutron vibrational spectroscopy.

Dynamics of μ+ in Sc and ScHx

Abstract Muon spin rotation (μSR) experiments on single-cystal samples of ScHx, with x = 0, 0.05, 0.10 and 0.25, were performed in order to investigate the muon location and possible systematic variations in muon dynamics in these systems. The angular and magnetic field dependance of the μSR relaxation rates shows that at temperatures below 50 K the muon is localized at the interstitial tetrahedral (T) sites. The results for pure Sc, covering a wide range of high and low fields, are coherent with the picture that the muon performs fast tunnelling between two adjacent T sites, separated by 2z = 0.18c, above and below the a-b plane. This value for the muon position along the c axis is remarkably near the 2z = 0.194c found for D in Sc by neutron diffraction. The anisotropy of the relaxation rate at low magnetic fields varies sensitively with the H concentration, indicating a change in the ratio of the radial, muon-induced and the partly inherent, uniaxial electric field gradients at the position of the Sc nuclei.