Local lattice distortions and ligand hyperfine interactions in Eu2+-and Gd3+-doped fluorites

A computer simulation of lattice distortions around an impurity ion Eu²⁺ in MeF₂ fluorites (Me=Ca, Sr, Ba) is reported. ENDOR data on displacements of F⁻ ions distant from an impurity center were used to determine the parameters of the Eu²⁺-F⁻ short-range interaction potential. A theoretical study of the effect of hydrostatic pressure on the impurity-center local structure has been made. A comparison with experimental data permits a conclusion that the calculated ligand displacements are reliable. An experimental ENDOR investigation of the ligand hyperfine interaction (LHFI) in MeF₂:Gd³⁺ crystals (Me=Ca, Sr, Pb, Ba) has been performed. The results obtained in the simulation are used to describe the LHFI of impurity ions with the nearest-neighbor fluorine environment. The contributions to LHFI associated with ligand polarization are shown to constitute 10–50% of the experimental LHFI constants. The inclusion of such contributions results in practically linear dependences of the remaining short-range part of the LHFI on distance. Fiz. Tverd. Tela (St. Petersburg) 40, 2172–2177 (December 1998)     

Ligand hyperfine interactions and the model of the paramagnetic center Gd3+ in α-LiIO3 single crystals

The ligand hyperfine interaction (HFI) of Gd³⁺ ions in α-LiIO₃ single crystals is studied by the method of rf discrete saturation, and the tensor components of ligand HFI are determined. The model of a paramagnetic center is proposed on the basis of the analysis of the obtained results, and the mechanism of lattice distortion is discussed.     

Theory of hyperfine interactions in aqueous Mn3+-porphyrin system

The electronic structure and associated magnetic hyperfine properties of Mn³⁺-porphyrin with two ligand water molecules are studied using the Unrestricted Hartree-Fock procedure with an aim to understand the observed anomalous water proton relaxivity associated with Mn³⁺ (TPPS₄). Our results show that the electron spin distribution is strongly localized on the metal ion and the point dipole model seems to be a good approximation for the dipolar hyperfine interactions in this system. It appears that the anomalous relaxivity observed in the Mn³⁺ (TPPS₄) is not due to the strong delocalization of the spin distribution associated with the metal ion and may result from unusual dynamical effects.     

Magnetic hyperfine interactions in GdAl3

We have studied the magnetic hyperfine interactions in GdAl₃ using¹⁵⁵Gd Mössbauer spectroscopy between the temperatures of 4K and 90K. Previous studies on GdAl₃ have shown that antiferromagnetic ordering occurs at 18K, and that a fit of the susceptibility to 1/(T-θ_p) yields a θ_p value of ?89K. The large ratio of θ_p to T_N is indicative of magnetic frustration between competing ferro-and antiferromagnetic interactions, which may be due to a combination of the oscillatory nature of the RKKY interaction and the geometry of the hexagonal lattice. Our studies show that the saturation magnetic hyperfine field at the Gd site is ?24.0 T, with the moments lying in the basal plane. The efg at the gadolinium site is 2.55(1)×10¹⁷V cm^?2 which is considerably larger than the value predicted by a point charge calculation. This difference may indicate that there is a conduction electron contribution. A helical magnetic structure has been calculated from RKKY theory.     

The 693 keV 2+′→2+ transition in154Gd

The multipolarity of the 2²⁺′→2⁺ transition in¹⁵⁴Gd has been investigated. The correlation coefficientA ₂₂=? 0.04±0.10 was determined on an electron — gamma directional correlation spectrometer, while the conversion coefficient α _K =0.046±0.002 was deduced by comparing the normalised electron and gamma intensities. From these values the quantities δ,q _K andX were calculated and compared with the theoretical predictions.     

Low-energy coexisting band in 154Gd

A low-energy coexisting band Jpi (Ex keV) 0(+) (1182), 2(+) (1418), 4(+) (1701) is identified in the deformed nucleus, 154Gd. Detailed gamma-ray spectroscopy following the beta decays of 154Eu (J=3), (g,m(1),m(2))154Tb (J=0,3,7) is used to establish this structure. The structure is explained in terms of the pairing and deformation degrees of freedom, a "pairing isomer," which results from the nu[505] upward arrow Nilsson intruder orbital.     

Angular correlation measurements in154Gd

γ-γ angular correlation measurements were performed to determine the mixing parameters δ of some γ-transitions in¹⁵⁴Gd. High energy compton background could easily be eliminated using a two dimensional data aquisition system. The relativeB (E2)-values of (K=2→K=0) interband transitions were determined and compared with collective model predictions. Radioactivity:¹⁵⁴Eu[from¹⁵³Eu(n, γ)], enriched target. \(^{154} Eu\xrightarrow{{\beta ^{ --} }}^{154} Gd.\)      

Orbital following and hyperfine interactions in CH3

In a recent paper, Beveridge and Miller conclude that for the calculation of isotropic hyperfine interactions in simple planar hydrocarbon radicals, a semi-empirical molecular orbital method has some advantages over a semi-empirical valence bond method. In the present work we examine this conclusion and find that the arguments which support it are questionable. Our own critical comparison of the two methods does not lead to a clear conclusion. In addition, we report a method for calculating the degree of orbital following in a spin-unrestricted Hartree-Fock molecular orbital wave function and give numerical results for CH₃ in its out-of-plane bending mode.     

Laser spectroscopy applied to the study of hyperfine interactions

The impact of laser-spectroscopy techniques on the study of hyperfine interactions is illustrated by chosen examples. A brief survey of the theory of hyperfine interactions is given as well as an overview of the available experimental techniques. Results from alkali atoms, alkaline-earth elements and Group III atoms are given with special emphasis on the studies of electronic properties in sequences of Rydberg states and the investigation of nuclear properties for long chains of isotopes of the same element. Recent developments on the theoretical side, particularly what regards the many-body perturbation approach, are also discussed.     

The hyperfine field on impurities in Gd

Predictions are given for the hyperfine fields on s-p impurities in Gd.     

Electron paramagnetic resonance of Gd3+ ions in powders of LaF3:Gd3+ nanocrystals

The observation of electron paramagnetic resonance of Gd³⁺ ions in nanosized powders of rare-earth fluorides LaF₃:Gd³⁺ has been reported. The measurements have been performed on a single crystal and micro- and nanosized powders at room temperature. Electron paramagnetic resonance spectra and spin-Hamiltonian parameters of Gd³⁺ ions have been obtained. A qualitative difference of spectra in nano- and micropowders due to the increase in the spread of the crystal field parameters with the decrease in the particle size has been found. The relationship between the single-crystal domain size and the hydrothermal treatment time has been established.     

A study of platinum-supported catalysts through hyperfine interactions

The effect of indium addition on alumina-supported platinum catalysts was investigated by measurements of hyperfine interactions. Via lime differential perturbed angular correlation spectroscopy (TDPAC) on¹¹¹Cd, Pt/Al₂O₃ catalysts were studied in the flow of a heptane/H₂ gas stream. The results indicate that some amount of indium sticks to platinum which is then dispersed on the support surface. The amount of In that is free from platinum is mobile under reaction conditions, being capable of diluting platinum particles and draining off coke precursors from the platinum surface.Researcher from Consejo Nac. Invest. Cient. Tecn., CONICET, Argentina.     

The hyperfine interaction of As in Gd metal

The temperature dependence of the internal magnetic hyperfine field at As in Gd metal has been measured by IPAC technique at 90, 140, 193, 230 and 300 K. The reduced hyperfine magnetic field shows slight deviation from the Brillouin function for J=7/2.     

Theoretical study of the rovibrational dependence of nuclear hyperfine interactions in the v 3 band of SF6

The dependence of the spin-rotation hyperfine interaction on the vibration-rotation molecular state detected in the analysis of the hyperfine structures of the v ₃ band of SF₆ can be taken into account by introducing some of the formal operators given by Michelot. We have established the relationships between the coefficients of Michelot's formal operators and the molecular constants. This has enabled us to understand how far the formal coefficients are independent and to check the order of magnitude of spectral effects due to other hyperfine operators described by Michelot.     

Description of multi-band structures in154Gd and156Gd on symmetry considerations

The observed similarities of the yrast band level energies of several pairs of lighter rare-earth even-even nuclei are indicative of a particle-hole type symmetry for nuclei withA =164±a. This feature is sought to be described in terms of the representations of theSU(7) group in the interacting Fermion picture.SU(3) reduction of this group leads to the prediction of characteristic multi-band structures in these nuclei. Detailed comparison of the single-parameter predictions of this scheme with the available experimental informations on the level schemes for the nuclei¹⁵⁴Gd and¹⁵⁶Gd reveals striking agreement between the theory and the experiments. The results are discussed in comparison with the predictions of the simple Interacting Boson Model.     

Pac studies of spin deviations,pressure induced hyperfine field shifts,and temperature variation of supertransferred hyperfine fields

A PAC study of¹¹¹Cd substiuted antiferromagnetic transition metal salts is reported. Supertransferred hyperfine fields at¹¹¹Cd nuclei were used for the first time to observe three effects: zero-point spin deviations, temperature variation, and pressure shift of the hyperfine field. Comparison of KNiF₃ and RbMnF₃ with their corresponding quadraticlayer fluorides K₂NiF₄ and Rb₂MnF₄ yields an estimate for the magnitude of the zeropoint spin deviation in doped antiferromagnets. The temperature dependences of the Cd hyperfine fields in RbMnF₃/Cd and MnF₂/Cd have been determined. The shift in hyperfine field observed for -MnS/Cd under external pressure further supports our model for the origin of the supertransferred hyperfine fields.This work was supported by the US Energy Research and Development Administration.     

Theoretical study of hyperfine interactions at the Ta site in Hafnia polymorphs

The electric field gradient resulting from Ta substitutional defect in normal monoclinic phase is studied using all-electron ab initio NFP-LMTO method. Hyperfine parameters in Pbca and Pnma phases have also been calculated to determine the usefulness of quadrupolar interactions in the investigation of phase diagrams under hydrostatic pressure. Predictions for hyperfine parameters in high temperature P42nmc and Fm3m phases were also developed. Given the donor behavior of Ta in HfO₂, two charge states, 0 and +1, have been studied for each phase. Although HFI do not vary significantly with charge, it was determined that for a Ta+1 in P21/c phase hyperfine parameters is consistent with experimental results. Quadrupolar interactions for transitions to denser phases show important variations with respect to that of the normal phase: asymmetry parameter for Pbca and the electric field gradient for Pnma both increase substantially. At high temperature phases, drastic decrease in both EFG and asymmetry parameter in P42nmc is observed, while they almost vanish in Fm3m.     

The hyperfine interaction of Ho3+ in gold

Summary Ho in Au exhibits interesting paramagnetism. The full J=8 angular momentum is partially quenched by the crystal field. At sufficiently low temperatures, however, the ion acts like a Van Vleck paramagnet although it has a non-magnetic doublet at ground state instead of a singlet ground state. An applied magnetic field definitely is enhanced by the large hyperfine interaction and excited ground state mixing. This results in anisotropy all the way up to 1 K which makes^166m HoAu an excellent candidate for a secondary thermometer in the 10mK-1 K region. The complete interpretation of the^166m Ho³⁺ Au experiments will require sizeable computing times.