Masanori Arakawa

EPR and 19F-ENDOR of Cr3+ Impurity Centres in K2ZnF4 and K2MgF4

The EPR spectra of three kinds of Cr 3+ centres in K 2 ZnF 4 and K 2 MgF 4 are reported. The Cr 3+ ion in each centre replaces the divalent cation substitutionally. The centre I (tetragonal symmetry) is ascribed to a vacancy-free Cr 3+ ion. In centre II (nearly-trigonal) a vacancy is present at the nearest K + site and in centre III (nearly-tetragonal) at divalent cation site. For each centre the values of the parameters ( g // - g ⊥ ) and D in the spin Hamiltonian are discussed by considering the effective negative charge distribution surrounding the Cr 3+ ion. In the vacancy-associated centres the values of D are interpreted by superposition of two uniaxial spin-Hamiltonian terms. Results of 19 F-ENDOR of K 2 MgF 4 : Cr 3+ are also presented.

EPR Study of Cr3+ Centres in Rb2ZnF4, Rb2CdF4 and Cs2CdF4 Crystals

Results of EPR measurements on Cr 3+ centres in Rb 2 ZnF 4 , Rb 2 CdF 4 and Cs 2 CdF 4 are reported for crystals co-doped with Li + ions. Due to charge compensation some of Cr 3+ ions are associated with a cation vacancy (V M ) or a Li + ion at the nearest divalent-cation site in the c -plane. The fine structure parameters D and E are interpreted by two uniaxial spin-Hamiltonian terms along the c -axis of the host crystal and along the direction of the Cr 3+ –V M or Cr 3+ –Li + pair. The validity of this interpretation is discussed by the superposition model.

EPR of Cr3+ Centres in K2ZnF4 Crystals Doped with Na+ and Li+ Ions

Results of EPR measurements of Cr 3+ in K 2 ZnF 4 are reported for crystals co-doped with Na + or Li + ions. A few kinds of Cr 3+ centres are observed in both crystals, where parts of Cr 3+ are associated with charge compensators. In Na + -doped crystal, the nearest Zn 2+ -vacancy centre has comparable intensity with the uncompensated centre, but the nearest K + -vacancy centre is not observed. In Li + -doped crystal, the Zn 2+ -vacancy centre is weak compared with the uncompensated centre, and the K + -vacancy centre is also not observed, but a new centre having comparable intensity with the uncompensated centre appears. The new centre is attributed to Cr 3+ associated with Li + at the nearest Zn 2+ site. Fine structure parameters in the spin Hamiltonian are discussed by superposition of the two uniaxial terms along the crystal-tetragonal axis and bond axis to the charge compensator.

EPR Study of Cr3+-Li+ Centres in Several Perovskite Fluorides

Centres with tetragonal symmetry in KZnF 3 , KCdF 3 , RbCdF 3 and CsCdF 3 co-doped with Cr 3+ and Li + ions have been studied by EPR in the cubic phase of host crystals. Due to charge compensation some of Cr 3+ ions are associated with a cation vacancy (V M ) or a Li + ion at the nearest divalent-cation site. The magnitude of the fine structure parameter D in the spin Hamiltonian increases with lattice constant for the Cr 3+ -Li + centre. In KZnF 3 the magnitude of D for the Cr 3+ -Li + centre is smaller than that for the Cr 3+ -V M centre as expected from point charge model, whereas in KCdF 3 , RbCdF 3 and CsCdF 3 the former is larger than the latter against the expectation from the point charge model. The dependence of D on lattice constant is discussed.

Anomaly in the fine-structure splitting of EPR spectra of Cr3+ centres in Tl2ZnF4 crystals

EPR measurements have been made at room temperature on as-grown single crystals of Tl2ZnF4. For crystals doped with Cr3+, signals from the Cr3+ centres with monoclinic (centre II) and orthorhombic (centre III) symmetries are observed together with ones from the uncompensated tetragonal centre (centre I). On the basis of spin-Hamiltonian separation analysis, the centre II is ascribed to a Cr3+ ion substituted for a Zn2+ site with a vacancy at its nearest Tl+ site. The centre III is ascribed to a Cr3+ ion associated with a vacancy at the nearest Zn2+ site. Anomalies were revealed in the separated parameters b2a(1) for the centres II, III and in b2a(2) for the centre III, which have about double the magnitude of those in the other layered perovskite fluorides Rb2ZnF4, K2ZnF4 with different monovalent cations. These anomalies may be due to some effect of the nearest and the next-nearest Tl+ ions.

EPR study of Fe3+−VCd and Fe3+−Li+ centres in RbCdF3 and CsCdF3 crystals

EPR measurements on Fe 3+ centres in RbCdF 3 and CsCdF 3 for crystals co-doped with Li + ions have been made in the cubic phase of host crystals. In both the crystals spectra from two kinds of Fe 3+ tetragonal centres associated with Cd 2+ vacancy (V Cd ) and Li + ion at the nearest-divalent-cation sites have been observed. From obtained fine structure parameters in the spin Hamiltonian the metal-ligand distances have been determined by the superposition model. The fine structure parameters have been explained well by the obtained metal-ligand distances, which show the contraction ordistortion of fluorine octahedra surrounding Fe 3+ ions.

Host Lattice Effects in the EPR Spectra of Cr3+-Li+ and Cr3+-VMg Centres in A2MgF4 (A=K, Rb) Crystals

EPR measurements on Cr 3+ centres in K 2 MgF 4 and Rb 2 MgF 4 for crystals co-doped with Li + have been made at room temperatures. In both crystals spectra for Cr 3+ centres associated with a cation vacancy (V Mg ) and a Li + ion at the nearest Mg 2+ site in the c plane have been observed. The obtained fine structure parameters D and E are interpreted by two uniaxial spin-Hamiltonian parameters D 1 and D 2 using the separation method. The systematic relations among the tetragonal Cr 3+ -V M and Cr 3+ -Li 1 centres in cubic perovskite fluorides are discussed by the separated parameters D 2 . The relation of the separation method to the Rudowiczs net charge compensation model is also discussed.

Electron Paramagnetic Resonance of Gd3+ Centres in Several Perovskite Fluorides

EPR measurements for substituted Gd 3+ centres in CsCaF 3 , CsCdF 3 , RbCdF 3 and KCdF 3 have been made at several temperatures in a range 77-487 K. In all the crystals the spectrum with tetragonal symmetry for Gd 3+ centre associated with the nearest-neighbour cation vacancy has been observed together with that for cubic centre. The observed fourth-order parameters in the spin Hamiltonian of both cubic and tetragonal centres are discussed in the series of perovskite fluorides. The result suggests that the parameter b 4 for Gd 3+ in 6-fold cubic coordination changes its sign as the lattice constant decreases. The superposition model for the fourth-order parameter is examined.

EPR study of Cu2+ impurity centre in WO3

EPR measurements on Cu 2+ centre in WO 3 arc made at 131 K using an X-band spectrometer. The determined g and A parameters in the spin Hamiltonian have strong c -axis anisotropy, as g x =2.053, g y =2.050, g z =2.415 and A x =24.5 ×10 -4 cm -1 , A y =18.9×10 -4 cm -1 , A z =-99.0×10 -4 cm -1 (Eulerian angles; θ 1 =11.7°, φ 1 =44.5°, ψ 1 =6.5°). The ground state of Cu 2+ ion is determined to be ∣ x 2 - y 2 > by g -value analysis. Two models for Cu 2+ centre are discussed by comparing with previous EPR results for V 4+ (3d 1 ) ion in WO 3 ; one is Cu 2+ centre associated oxygen vacancy at the nearest site along the c -axis and the other is Cu 2+ centre located at interstitial site.

EPR of Several Iron-Group Impurities in LiBaF3 Having Inverse Perovskite Structure

Using 34 GHz band, the spin-Hamiltonian parameters of Cr 3+ , Mn 2+ , Fe 3+ and Co 2+ centres with cubic symmetry and Ti 3+ centre with tetragonal symmetry in LiBaF 3 have been determined. Co 2+ ion with g =4.316 indicates that it substitutes for the Li + ion which is surrounded by a fluorine octahedron.