Ligand hyperfine interaction in Gd<Superscript>3+</Superscript> tetragonal centers in CaF<Subscript>2</Subscript> and SrF<Subscript>2</Subscript> and the structure of the impurity nearest surroundings

ENDOR experimental spectra of Gd³⁺ tetragonal impurity centers in CaF₂ and SrF₂ crystals were used to determine the superhyperfine interaction (SHFI) constants of the impurity with ¹⁹F nuclear spins of its first coordination sphere and the compensator ion. The distances in the Cd³⁺F₉ complex were estimated within the model of isotropic SHFI constants suggested in [1]. An analysis of the data on the SHFI and spin-Hamiltonian constants [2] in terms of the superposition model indicates significant changes in the contributions (due to the Gd³⁺ mixed states) to these parameters for the tetragonal centers in comparison with the corresponding contributions for the cubic and trigonal centers in the same crystals.     

Superhyperfine structure in the EPR spectra and optical spectra of impurity f ions in dielectric crystals: A review

The results of observation and simulation of the superhyperfine (ligand hyperfine) structure (SHFS) of the electron paramagnetic resonance (EPR) spectra of rare-earth and uranium impurity ions in dielectric crystals have been systematized. The resolved SHFS of the EPR spectra of doped cubic crystals (with the fluorite and perovskite structures) has been observed for orientations of a constant magnetic field along the crystallographic axes. Most attention has been paid to tetragonal double fluorides LiRF₄ (R = Y, Lu, Tm), in which the SHFS of the EPR spectra has also been found for intermediate orientations of the magnetic field. For the LiYF₄: Nd³⁺ single crystal, the splitting of optical spectral lines due to the interaction of Nd³⁺ ions with nuclear magnetic moments of the nearest neighbor fluorine ions has been observed for the first time. The Van Vleck paramagnet LiTmF₄: U³⁺ is characterized by the SHFS with clearly distinguishable components due to the interaction of uranium ions both with nuclei of the fluorine ions and with enhanced magnetic moments of the thulium nuclei. The SHFS envelopes of the EPR spectra of Yb³⁺, Ce³⁺, Nd³⁺, and U³⁺ ions in LiYF₄ and LiLuF₄ crystals are well reproduced by numerical calculations based on the microscopic model using only three fitting parameters: the width of transitions between the electron-nuclear sublevels of the complex containing the paramagnetic ion and nuclei of the ligands and two constants of covalent bonding of the f electrons with 2s and 2p electrons of the nearest neighbor fluorine ions.     

Enhanced superhyperfine interaction on Ru3+ in Tm3Al5O12 and on Ru3+ and Yb3+ in Tm3Ga5O12

Measurements of the enhanced superhyperfine interaction linking the electron spin of an impurity EPR ion and the nuclear moments of Van Vleck host lattice ions are reported for Ru³⁺ in Tm₃Al₅O₁₂ and for Ru³⁺ and Yb³⁺ in Tm₃Ga₅O₁₂. The Ru³⁺ results appear to be the first reported where this interaction is large enough to give rise to a well resolved superhyperfine structure. For Yb³⁺ the observed temperature dependent line narrowing evidences the role of Tm spin fluctuations.     

EPR of Pb3+ ion in LiBaF3 crystals

Complex EPR spectra of paramagnetic centers Pb³⁺ formed in LiBaF₃:Pb²⁺ crystals under X-ray irradiation are studied in the temperature range of 10–150 K. It is shown that lead ions substitute Ba²⁺ ions in the LiBaF₃ crystal and are in the cubic-octahedral 12-fold environment of the fluorine ions. The hyperfine structure constants describing the observed spectrum are determined and parameters of superhyperfine interaction with the nearest fluorine ions are estimated.     

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)     

Electron nuclear double resonance of the cubic Dy3+ center in the KZnF3 single crystal

ENDOR measurements on the¹⁹F^? nuclei in the first four shells of KZnF₃ containing Dy³⁺ ions in the cubic site are reported. The values and signs of the hyperfine and transferred hyperfine interaction parameters are determined. The local deformation of the crystal lattice in the vicinity of the impurity ion is estimated. The theoretical analysis of the THFI parameters for the first coordination shell of the F^? ions has been carried out. For the Dy³⁺ ion the influence of spin polarization of the closed 5s and 5p shells is considered for the first time. Spin polarization is shown to play a significant role in the mechanisms of rare-earth ion-ligand coupling.     

Local structure of Gd3+ impurity center at cubic sites in fluorites

A computer controlled ENDOR spectrometer and special software are used to determine the coordinates of¹⁹F nuclei in four anionic spheres of cubic MeF₂:Gd³⁺ (Me = Ca, Sr, Ba, Pb) centers. The computer simulation of Gd³⁺ cubic impurity center in crystals MeF₂ (Me = Ca, Sr, Ba) has been also performed. The analysis of lattice relaxation near the impurity center carried out on the base of ENDOR experiments data has allowed us to obtain potential of interaction Gd³⁺-F^?. For the first time not only radial displacements but also angular displacements of F^? ions of the third sphere have been taken into account and it has been shown that it influences the determination of lattice relaxation around the impurity center essentially. The influence of hydrostatic pressure (up to 30 kbar) on the local structure of the impurity center has also been investigated using computer simulation. The comparison of the experimental and calculated displacements of distant ions gives a reliable test of the validity of theoretical lattice relaxation model and accuracy of calculations of impurity-directed shifts of ligands.     

Analysis of the structure of tetragonal oxygen centers associated with Yb<Superscript>3+</Superscript> ions in the KMgF<Subscript>3</Subscript> crystal

The parameters of the crystal field of the tetragonal oxygen center associated with a Yb³⁺ ion in the KMgF₃ crystal found previously in a study of optical and ESR spectra are applied to analyze lattice distortions in the vicinity of the impurity ion and the O^2? ion compensating for the excess positive charge. Within the superposition model, it was ascertained that the Yb³⁺ ion and the neighboring ions of fluorine and oxygen on the axis of the center shift significantly along the direction from the O^2? ion to the Yb³⁺ ion during the formation of the tetragonal oxygen center. As this takes place, the distances of both (fluorine and oxygen) ions from the impurity ion increase. The four F^? ions of the nearest octahedral neighborhood of Yb³⁺ that are arranged symmetrically in the plane perpendicular to the axis of the center slightly recede from the axis.     

Local structure of Gd<Superscript>3+</Superscript> and Eu<Superscript>2+</Superscript> impurity centers in a CdF<Subscript>2</Subscript> crystal

The local crystal structure of Gd³⁺ and Eu²⁺ cubic impurity centers in cadmium fluoride is calculated within the shell model in the pair potential approximation. The local compressibility of the cationic and anionic sublattices of the host lattice is determined in the vicinity of the Gd³⁺ (Eu²⁺) impurity ion.     

EPR and optical spectroscopy of Yb<Superscript>3+</Superscript> ions in CaF<Subscript>2</Subscript>: an analysis of the structure of tetragonal centers

CaF₂ crystals doped with Yb³⁺ ions have been studied by electron paramagnetic resonance (EPR) and optical spectroscopy. EPR spectra of paramagnetic centers (PCs) for cubic (T_c) and tetragonal (T_tet) symmetries were identified. Empirical energy level diagrams were established and crystal field parameters were determined. Information on the CaF₂∶Yb³⁺ phonon spectra was obtained from the electron-vibrational structure of the optical spectra. The crystal field parameters were used to analyze the crystal lattice distortions in the vicinity of the Yb³⁺ ion. Within the framework of a superposition model, it is established that four F⁻ ions located symmetrically with respect to the fourfold axis from the side of the ion-compensator approach the impurity ion and deviate from the PC axis. The second set of four fluorine ions also approaches the Yb³⁺ ion and the PC axis. The ion-compensator F⁻ also approaches considerably the impurity ion.     

Tetragonally distorted Cr2+ ions in BaF2 and SrCl2 studied by ESEEM spectroscopy

Electron spin echoes on tetragonally distorted non-Kramers Cr²⁺ ions in chromium doped BaF₂ and SrCl₂ single crystals have been observed. The superhyperfine interaction (shf) with the nearest halide ions has been studied using the electron spin echo envelope modulation (ESEEM) signal. For Cr²⁺ in BaF₂ the nearest coordination shell consists of four equivalent fluorine ions in a plane perpendicular to the defect tetragonal axis, the Cr²⁺ ion being off-centre displaced towards the fluorine plane. The Cr²⁺ shf tensor for BaF₂ has been determined. For SrCl₂:Cr ESEEM signals associated to the shf interaction with nearest chloride ions were observed but the measurements did not allow us to give a definitive defect model in this case.     

Tetragonal distortions of the octahedral environments of Cr3+, Fe3+ and Gd3+ ions in A2CdF4 (A = Rb,Cs) and ACdF3 crystals

Abstract

The tetragonal distortions of local octahedral environments of Cr³⁺, Fe³⁺ and Gd³⁺ ions in Rb₂CdF₄, Cs₂CdF₄, RbCdF₃ and CsCdF₃ crystals have been studied by analyzing their EPR spectra. From the studies, it is found that the tetragonal distortions for Cr³⁺ and Fe³⁺ impurity ions, which substitute Cd²⁺ and have nearly the same ionic radius, are close to each other, whereas that for Gd³⁺ impurity ion, having a larger ionic radius, is larger than those for Cr³⁺ and Fe³⁺ ions in the same crystal. It appears that not only the impurity-ligand distance, but also the tetragonal distortions of impurity centres in crystals are closely related to the size of impurity.     

Ligand electron-nuclear double resonance of <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> Trigonal Gd<Superscript>3+</Superscript> centers in CaF<Subscript>2</Subscript> with a mixed oxygen-fluorine environment

This paper reports on the results of ligand electron-nuclear double resonance (ENDOR) investigations of T₁ trigonal ¹⁵⁷Gd³⁺ centers in the CaF₂ compound. It is experimentally found that the nearest environment of an impurity center contains only one ¹⁹F ion. Anions in the other coordination shells are identical to those in the pure CaF₂ crystal. However, ¹⁹F ions in these shells are displaced from their ideal positions in the lattice. The parameters of the ligand hyperfine interaction (LHFI) for ¹⁹F nuclei and their coordinates and displacements with respect to the positions in the lattice of the pure CaF₂ crystal are determined. It is demonstrated that the unusual isotropic LHFI constant A _s >0 for Gd³⁺ ions in the lattice with a mixed oxygen-fluorine nearest environment can be associated with the strong polarization of impurity centers in accordance with the empirical model proposed in [1], provided the structural model of the nearest environment of impurities in the T₁ centers [2] is correct. This structural model is confirmed by the analysis of the isotropic hyperfine constant A(s) for ¹⁵⁷Gd³⁺ centers.     

EPR,ENDOR and optical spectroscopy of Yb3+ ion in KZnF3 single crystals

The paramagnetic center of tetragonal symmetry formed by the Yb³⁺ ion in the KZnF₃ crystal has been studied using methods of EPR, ENDOR and optical spectroscopy. The location of the impurity ion and the structural model of the complex differing from the model of the Yb³⁺ center in KMgF₃ have been established. The empirical scheme of the energy levels of the Yb³⁺ ion has been found. The parameters of its interaction with the crystal electrostatic field and the hyperfine interaction with ligands of the nearest environment have been determined. The parameters of the crystal field were used for the analysis of the distortions of the crystal lattice in the vicinity of Yb³⁺. The parameters of the transferred hyperfine interaction have been calculated for the distances between Yb³⁺ and F⁻ ions of the nearest environment obtained taking into account the found distortions. They are in good agreement with the experimental values.     

Influence of gadolinium concentration on the EMR spectrum of Gd in zircon

Electron magnetic resonance (EMR) spectra of gadolinium-doped zircon (ZrSiO₄) powders have been studied at room temperature for gadolinium concentrations between 0.20 and 1.0 mol%. The results suggest that Gd³⁺ ions occupy substitutional sites in the zircon lattice, that the electron magnetic resonance linewidth increases with increasing gadolinium concentration and that the range of the exchange interaction between Gd³⁺ ions is about 1.17 nm, larger than that of the same ion in other host lattices, such as ceria (CeO₂), strontium oxide (SrO) and calcium oxide (CaO). The fact that the electron magnetic resonance linewidth of the Gd³⁺ ion in polycrystalline zircon increases, regularly and predictably, with Gd concentration, shows that the Gd³⁺ ion can be used as a probe to study, rapidly and non-destructively, the crystallinity and degradation of ZrSiO₄.     

Superhyperfine Structure of the EPR Spectra of Nd<Superscript>3+</Superscript> Impurity Ions in Fluorite CaF<Subscript>2</Subscript>

EPR spectra of a CaF₂ single crystal that was grown from melt containing a small addition of NdF₃ were studied. Signals corresponding to tetragonal centers of Nd³⁺ ions and cubic centers of Er3+ and Yb3+ ions were found. Superhyperfine structure (SHFS) in the spectra of the Nd³⁺ ions was observed for the first time in this crystal; parameters of the superhyperfine interaction of the Nd³⁺ ions with the nearest nine fluorine ions were determined. The dependence of the resolution of the Nd³⁺ EPR spectrum SHFS on the incident microwave power at the temperature of T ≈ 6 K was studied. Obtained results are discussed and compared with the literature data.     

Gd 3+ -F − dimer centers in the ferroelectric lead germanate

The Gd³⁺ centers formed in lead germanate doped with fluorine have been studied using electron paramagnetic resonance. The parameters of the triclinic spin Hamiltonian are found at room temperature. The center is shown to be a gadolinium ion having the nearest environment in which the fluorine ion is substituted for the oxygen ion. The Gd-F dimer binding energy relative to the energy of a single gadolinium ion is determined. The interaction of the dimer center with the spontaneous polarization is lower than the measurement error.     

On the transferred hyperfine interaction in substituted yig

NMR of⁵⁷Fe is studied in a number of (M_xY_3–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⁺–O^2–-Fe³⁺ vs. Y³⁺–O^2–-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.     

Superhyperfine Structure of EPR Spectra in LiLuF<Subscript>4</Subscript>:U<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript> Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF₄:U³⁺ and LiYF₄:Yb³⁺ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U³⁺ ion spectrum is characterized by g-factors g _∥ = 1.228 and g _⊥ = 2.516, and contains the hyperfine structure due to the ²³⁵U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A _∥ = 81 G and A _⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U³⁺ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF₄:Yb³⁺ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed. Authors' address: Igor N. Kurkin, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

β-irradiation effect in aluminoborosilicate glasses: The role of <Emphasis Type="Italic">RE</Emphasis> codoping (<Emphasis Type="Italic">RE</Emphasis> = Sm,Gd)

The effect of Sm and Gd codoping on the structural modifications of β-irradiated aluminoborosilicate glasses has been studied by electron paramagnetic resonance (EPR) and Raman spectroscopy. The EPR spectra showed that the relative amount of Gd³⁺ ions occupying network former positions (Gd_[n.f.]³⁺) follows a nonlinear behavior as a function of the Sm/Gd ratio. This suggests that codoping favors the occupation by Gd³⁺ ions of the network former positions rather than the modifier positions in aluminoborosilicate glasses. The appearance of a superhyperfine structure of EPR lines attributed to boron-oxygen hole centers (BOHC) with increasing Sm/Gd ratio was observed. This suggests that Gd³⁺ ions are diluted in the vicinity of the BOHC defects. The concentration of defects created by irradiation reveals a nonlinear dependence on Sm and Gd codoping for the lowest irradiation dose (10⁵ Gy). Therefore, codoping also affects the defect creation processes at least at the lowest irradiation dose. Raman spectroscopy measurements suggest that the irradiation-induced structural changes vary nonlinearly with the Sm/Gd ratio. In fact, the shift of the Si-O-Si bending vibration modes reveals a clear minimum for samples containing equal amounts of Sm and Gd (1: 1) in the investigated glasses. The text was submitted by the authors in English.