Tetragonal Gd3 Research Articles

Spin-Hamiltonian parameters and defect structures for two tetragonal Gd3+ centers in Gd3+-dpoed Tl2ZnF4 crystal

Seven spin-Hamiltonian parameters (g factors g//, g⊥ and zero-field splittings b20, b40, b44, b60, b64) of two tetragonal Gd3+ centers, denoted A and B, in layered perovskite fluoride Tl2ZnF4 crystals doped only with Gd3+ ion and co-doped with Gd3+ and Li+ are calculated using the diagonalization (of energy matrix) method based on the one-electron crystal-field mechanism. In the calculations, the defect models suggested in the previous paper that center A is due to Gd3+ ion at the octahedral Zn2+ site without any local charge compensation and center B is due to Gd3+ ion at the nine-coordinated Tl2+ site associated with a Li+ ion at the nearest Zn2+ site along C4 axis for charge compensation are applied. The calculated results are in reasonable agreement with the experimental values. The suggested defect models of both Gd3+ centers are therefore confirmed and the respective defect structural data are obtained. The results, including the validity of defect structural data, are discussed.

Electron paramagnetic resonance of Gd3+ ions in Ca1 − x − y Y x Gd y F2 + x + y crystals

Electron paramagnetic resonance of Ca1 − x − y Y x Gd y F2 + x + y single crystals has revealed spectra that are not typical of gadolinium-doped CaF2 crystals. These spectra have a nearly tetragonal symmetry and are most probably caused by Gd3+ ions localized in yttrium clusters. Weak spectra of tetragonal Gd3+ centers, whose parameters are close to those of a cubic gadolinium center caused by an isolated Gd3+ ion, have been also detected. These centers are attributed to isolated Gd3+ ions localized near octahedral rare-earth clusters or their associations.

Theoretical studies of the spin-Hamiltonian parameters and defect structure for the tetragonal Gd3+ center in cubic c-RbZnF3 crystal

A diagonalization (of the energy matrix for which the Hamiltonian contains the free ion, crystal-field interaction and Zeeman interaction terms) method was applied to calculate the spin-Hamiltonian parameters (g factors, g // and , and five zero-field splittings, ) for the tetragonal Gd3+ center in cubic c-RbZnF3 perovskite. The calculated results are in reasonable agreement with experimental values. From the calculation, the defect model that the tetragonal Gd3+ center is attributed to Gd3+ occupying the 12-fold coordinated Rb+ site associated with a nearest Rb+ vacancy, VRb, along C 4 axis owing to charge compensation is confirmed and the defect structural data of this Gd3+ impurity center in c-RbZnF3 are acquired. The results are discussed.

Studies of the spin Hamiltonian parameters forGd3+ inPr1.98Gd0.02CuO4

The spin Hamiltonian parameters (i.e., zero-field splittings (ZFSs)b20,b40,b44 and theg factor) forthe tetragonal Gd3+ in Pr1.98Gd0.02CuO4 arestudied from the superposition model and the approximation formula including admixture of the ground8S7/2 and theexcited 6L7/2 (L = P,D, F, G) states via the spin–orbit coupling interactions, respectively. The calculated ZFSs andg factor are in reasonable agreement with the experimental data. It isfound that the impurity–ligand bonding angles related to the fourfold(C4) axis in theimpurity Gd3+ centreare about 0.71° smaller than those in the host.

Local structures of the tetragonal Gd3+−VMand Gd3+−Li+centers in perovskite fluorides

The zero-field splittings b(2)(o) of the tetragonal Gd3+-V-M and Gd3+-Li+ centers for Gd3+ ions in fluoroperovskite crystals have been studied on the basis of the superposition model in which the value of t(2) (b) over bar(2) is Obtained from the stress-dependence of EPR spectra for RbCaF3:Gd3+ crystal. From the studies, the local structures of both tetragonal centers in fluoroperovskite crystals are obtained and the results are discussed.

Observation of defect clustering in Gd-doped calcium fluoride

Abstract Solid solutions Ca1-xGdxF2+x for 3 × 10−7≤ x ≤10−1 have been studied by electron paramagnetic resonance (EPR) and ionic thermal currents (ITC). The EPR experiments show the presence of two single-ion sites a cubic and a tetragonal Gd3+ center which co-exist with comparable abundances for intermediate impurity concentrations. The cubic center predominates at very low and high concentrations. Seven different relaxation processes have been identified from the ITC spectra and the variation of their intensity vs. x was measured. The absolute concentrations of the cubic and nn Gd3+ dipoles were calculated. The scavenging of interstitial fluorines by the neutral clusters explains both the abundance of cubic sites at high concentration and the variety of orientable clusters detected by ITC.

Dielectric relaxation and EPR studies of Gd3+-FI-dipoles in strontium and barium fluoride

Dielectric relaxation measurements on SrF2:Gd3+ show four absorption peaks. By correlation with EPR line broadening a relaxation (RI) with activation energy 0.454 eV is associated with a tetragonal Gd3+ site, and a second (RIV) with activation energy 0.64 eV is associated with a trigonal Gd3+ site. A two-shell model of ionic relaxation processes links RI and RIV but does not adequately account for the observed concentration dependence of the ratio of peak intensities and of the width of the RIV peak, nor for small shifts between the relaxation frequency of the RIV peak and the trigonal site. Two relaxations (RII and RIII) with activation energies 0.38 eV and 0.14 eV are assigned to aggregates on the basis of their concentration dependence. In BaF2:Gd3+ only two dielectric absorptions are found. One, with an activation energy 0.61 eV, is assigned to a trigonal Gd3+ site and the second, with activation energy 0.36 eV, is assigned to aggregates.

Effect of strain on the crystal field of tetragonal Gd3+in CaF2

The EPR longitudinal and shear strain parameters for tetragonal Gd3+ in CaF2 have been measured using the ultrasonic strain modulation technique (UMER). A comparison has been made with the results for the cubic Gd3+ site using the superposition model as a guide, but as the local elastic constants for the tetragonal defect are unknown, the conclusions are tentative.

Dielectric loss and EPR studies of CaF2:Gd3+

Dielectric loss measurements on CaF2:Gd3+ show four relaxation absorptions. By correlation with EPR the assignment of a relaxation with activation energy 0.396 eV to a tetragonal site is confirmed. Evidence against assignment of a relaxation with activation energy 0.68 eV to a trigonal site is obtained. Relaxations with activation energy 0.15 eV and 0.076 eV are tentatively assigned to aggregates. Relaxation of the tetragonal Gd3+-H1- and Gd3+-D1- sites is shown by EPR to have an activation energy close to the Gd3+-F1- site but is not observed in the dielectric spectrum.