ALBERT

Notes: The structure of 3Na2O⋅R2O3⋅6SiO2 (R=Nd, Yb and Lu) glasses has been studied by time-of-flight neutron diffraction. The average Si-O and O-O nearest-neighbor coordination is found to be similar to those of the SiO4 tetrahedral units in vitreous SiO2 and other silicate glasses. The rare-earth (RE) ions coordinate with no more than seven nearest-neighbor oxygen atoms. The dynamic and static response of these glasses and their isochemical crystalline analogs has been investigated by inelastic neutron scattering and magnetic susceptibility measurements. The magnetic excitation spectra were analyzed by a ligand-field model using a method of descending symmetry. The correlation between short-range atomic order in a network glass and magnetic interactions of the RE ions with the local environment is discussed.

Notes: A Raman heterodyne detection of magnetic resonance has been performed for probing the local structure of a sodium phosphate glass doped with trivalent europium. High resolution rf modulation spectra between 0.5 and 10 MHz were observed without the laser field in resonance with electronic transitions of the glass. The intensity of the observed Raman heterodyne signal depends on the external static magnetic field and sample temperature as well as the intensity of the laser and rf fields. The ability of monitoring the rf resonance spectra with micrometer spatial resolution may offer a potentially important means to probe the variation of local structure in disordered solid state materials. © 1996 American Institute of Physics.

Notes: In this paper we present and discuss experimental results on molecular mobility in propylene glycol and its three oligomers confined to the ∼100 A(ring) pores of a controlled porous glass. The objective is to elucidate the finite size effects on the dynamics of hydrogen-bonded liquids of different molecular weights but identical chemical composition. The methods of dielectric and neutron spectroscopy have been employed to investigate both the low- and high-frequency features as a function of temperature. We find that all fluids in pores separate into two distinct liquid phases. (i) molecules physisorbed at the surface which exhibit a dramatic frustration of their mobility related to a substantial positive shift of the glass transition temperature Tg by up to ΔTg≈+47 K; and (ii) relatively "free'' molecules in the inner pore space subject to only moderate retardation of the α and normal mode relaxation and substantial broadening of the distribution of relaxation times. The shift in Tg for the α process with ΔTg≈+5 K is maximal for the monomer liquid and gradually diminishes with increasing molecular weight or decreasing intermolecular hydrogen bonding. The inelastic neutron spectrum of confined propylene glycol shows the boson peak as expected in bulk strong and intermediate glass formers in the vicinity of Tg. This effect can be attributed to the finite-size induced crossover from long wave vibrations characteristic of a continuous medium to localized vibrations in a confined geometry. © 1995 American Institute of Physics.

Notes: The diffusion of hydrogen molecules in hexagonal D2O ice (Ih) in the temperature range of 25–60 K was investigated by quasielastic neutron scattering. The observed spectra consist of a narrow elastic peak and a broad quasielastic component. The single-particle diffusion coefficient for H2 in D2O ice was obtained using a simple model of diffusion and is remarkably high—comparable to the coefficient of H2 in a liquid.

Notes: Neutron inelastic scattering experiments have been performed on UF4 and NpF4. We had anticipated observing a number of sharp magnetic transitions in these experiments because the tetravalent metal ions are expected to have localized f states. However, very few transitions were observed, three and two in UF4 and NpF4, respectively. Except for the lowest-lying level (at 1.5 meV) in UF4 the transitions are wider than the instrumental resolution, suggesting a strong interaction between the electronic states and lattice vibrations. The levels observed do not correspond to those predicted by a crystal-field scheme recently proposed from optical spectroscopy. However, the paucity of observed levels in our experiments excludes us proposing an alternative scheme. The 1.5 meV energy level in UF4 is in good agreement with that deduced from specific heat and magnetic susceptibility data. © 1994 American Institute of Physics.

Notes: Early experiments at the pulsed neutron source at Argonne National Laboratory (ANL) showed that it was possible to observe crystal-field (CF) levels at energy transfers above 150 meV in UO2. More recent investigations at the UK pulsed source at Rutherford Laboratory have improved the resolution and seen a series of well-resolved peaks. Based on these measurements the CF parameters are V4=−130 meV and V6=+30 meV. Since these parameters should be independent of the precise An ion, there is interest in looking at the tetravalent oxides UO2, NpO2, and PuO2. We have now performed experiments at ANL on NpO2 at temperatures between 15 and 200 K. We find no sharp peak in the excitation spectrum up to energy transfers of about 250 meV. This result is contrary to the expected strong transition at about 53 meV using the above CF parameters in a complete intermediate coupling calculation. The observed phonon spectrum of NpO2, in comparison with that of ThO2, shows larger widths in all of the three vibrational bands above ∼30 meV, which may indicate strong interaction between CF levels with optic phonons in NpO2.

Notes: We have performed inelastic neutron scattering study of the cerium γ↔α valence transition (T0(approximately-equal-to)150 K) in polycrystalline Ce0.74Th0.26 at the Intense Pulsed Neutron Source (IPNS) of Argonne National Laboratory. An incident neutron energy of 300 meV was used to measure the excitation energy spectra of Ce0.74Th0.26 at 100, 140, 155, and 200 K by chopper spectrometers. By comparing the neutron total scattering of Ce0.74Th0.26 with that of La0.74Th0.26, an isostructural nonmagnetic alloy measured under idential experimental conditions, the magnetic scattering function of Ce0.74Th0.26 averaged over all q in the Brillouin zone, Savemag (Q,E), is derived. We find that the magnetic response of Ce0.74Th0.26 at all temperatures is predominately due to moments of 4f character. The obtained magnetic scattering function in this temperature region consists of a broad quasielastic peak, which is well fitted by a spin relaxational spectral function, namely, a Lorentzian centered at 0 energy. The peak shifts to higher energies and broadens as the temperature is lowered. As the temperature decreases across the transition temperature, the magnetic intensity drops sharply, accompanied by an abrupt broadening of the linewidth corresponding to a spin fluctuation energy much higher than the thermal energy. Γ, the Lorentzian HWHM's, were found to be 16, 25, 63, and 110 meV at T=200, 155, 140, and 100 K, respectively. Within experimental precision, we find no evidence of additional inelastic peaks due to crystal-field excitations. These results are in good agreement with those from an earlier neutron experiment1 using thermal-energy neutrons in which the measured spectra were limited to about 70 meV. The static single-site susceptibility obtained by a Kramers–Kronig analysis agrees well with the bulk susceptibility.1 The 4f occupation per Ce atom, deduced by summing theneutron data from −100 to 230 meV, is about 0.6 and 0.4 for the γ and α phase, respectively. These values are considerably smaller than those obtained from photoemission2 and other measurements.1 This indicates that the neutron experiment did not extend to high enough energies to account for all the intensity. In the high-temperature γ phase the relaxational model represents a reasonable approximation to the spin dynamics. By integrating the Lorentzian scattering function obtained from the fits of the neutron data over an energy interval of −0.1 to 2 eV, we obtained an f occupancy close to unity in the γ phase. For the α phase there is currently no analytical expression of the magnetic scattering function from first-principle calculations. Model calculations3,4 of the ground state (T=0) for a single f impurity in the metal, on the other hand, predicts a magnetic response function having a thresholdlike rise at a finite energy, followed by a long tail extending to high energies. In order to be able to compare with the theory, we have also undertaken the measurements of the magnetic scattering function of Ce0.74Th0.26 at 10 K with an incident neutron energy of 1.2 eV. We find that Savemag shows an inelastic peak at about 138 meV and a tail at higher energies. The line shape of the measured spectrum agrees qualitatively with the single-impurity theory3,5,6 and the result of a recent polarized neutron study7 of α-Ce. By summing the measured magnetic intenstiy up to 500 meV, we estimated a 4f occupancy of 0.76 per Ce atom. The static susceptibility at 10 K and the electronic specific heat coefficient obtained from the neutron data and the theory3,8 agree well with the values obtained from bulk measurements.1,9,10 The above results are also in fair agreement with the electronic spectroscopy data.2 Since the neutron measurements were made using polycrystalline samples, we are unable to detect, if any, coherence effects6,11 due to interaction of the f electrons in the lattice. A detailed report on the inelastic neutron scattering investigations is presented elsewhere.12

Notes: Polarized neutron scattering techniques have been used to study the spatial distribution and temperature dependence of the magnetization induced by an externally applied magnetic field in a single crystal of CeCu6, a heavy-fermion compound that remains paramagnetic and nonsuperconducting down to ∼10 mK. The measurements were performed at 4.2 K and 92 mK in an applied magnetic field of 50 kOe. We find that the induced magnetization is predominately of 4f electronic character and exhibits Pauli-like behavior in the 4.2 K–92 mK temperature region.

Notes: The crystal-field splitting of the Er3+ ground multiplet 4I15/2 in ErPO4 is investigated by inelastic neutron scattering. Four excitations from the Γ7 ground state to the excited states and several transitions between the excited states have been identified. The observed transition energies and intensities are used to refine the parameters of the crystal-field potential. The calculated magnetic susceptibility χ(T) agrees well with experimental values from single-crystal measurements. A comparison of the neutron data with optical absorption and both nonresonance and resonance Raman scattering measurements has been made.

Notes: A 5.4-meV excitation has been measured in PrO2 using inelastic neutron scattering. The excitation is identified as the energy splitting of the Γ8 ground state, corresponding to the dynamic Jahn–Teller process responsible for producing a reduced magnetic moment below the 14-K Néel point. Its magnitude and temperature dependence are in keeping with the predictions of Allen and of Sasaki and Obata for UO2, as scaled to PrO2. We had previously invoked this mechanism to explain the broadening of the 130-meV Γ8-to-Γ7 transition. Implications of these results for the magnetic response of PrO2, and other f-electron systems will be discussed.