C.-K. Loong

Crystal-field properties of f-electron states in RBa2Cu3O7 for R = Ho, Nd and Pr

The authors present details of their predictions for the crystal-field split levels in RBa2Cu3O7 compounds with R=Nd3+ (f3 4I9/2) and Pr3+ (f23H4) based on the previously reported crystal-field split energy levels of f105I8 for Ho3+ in HoBa2Cu3O7. Their predictions of the overall distributions of crystal-field levels provide the framework for interpretation of inelastic neutron scattering spectra of NdBa2Cu3O7 and PrBa2Cu3O7. For NdBa2Cu3O7 the inelastic scattering peaks are clearly seen and can be well assigned in terms of the energy levels of f34I9/2 by a small re-scaling of the predicted crystal-field parameters to adjust the overall width of the calculated spectrum. In the case of PrBa2Cu3O7 several important inelastic scattering features, particularly the lowest energy ones, are so unusually broad and weak that it was difficult to identify them without predictions of this type. Enough features in the scattering spectra have now been assigned for us to obtain fitted values for the crystal-field parameters of Pr3+ and Nd3+ in RBa2Cu3O7. As tests of these new assignments they discuss predictions of the anisotropies of magnetic susceptibility, and the thermodynamic functions for the paramagnetic phase of PrBa2Cu3O7. Finally, a primitive model for interactions between Pr3+ ions is used to give an initial interpretation of the saturation moment that has been reported for the antiferromagnetically ordered phase of PrBa2Cu3O7. They conclude that standard crystal-field theory correctly applied to the f2 configuration of the Pr3+ ions fully explains the unusual magnetic properties of PrBa2Cu3O7.

Crystal structure of (LixK1 − x)2CO3 (x = 0, 0.43, 0.5, 0.62, 1) by neutron powder diffraction analysis

Abstract The crystal structures of (LixK1 − x)2CO3 (x = 0, 0.43, 0.5, 0.62, 1) were determined by neutron powder diffraction at room temperature. These samples were obtained by a melt and quench process. (LixK1 − x)2CO3 (x = 0, 0.5, 1) formed as single phases and (LixK1 − x2CO3 (x = 0.43, 0.62) contained two phases at room temperature. In the Rietveld refinements, the improvement from 7Li enrichment is due to the lower absorption coefficient of 7Li than of 6Li. A final weighted R-factor of 4.54% was obtained for the refinement of 2373 reflections by the Rietveld method from a (Li0.5K0.5)2CO3 synthesized using 7Li2CO3 and K2CO3 (99.9% pure). Slight distortion of the CO32− units in the monoclinic cell was observed; the O(1)CO(2) angle and CO(3) length are larger than those for the other CO bonds and OCO angles. These local structure characteristics can be explained by the difference in the ionic size of Li+ and K+ and the different electrostatic interactions between the cations and CO32− units.

Raman study of phonon modes in ErVO4 single crystals

The phonon modes of a pure ErVO4 crystal were determined at room temperature using Raman scattering methods, and the observed frequencies were assigned according to group theory in terms of the internal modes of the VO43− ions and the external modes of the Er(VO4) lattice. The assignments of the phonon modes match well with the overall phonon systematics of the rare-earth orthovanadate series, and the results presented here reinforce the general trend of bonding strength in the zircon series of RVO4, RAsO4, and RPO4 materials.

Neutron scattering investigation of the magnetic ground state of PrO2

Abstract We report on a series of neutron experiments on stoichiometric PrO 2 . The material, which has the cubic CaF 2 structure, orders antiferromagnetically at 14 K with a type I magnetic structure as found in UO 2 . The ordered moment is 0.6 ± 0.1 μ B /Pr atom. No lattice distortion has been observed below T N with a high-resolution neutron-diffraction experiment. Neutron inelastic scattering at the Intense Pulsed Neutron Source, Argonne National Laboratory, has been used to determine the crystal-field splitting of 130 ± 5 meV between the Γ 8 ground state and the Γ 7 excited state of the J = 5 2 multiplet. This represents the first direct observation of a crystal-field splitting in the rare-earth or actinide dioxides, and the largest such electronic splitting ever observed by neutron scattering. We propose that the ground state of PrO 2 is the Γ 8 quartet with the degeneracy lifted by a dynamic Jahn-Teller effect. The A 4 〈 r 4 〉 crystal field potential term for PrO 2 is - (57 ± 3) meV, and the significance of this determination for other oxides is discussed.

Anisotropic thermal expansion of MgSiN2 from 10 to 300 K as measured by neutron diffraction

The lattice parameters of orthorhombic MgSiN2 as a function of the temperature have been determined from time-of-flight neutron powder diffraction. The results indicate that MgSiN2, just like several other adamantine-type crystals, exhibits a relatively small thermal expansion coefficient at low temperatures. This is ascribed to a strongly bonded 3D, relatively open, crystal structure which is characteristic of highly covalent bonded materials. The anisotropic linear thermal expansion behaviour could be qualitatively related to the characteristics of the crystal structure. The least dense packed crystallographic direction showed the smallest anisotropic linear expansion coefficient.

Raman scattering study of Tb(V1−xPx)O4 single crystals

The polarized Raman spectra of a series of isostructural vanadate/phosphate compounds of the form: Tb(V1−xPx)O4 with 0⩽x⩽0.75 have been investigated at room temperature. While the observed Raman spectra are generally consistent with group theory predictions for a body-centered tetragonal crystal structure (D4h19), due to the replacement of P for V, additional spectral features induced by the disorder of the mixed vanadate/phosphate system are also observed. In particular, all of the external lattice modes are characterized by a one-phonon-like behavior, while the behavior of the internal modes of the (V,P)O4 tetrahedron is two-phonon-like.

Optical characterization of Nd3+- and Er3+-doped lead-indium-phosphate glasses

In this work, Judd–Ofelt analysis is applied to rare-earth-doped lead-indium-phosphate glasses (RE-PbInPO4, where RE=Er3+ and Nd3+) in order to evaluate their potential as both glass laser systems and amplifier materials. The phenomenological Judd–Ofelt parameters Ω(2), Ω(4), and Ω(6) are determined for both rare-earth ions together with their quality factors and compared with the equivalent parameters for several other host glasses. The spontaneous emission probabilities and the lifetimes of the Nd3+ F43/2 laser transitions are determined and analyzed as a function of the optical quality factor. For Nd3+-PbInPO4, glass fluorescence emission (890, 1058, and 1330 nm) lines are observed. Highly efficient infrared-to-visible frequency up-conversion at 530, 550, and 670 nm as well as an intense infrared fluorescence emission (∼1540 nm) is observed in Er3+-doped PbIn(PO4) glasses pumped using 800 nm radiation excitation.

Crystal field excitations in RE2CuO4 (REPr, Nd)

Abstract The magnetic excitation spectra of Pr 2 CuO 4 and Nd 2 CuO 4 determined by inelastic neutron scattering over the temperature range 15–300 K are reported. Well-defined transitions between the rare earth ion crystal field levels have been observed up to about 100 meV for both compounds. The results are analyzed in terms of a crystal field model under the scheme of intermediate coupling calculated by spherical tensor techniques. The calculated crystal field excitation spectra and magnetic susceptibilities agree well with experiments. No measurable influence of the rare earth crystal field structures by the antiferromagnetic ordering of copper spins is found in these materials.

Anomalous temperature dependence of the lattice parameters in HoPO4 and HoVO4: rare earth quadrupolar effects

Abstract The temperature dependencies of the lattice parameters in tetragonal HoPO 4 and HoVO 4 were measured using neutron powder diffraction techniques. Below about 100 K, the lattice parameter a of HoPO 4 increases with decreasing temperature while c decreases. In HoVO 4 , the above behaviour is reversed, that is a decreases with decreasing temperature while c increases. Similar measurements on non-magnetic LuPO 4 and LuVO 4 do not show any anomaly. This observation indicates that the unusual temperature dependence of the lattice constants is magnetic in origin. The observed effect can be explained in terms of an Ho 3+ quadrupole interaction with the crystalline lattice. In particular, the calculated electronically generated quadrupole moment of Ho 3+ in HoPO 4 and HoVO 4 exhibits a temperature dependence similar to that observed for the lattice parameters.

Crystal-field splitting, magnetic interaction, and vibronic excitations of 244Cm3+ in YPO4 and LuPO4

The electronic and lattice-dynamical properties of 244Cm3+ doped orthophosphate crystals of YPO4 and LuPO4 were studied using site-selective fluorescence line narrowing (FLN) techniques. Although radiation damage caused substantial inhomogeneous line broadening, FLN gave rise to sharp resonant and satellite lines. Samples were annealed at 500 °C to eliminate satellite lines due to Cm3+ ions at different lattice environments. The crystal–field splitting of the nominal 8S7/2 ground state and the 6D7/2 excited state was determined from an analysis of the excitation and fluorescence spectra using an exchange charge model of crystal field interaction within the scheme of intermediate coupling. The effective g factors of the four Kramers doublets of the J=7/2 ground state and the lowest doublet of the excited state were evaluated from a least-squares fit of the observed Zeeman splitting to an effective-operator Hamiltonian. The energies of phonon modes were estimated from the observed vibronic spectra of the 24...