Transferred Hyperfine Interaction Research Articles

Transferred hyperfine interaction in the ESR spectra of Eu2+ in CdTe.

Transferred hyperfine interaction in the ESR spectra of Eu2+ in CdTe.

On the transferred hyperfine interaction in substituted yig

NMR of57Fe is studied in a number of (MxY3−x) Fe5O12 garnets for small concentrations of M (M is either trivalent RE ion −Ho 3+, Gd 3+, Nd3+, Pr 3+, La 3+ or Bi 3+ ion). Beside the main resonance lines, the satellites were observed, which correspond to those Fe, in vicinity of which the impurity M is located. After correcting for the dipolar field, the field corresponding to the change of the transferred hyperfine interaction in M3+−O2−-Fe3+ vs. Y3+−O2−-Fe3+ triad was deduced from the satellites splitting. The analysis of the results indicates that the observed change in the transferred hyperfine field is mainly connected with the transfer of electrons between M3+ and Fe3+ ions and not with the local deformation around the impurity.

Hyperfine interaction in the mixed conductor γ LiV2O5

Transferred hyperfine interaction effects as studied from51v NMR in γ LiV2O5 (bronze) between the non-magnetic V5+ and paramagnetic V4+ lead to differing non-integral valences (neither 5+ nor 4+) on these sites at room temperature and below. However, at high temperatures exchange between two sites leads to a single mixed-valence state. d-d (V4+ −V4+) pairing along with relaxation effects show up in the low temperature spectra. Though susceptibility remains almost constant from 300–100K, there is considerable narrowing as well as intensification of the lines.

19F ENDOR measurements of transferred hyperfine interactions in CsCdF3:Gd3+and CsCaF3:Gd3+

The transferred hyperfine interaction parameters have been measured at 4.2K by ENDOR for five shells of 19F neighbours around Gd3+ in CsCdF3:Gd and CsCaF3:Gd. The parameters for the octahedrally coordinated ligands are rather different from those previously determined from the transferred hyperfine structure of EPR spectra at 300K by Buzare and co-workers (1977). The authors' own EPR measurements show that the separations of the transferred hyperfine-structure lines are rather smaller than those predicted using the ENDOR data. They are only just consistent for CsCdF3, and they are slightly outside the estimated uncertainty of the EPR data for CsCaF3. There is no evidence of temperature dependence of the separation of the transferred hyperfine-structure lines in the EPR spectrum. Local distortions are deduced from 19F second-shell parameters and these distortions and the ligand-shell parameters are compared with those for octahedrally coordinated 19F ligands in CaF2 and CdF2.

Transferred hyperfine interaction of cubic Gd3+centres in CsCaF3

The transferred hyperfine (THF) interactions with two shells of 19F neighbours around Gd3+ centres in CsCaF3 were measured with ENDOR. The isotropic SHF constant of the nearest neighbours was found to be -3.86 MHz. The anisotropic constant b=5.80 MHz, which is smaller than the expectation for b from a classical point-dipole interaction. These results point to the observation of a negative spin density at the nearest neighbours. The measured THF constants can be explained satisfactorily by a simple overlap model in which the spin polarisation of the Gd3+ 5s and 5p shells due to the inner unpaired 4f7 electrons is transferred via overlap to the nearest neighbours. A similar good explanation with this model was also achieved for the negative spin density observed around Gd3+ centres in CsCdF3.

31P and51V ENDOR in LuPO4:Gd3+, YVO4:Gd3+and LuVO4:Gd3+

ENDOR measurements of 31P and 51V in LnPO4 and LnVO4 have shown three features. First they have shown that there are large non-dipolar contributions to the transferred hyperfine interaction between a Gd3+ impurity ion and the nearest neighbour positive ion, as was also found in alkaline earth fluorides by Baker and Bluck (1985). Secondly, they have shown that changes in the electric quadrupole interaction of 51V may be used to deduce local distortions produced by a Gd3+ impurity. Thirdly, they have demonstrated a large deviation in the angular dependence of the ENDOR spectra from the commonly used first-order expressions, due to effects of fine structure.

Theory of the transferred hyperfine interaction between well-separated atoms and the influence of soft-phonon modes.

We have calculated the transferred hyperfine interaction at the ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ion in ${\mathrm{PbF}}_{2}$ in which the distance between ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{Pb}}^{2+}$ is 30% larger than the bond lengths in ${\mathrm{MnF}}_{2}$. The contributions arising from the overlap of wave functions and the charge transfer and the charge redistribution effects have been calculated from first principles and the s, ${p}_{\ensuremath{\sigma}}$, and ${p}_{\ensuremath{\pi}}$ spin densities have been found. The change in the transferred hyperfine coupling due to a soft-phonon mode which goes to zero at a temperature of ${T}_{c}$=50 K has been calculated and verified experimentally.

The indirect hyperfine interaction of a29Si nearest neighbour to an oxygen defect in SiO2

A recent calculation determined the indirect aluminium hyperfine interactions for a number of oxygen-aluminium trapped-hole centres (Al3+-O-) in ionic crystals. One of these crystals was SiO2 where Al3+ is incorporated as an impurity. The theory showed that the dominant mechanism of the Al3+ transferred hyperfine interactions is exchanged polarisation. Since experimental data also exist for the Si4+ magnetic nuclei that are nearest neighbours to the Al3+-O- centre in SiO2, it is of interest to see if this same theory can account for the Si4+ transferred hyperfine interactions. This Letter reports on the results of calculating the Si4+ transferred hyperfine interactions for this system. The model has much the same level of success for the Si hyperfine interactions as for the Al.

ENDOR of cations in CdF2:Eu2+and PbF2:Gd3+

ENDOR has been measured for nearest cation nuclei in CdF2:Eu2+ and PbF:Gd3+. In the latter the transferred hyperfine interaction has axial symmetry about the Gd-Pb direction, corresponding approximately to the superposition of the dipolar interaction and an isotropic term AsS.I with As=-0.578(36) MHz. Assuming, as seems likely in comparison, that CdF2:Eu2+ also has an interaction with axial symmetry, it also corresponds approximately to a superposition of the dipolar interaction and an isotropic term with As=-0.346(24) MHz for 111Cd and -0.364(24) MHz for 113Cd. The magnitudes of As are unexpectedly large, corresponding to an interaction comparable with that with the 19F ligand nuclei. Also unexpected, and unexplained, is the same sign for As in the two materials, as the nuclear moments of both isotopes of Cd and that of 207Pb are of the opposite sign.

High-field 19F NMR relaxation studies in the superionic conductor Mn:PbF2.

The $^{19}\mathrm{F}$ NMR spin-lattice relaxation rate ${T}_{1}^{\mathrm{\ensuremath{-}}1}$ in ${\mathrm{Mn}}^{2+}$:${\mathrm{PbF}}_{2}$ was measured in applied fields such that \ensuremath{\gamma}${H}_{0}$>A, the ${\mathrm{Mn}}^{2+}$${\mathrm{\ensuremath{-}}}^{19}$F transferred hyperfine interaction. The results obtained generally agree with the predictions of the impact-model theory of nuclear relaxation in magnetically tagged superionic conductors. It is shown that for reasonable choices of the isotropic ${A}_{s}$ and anisotropic ${A}_{p}$ $^{19}\mathrm{F}$ transferred hyperfine interaction and the ${\mathrm{Mn}}^{2+}$ electron spin-lattice relaxation rate ${\ensuremath{\tau}}_{\mathrm{SL}{}^{\mathrm{\ensuremath{-}}1}}$, one can obtain agreement between theory and experiment at both low (T550 K) and high (T>700 K) temperatures, but not in the range 550T700 K, using the conductivity-determined hopping rate. This discrepancy is used to determine an ``NMR effective hopping rate.''

Nuclear-magnetic-resonance study of spin transfer to H in Rb2CuCl4∙2H2O and Rb2CuBr4∙2H2O

The nuclear magnetic resonance of 1H in Rb2CuCl4∙2H2O and isomorphic Rb2CuBr4∙2H2O has been measured at room temperature, near 165, and 77 K. The fractional spin transfer from the copper ion to hydrogen due to the transferred hyperfine interaction is analyzed: 0.22 ± 0.03% at 300 K, 0.25 ± 0.02% at 168 K, and 0.29 ± 0.02% at 77 K in Rb2CuCl4∙2H2O; and 0.22 ± 0.03% at 300 K, 0.28 ± 0.02% at 163 K, and 0.32 ± 0.02% at 77 K in Rb2CuBr4∙2H2O. The temperature-dependent spin transfers are discussed in terms of the molecular mass and the bond strength of the two compounds.

Ligand electron-nuclear double resonance ofF19ions aroundYb3+impurities in fluorites

Results of electron-nuclear double-resonance experiments performed on four shells of fluorine nuclei surrounding ${\mathrm{Yb}}^{3+}$ impurities at cubic sites in the four fluorite lattices ${\mathrm{CdF}}_{2}$, ${\mathrm{CaF}}_{2}$, ${\mathrm{SrF}}_{2}$, and ${\mathrm{BaF}}_{2}$ are reported. The data are described in terms of a Hamiltonian for the transferred hyperfine interactions (THFI) including an anisotropic nuclear Zeeman term. The contribution of the dipole-dipole magnetic interaction ${A}_{d}$ is analyzed for different coordination spheres using calculated values for the impurity-ligand distance. These are obtained from a model for the local distortions around trivalent rare-earth impurities. The covalent contributions to the THFI constants are important for nearest neighbors, and a comparison of the values determined for ${\mathrm{Yb}}^{3+}$ is made with a similar set for the isoelectronic (4${f}^{13}$) ${\mathrm{Tm}}^{2+}$ ions in the same lattices.

Semiempirical model of the exchange polarization mechanism of transferred hyperfine interactions in ionic radicals

This paper presents a semiempirical method for calculating metal hyperfine interaction constants in ionic radicals whose zero-order wave function places the unpaired electron completely on the anion with zero overlap between it and the relevant cation orbitals. The mechanistic basis of the calculation is spin polarization of closed-shell anion orbitals by their exchange interaction with the unpaired electron, followed by partial transfer of the resulting unpaired electron density in these anion orbitals to the cation orbitals via overlap. The exchange polarization is described by a perturbation theory expansion in anion excited states, which expansion is summed by introducing semiempirical average excitation energies. The contribution of direct exchange polarization of the cation orbitals to the cation hyperfine interactions is found to be negligible, due primarily to the very large excitation energies of the cation core orbitals. Calculated results are compared with experiment for the ionic radicals Na(C=CH2) and NaO2.

ENDOR and transferred spin densities of the 4f11ions in fluorides

The ENDOR of Tm2+ cubic paramagnetic centres in KMgF3 is investigated. The data on the measurements of the transferred hyperfine interaction (THFI) parameters are compared with the THFI data for Tm2+ in CaF2 and Yb3+ in KMgF3, CaF2. For the interpretation of the total experimental data a new model in which, along with the covalency, the processes of charge transfer to the empty 5d shell and the processes of excitation of the outer 5p6 shell with its successive occupation by the electron of the nearest fluorine shell play an essential role is proposed.

Lattice effects on bonding, covalency, and transferred hyperfine interaction in K2NaCrF6 and CrF3

The effects of the crystal lattice potential on bonding and covalency parameters of K2NaCrF6 and CrF3 have been investigated in terms of a partially relaxed lattice model described earlier. The theoretical values of different quantities in these two crystals are compared with the Hartree–Fock predictions on the CrF3−6 cluster in vacuo. In K2NaCrF6, the lattice potential reduces the orbital energies of the metal and ligand valence MO’s by 16.8 and 15.7 eV, respectively. In CrF3 these figures are 0.5 eV smaller. The 3d states, which have positive orbital energies in the cluster-in vacuo calculation, become bound states by the action of the lattice potential. The overlap populations suffer different changes in MO’s of different symmetries. The bonding MO’s turn out to be less bonding and the antibonding MO’s still less antibonding, the net effect being an increase in the total overlap population. The covalency parameters λπ and γπ are proportional to the square root of the ligand–metal π overlap and change with the internuclear distance RML as R−3ML and R−2ML , respectively. This calculation suggests that some 15% of the value of these parameters can be attached to the 3d radial expansion. CrF3 appears to be slightly less covalent than K2NaCrF6 according to the values of both γπ and the theoretical nephelauxetic ratio. The observed spin-transfer parameter fπ cannot be reproduced by simple models such as the metal–ligand orthogonalization or the first-order perturbation mixing of the 3dπ and 2pπ states. Our spin-restricted cluster calculation gives fπ values in reasonable agreement with neutron diffraction and resonance spectroscopy data and predicts that fπ varies as R−6ML . The cluster–lattice interaction gives a small contribution to fπ but it seems to be appropriate for explaining the differences in fπ which a given cluster shows in different lattices.

Spin coherence in crystalline complexes. Naphthalene: 2(1,4-diiodotetrafluorobenzene)

The weak crystalline complex of naphthalene : 2(1,4-diiodotetrafluorobenzene), which exhibits naphthalene phosphorescence with a lifetime in milliseconds, is conveniently investigated by a combination of conventional and pulse techniques of optically detected magnetic resonance. The D + E and D − E resonances, investigated here, reveal no transferred hyperfine interaction, but increased D and E values because of the external spin-orbit coupling which is found to be anisotropic. Measurements are made of T 2, T 2*, and T 2ϱ for this complex. This coherence study finds T 2 to be similar in magnitude to what has been observed for mixed organic crystals. T 2*, when compared with T 2, confirms the earlier suggestion that the crystalline complex is highly ordered. T 2ϱ measurement suggests interaction with the nuclear spins of C 6I 2F 4.

NMR investigation of spin transfer to H in K2CuCl4∙2H2O

Nuclear magnetic resonance of 1H in K2CuCl4∙2H2O has been measured with frequencies of 3 to 35 MHz at room temperature, 163, and 77 K. The fractional spin transfer from the copper ion to hydrogen due to the transferred hyperfine interaction is analyzed: 0.19 ± 0.03% at 300 K, 0.23 ± 0.02% at 163 K, and 0.27 ± 0.02% at 77 K, showing that the spin transfer is temperature dependent. This temperature dependence is closely associated with the unusually strong temperature dependent super-exchange interaction between Cu2+ ions in this crystal.

Studies of radical pairs in potassium hydrogen malonate

The cause of irreversible temperature changes in the fine structure of radical pairs in potassium hydrogen malonate previously observed by E.S.R. has been examined in detail by ENDOR. The results show that these changes can be attributed to structural relaxations of the stressed molecules following the abstraction of hydrogens by X-rays at 77 K from the methylene carbons. The appearance of different modifications of pairs is then explicable on the basis of symmetry conditions, which change during annealing of the radicals. In addition the ENDOR results demonstrate that the hyperfine interaction in radical pairs differs from the appropriate coupling constant of the corresponding monoradical in virtue of the transferred hyperfine interaction. It is shown that the transferred hyperfine interaction could be a measure for the direct overlap of the unpaired electrons in radical pairs.

Transferred hyperfine interactions between25Mg nuclei and neighbouring Cr3+impurities in magnesium oxide (ENDOR study)

Using the ENDOR technique, magnetic hyperfine and quadrupole interactions of 25Mg nuclei close to Cr3+ and Ni2+ substitutional impurities in MgO have been measured. The results are discussed in terms of the amount of unpaired spin in the Mg2+ orbitals. It is shown that the size of the isotropic part of the magnetic hyperfine interaction is explained better if the Mg2+ 1s orbitals are included in the calculation that if only the outer, 2s orbitals are used. The anisotropic part is described well by a combination of the effects of the magnetic dipole interaction and the overlap of Mg2+ 2p orbitals with O2+ orbitals containing unpaired spin. The quadrupole interactions can only be partly analysed because of insufficient information but it is shown that charge transfer only accounts for a small part of the observed interactions.

Mössbauer and EPR studies of activated aconitase: development of a localized valence state at a subsite of the [4Fe-4S] cluster on binding of citrate.

During activation of aconitase a ferrous ion is incorporated into a [3Fe-4S] cluster to yield a structure with a [4Fe-4S] core. Using 57Fe or 56Fe for activation we have studied with Mössbauer spectroscopy the beef heart enzyme in the presence of citrate. Our studies show that the environment of one iron site (Fea) of the [4Fe-4S] cluster is drastically altered in the presence of citrate. Fea is the iron acquired during activation of aconitase. In the oxidized [4Fe-4S]2+ state two species with enzyme-bound substrate are observed, whereas only one is observed in the reduced [4Fe-4S]+ state. The Mössbauer parameters of Fea reveal that the site has acquired substantial high-spin ferrous character. This is most pronounced in the 1+ state where at Fea the cluster exhibits a localized valence state. The dramatic increase of the isomer shift upon substrate binding strongly suggests that the ligand environment of Fea has become at least five-coordinate and that the cluster may function as a Lewis acid. In the absence of citrate the EPR spectra of the active [4Fe-4S]+ enzyme (g1,2,3 = 2.06, 1.93, 1.86) show no hyperfine broadening in the presence of H2 17O. However, in the presence of citrate (g1,2,3 = 2.04, 1.85, 1.78) sizable transferred hyperfine interactions are observed; under the experimental conditions the hydroxyl groups of citrate and isocitrate as well as water are labeled with 17O. We did not detect broadening by 17O-labeled carboxyl groups of citrate in H2 16O. Implications for the mechanism of aconitase are discussed.