H Takeuchi

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.

Substitutional disorder and the ground state spectroscopy of gallogermanate crystals

Electron spin resonance (ESR) spectra at X-band microwave frequencies are reported for dopant ions in (CGGO), (SGGO), (LGGO) and (LGS). The ESR measurements identify two distinct sites for in CGGO and SGGO and a single site in LGGO and LGS characterized by spin S = 3/2 for ions in the ground state. In CGGO a sharp line ESR spectrum, fitted to an axially symmetric spin Hamiltonian with , and , is assigned to substituents at trigonally distorted octahedral sites with a regular array of and ions in second-nearest-neighbour sites. A second spectrum of broader lines, due to ions in orthorhombically distorted sites associated with random occupation of the second-nearest-neighbour sites by and ions, has slightly different spin Hamiltonian parameters; , and . In this spectrum the principal axis is rotated by about from the c-axis towards the a-axis. In SGGO there are also ESR spectra from ions in trigonal and orthorhombic symmetry sites with almost identical spin Hamiltonian parameters. The ground state splittings of the trigonal centres measured optically by fluorescence line narrowing gives for both crystals. In contrast, only the trigonal centres are identified in LGGO and LGS, their ESR spectra being very similar to those of the trigonal centres in CGGO and SGGO. These results are discussed in terms of the substitutional disorder in these crystals and the nature of the site environment of substituents. The spectra correspond to ions in strong and weak crystal field sites, respectively, in agreement with earlier optical studies.

EPR study of tetragonal and monoclinic Fe3+ centres in Rb2ZnF4 crystals

EPR measurements have been made on tetragonal and monoclinic Fe3+ centres in Rb2ZnF4 at room temperatures. The fine-structure terms for the monoclinic centre are separated into uniaxial and cubic terms for comparison with the Fe3+-VK centre in KZnF3 and for the uncompensated tetragonal centre. The monoclinic centre is ascribed to a Fe3+ ion associated with the nearest Rb+ vacancy and the principal z-axis is found to be 7 degrees from the (001)-axis in the opposite direction to that of the Rb+ vacancy. Rudowiczs NCC parameters for the monoclinic Fe3+ and Cr3+ centres are examined in relation to the separation of the spin-Hamiltonians. The calculated metal-ligand distances and deviation angles for the front and back fluorines by Newmans superposition model are in good agreement with, respectively, the distances for the uncompensated centre and angles for the Fe3+-VK centre in KZnF3 determined from 19F-ENDOR measurements. The relationships between the bnm and the axial and cubic parameters obtained by the superposition model are similar to those obtained from the spin-Hamiltonian separation.

Electron paramagnetic resonance study of centres in cubic and hexagonal perovskite crystals

EPR measurements have been made on centres in single crystals of at room temperature using an X-band spectrometer. In crystals doped with , both spectra with cubic and spectra with tetragonal symmetries are observed. These centres are ascribed to ions substituted for ions in cubic ) without any local charge compensation and with a vacancy at its nearest site, respectively. In some as-grown crystals co-doped with and , signals from a new trigonal centre are observed in place of those from the cubic and tetragonal centres in . The trigonal centre is identified as the - pair substituted for the - bond in a unit in hexagonal (), although the cubic perovskite structure is normally stabilized at room temperature. This indicates that the pairs in units may be related to the crystal growth of the hexagonal structure in the matrix.

A study of substitutional disorder in : II. Electron spin resonance and polarization of optical spectra

Electron spin-resonance (ESR) studies of in (CYA) reveal spectra consisting of fine-structure pairs of narrow symmetric lines and fairly broad asymmetric lines, corresponding to ions in sites with ordered and disordered configurations of the second-nearest-neighbour ions, respectively. The polarized intensities of the absorption band and the -line fluorescence, measured with the E-vector of the radiation parallel and perpendicular to the c-axis, are in the ratio . These polarization ratios suggest that the octahedra undergo an angular distortion such that the magnetic -axis of the spectrum is rotated away from the crystal c-axis. The microstructure of the surrounding is discussed in terms of the ESR and polarization results.

An EPR study of anisotropies of several Cr3+ centres in CsMgCl3

EPR measurements at 300 K and 1.6 K have been made on CsMgCl3 crystals doped with Cr3+ and Cr2+ and co-doped with Li+ using X- and Q-band spectrometers. Together with signals from the pair centre, signals from three trigonal Cr3+ centres (I, III, IV) and a monoclinic centre (II) are observed from the as-grown crystal. Accurate spin-Hamiltonian parameters at 300 K and 1.6 K are obtained for all the centres using the matrix-diagonalization method. The signs of the b20 parameters have been determined by the depopulation effect at 1.6 K. From a comparison of b20 on the basis of the ligand-distortion model for the centres I, III and IV and intensity change by x-irradiation, the trigonal centre with b20=1049.4*10-4 cm-1 is ascribed to the uncompensated Cr3+ ion and that with b20=-2807*10-4 cm-1 is ascribed to the Cr3+ ion associated with the nearest Mg2+ vacancy. From a spin-Hamiltonian separation analysis the monoclinic centre II is ascribed to a Cr3+ ion associated with the nearest Cs+ vacancy.

Electrostatic theory and spin-Hamiltonian separation for 3d3 ions in orthorhombic and tetragonal centres

The spin-Hamiltonian parameters for the 3d3 ion in an orthorhombic centre are calculated within the framework of the electrostatic model. Expressions for the three components of the g tensor and the fine structure parameters b20 and b22 are given in terms of the crystalline field energies and spin-orbit coupling parameters. The calculations are performed including the contributions from the 4T2 states in the ground term and the 2T2 states in the excited terms. The relationship between g/sub ///-gperpendicular to and b20 is derived. The basis of the spin-Hamiltonian separation for the orthorhombic centre of 3d3 ions is shown by electrostatic theory. The proper coordinate system for the orthorhombic centre is also discussed.

Angular variation of normal-type EPR intensities of V2+ in CsMgCl3

EPR measurements have been made at room temperatures on a trigonal V2+ centre in CsMgCl3 that was analysed as a substitutional centre by McPherson et al. to second-order in perturbation theory. The spectrum has been described by direct diagonalization of a 32*32-dimensional spin-Hamiltonian matrix using the computer software developed originally by the authors. The anisotropic parameters g/sub ///-gperpendicular to and b20 have opposite signs to each other contrary to the expectation from simple electrostatic theory. By rotating the sample while keeping the c-axis normal to the external magnetic field, signal intensities of low- and high-field fine-structure lines interchanged characteristically. From theoretical calculation the intensity variation has been ascribed to the variation of the transition probability. The intensity variation is considered to be the elementary process for the resonant field shift observed in magnetically concentrated materials. The uniaxial direction of a trigonal or tetragonal centre in a microwave cavity can be known from this type of intensity-variation measurement irrespective of the sign of the fine-structure parameter b20.

EPR and optical spectroscopy of Cr3+ ions in Y3Ga5O12 crystalline thin films

The emission spectrum of Cr3+ in Y3Ga5O12 crystalline thin films at 10K is composed of the sharp R1-line, its phonon sideband, and the very weak broadband. The lineshape of R1-line at the peak wavelength of 689.8 nm cannot be fitted to a single Gaussian. This suggests that there exist several different Cr3+ sites in the thin film. This fact is confirmed by lifetime measurements; the radiative decay of the fluorescence is decomposed into multiple-exponential curves. The dominant component (2.8 ms) of the decay curve for the broadband emission is equal to those (2.4 ms) for R1-line and its phonon sidebands. This result is discussed in terms of a tunneling model.

EPR study of centres in and crystals

EPR measurements have been made on as-grown single crystals of , and doped with using an X-band spectrometer. In each crystal a new spectrum having a large fine-structure splitting with orthorhombic symmetry is observed. The new centre is ascribed to a ion substituted for the divalent cation with an ion at its ligand site in the c-plane. In , signals from the uncompensated centre are not observed, in contrast with the case for and . Formation of the centre in is discussed using the spin-Hamiltonian separation analysis.