The superposition model was originally developed to separate the geometrical and physical information in crystal field parameters. Its success in the analysis of lanthanide spectra has been paralleled by the success of the related angular overlap model in the analysis of d-electron spectra. The basic ideas, method of application and reliability of the superposition model are discussed and its relationship with the angular overlap model is clarified. Developments described are the application of the superposition model to the ground (L=0) multiplet splittings of d5 and f7 ions, orbit-lattice interactions, transition intensities and correlation crystal fields. Special attention is paid to work which has been claimed to support or disprove the postulates of the model.

https://iopscience.iop.org/article/10.1088/0034-4885/52/6/002/pdf

The superposition model of crystal fields

Bi2W2O9, Bi2WO6, and Bi3TiNbO9 consisting of layered structure with perovskite slabs interleaved with Bi2O2 layers showed photocatalytic activities for H2 evolution from an aqueous methanol solution and O2 evolution from an aqueous silver nitrate solution. Bi2WO6 with the Aurivillius structure and a 2.8 eV band gap was active for the O2 evolution reaction under visible light irradiation (λ > 420 nm).

H2 or O2 Evolution from Aqueous Solutions on Layered Oxide Photocatalysts Consisting of Bi3+ with 6s2 Configuration and d0 Transition Metal Ions

Aurivillius structure Bi2MoO6 (BG:  2.70 eV) that is a low-temperature phase showed an intense absorption band in the visible light region and photocatalytic activity for O2 evolution from an aqueous silver nitrate solution under visible light irradiation, among various bismuth molybdates (Bi2MoO6, Bi2Mo2O9, and Bi2Mo3O12) synthesized by solid-state and reflux reactions. Bi2Mo3O12 (BG:  2.88 eV) also showed photocatalytic activity for O2 evolution under full-arc irradiation of a Xe lamp (λ > 300 nm). The photocatalytic activity of the Aurivillius structure Bi2MoO6 prepared by the reflux method was dependent on the annealing temperature after the preparation. The crystallinity was the important factor for the activity. Calculation by the density functional method indicated that the conduction band of Aurivillius structure Bi2MoO6 was made up of Mo 4d orbitals. It turned out that the visible-light absorption of this photocatalyst was due to the transition from the valence band consisting of O 2p orbitals to...

Photophysical properties and photocatalytic activities of bismuth molybdates under visible light irradiation.

Fe-ions modified mesoporous Bi2WO6 nanosheets with high visible light photocatalytic activity

Non empirical methods for obtaining information from EPR, ENDOR and optical data on the true impurity-ligand distance,R, as well as on the true variations, ΔR, induced by chemical and hydrostatic pressures, phase transitions in the host material and temperature changes are discussed through this work. Special attention is addressed to spectroscopic parameters of d-impurities whose dependence onR can reasonably be calculated theoretically for the superhyperfine (shf) tensor or the lowest optical transitions but not for fine structure terms. In the case of impurities with unpaired σ-electrons it is shown that the isotropic shf constant,A
 s, is specially sensitive toR changes. The microscopic origin of this fact is discussed in detail. Determination of trueR values from experimentalA
 s has been carried out for impurities like Mn2+ or Ni+ with encouraging results. In the case of Mn2+ in fluorides, results obtained by this method coincide with those reached through EXAFS and the analysis of the experimental 10 Dq. Finally, for some selected complexes, a view is offered on the dependence of several EPR and optical parameters upon metal-ligand distances. The main conclusion is that ΔR values of the order of 0.1 pm can be detected using EPR and optical parameters thus improving by an order of magnitude the sensitivity reached through EXAFS. In particular the use of ENDOR allows one to measure ΔR values close to 0.01 pm.

Impurity-ligand distances derived from magnetic resonance and optical parameters

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

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

This paper reports electron paramagnetic resonance (EPR) measurements of Cr3+ ions in YAlO3 (YAP) at 1.6 K and room temperature along with their optical absorption and luminescence spectra The polarization of the excitation spectrum of the Cr3+ luminescence has also been measured. The EPR spectrum is ascribed to substitutional Cr3+ ions at Al3+ sites. The spin-Hamiltonian parameters determined from the EPR spectrum of Cr3+ ions in nearly orthorhombic sites in YAP are gx = 1.981, gy = 1.981, gz = 1.981, b20 = +445 * 10-4 cm-1 and b22 = -278 * 10-4 cm-1. The anisotropy of the Cl3+ EPR spectrum is discussed in terms of the electrostatic potential of the surrounding O2- and Y3+ ions in YAP crystals and is compared with that of Ti3+ ions in the same host crystal.

https://iopscience.iop.org/article/10.1088/0953-8984/5/43/021/pdf

Electron paramagnetic resonance and optical spectra of Cr3+-doped YAlO3

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.

EPR Study of Cr 3+ -Li + 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.

Hideo Takeuchi

Papers

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

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

Electron paramagnetic resonance and optical spectra of Cr3+-doped YAlO3

EPR Study of Cr 3+ -Li + Centres in Several Perovskite Fluorides

Electron Paramagnetic Resonance of Gd 3+ Centres in Several Perovskite Fluorides