The electron paramagnetic resonance of an Fe 3ϩ ͑Sϭ5/2͒ impurity in the ferroelastic phase of a BiVO 4 single crystal has been investigated. The rotation patterns of the resonance fields measured on the crystallographic planes are analyzed with the monoclinic spin Hamiltonian. The determined parameters are g xx ϭ1.976, g xz ϭ0.0033, g yy ϭ1.995, g zz ϭ1.994, and B 2 0 ϭ467.3, B 2 2 ϭ2339, B 2 1 ϭ1693, B 4 0 ϭ0.12, B 4 1 ϭ Ϫ2.3, B 4 2 ϭϪ0.8, B 4 3 ϭ4, and B 4 4 ϭϪ0.3 in units of the 10 Ϫ4 cm Ϫ1 . The principal X, Y and Z axes of the D tensor are found to be along the crystallographic c*ϩ30.5°, aϩ30.5°in the c*a plane, and b axis, respectively. The zero-field splitting ͑ZFS͒ parameters D and E are calculated both within the point-charge electrostatic model and the superposition model. Comparison of experimental data with calculated second-order axial and rhombic ZFS parameters suggests that the Fe 3ϩ impurity substitutes for the V 5ϩ ion in BiVO 4 .
The electron paramagnetic resonance (EPR) of an Mn2+ (S=5/2) impurity in the ferroelastic and para-elastic phase of BiVO4 crystals with a single domain, grown by the Czochralski method, has been investigated using an X-band spectrometer. The rotation patterns of the resonance fields measured on the crystallographic planes are analysed: g=1.9940+or-0.0009, D/h=2.443+or-0.002, E/h=0.4912+or-0.0009, F/h=0.094+or-0.002, B42/h=0.0, Ax/h=0.27+or-0.02, Ay/h=0.25+or-0.02, and Az/h=0.33+or-0.02 GHz at room temperature (
X-band EPR spectra of Mn2+ in ferroelastic BiV04 single crystals at room temperature taken in three orthogonal planes are analyzed using a twestage least-squares fitting procedure. The absence of site splitting from the spectra shows unequivocally that Mn2+ lies on a site with two-fold rotation symmetry, whose two fold axis is parallel to the monoclinic crystal axis. The low symmetry effects arising from the monoclinic site symmetry are evidenced by the observed coincidence of the extrema for transitions between different Zeeman levels along the monoclinic axis, and by the 180" rotational symmetry in the monoclinic plane and a small noncoincidence of the turning points in this plane. In the first stage, the spectra are fitted using the monoclinic form of the electronic Zeeman and fine structure terms (of second and fourth degree) appropriate for the crystallographically determined orientation of the monoclinic axis. In the second stage, fitting of the nuclear hyperfine tensor (A) and the nuclear quadrupole tensor (P) is carried out with fKed values of the electronic Zeeman tensor (g) and fine structure terms, i.e. the tensor (D) and the fourth-degree terms, as obtained in the first stage. The principal axes of g, D, A, and P are found to be non-coincident in the monoclinic plane, as would be generally expected for a monoclinic site. I ) 83
The energy levels and wavefunctions including the two lowest-l~ng levels. namely 4A2 and :E, for which reliable cxperim~tal data exist for C ? ' ions at C3 s-etry sites in L i W . are calculated using the complete m&r diagonalization method wiiilhin the 3d3 configuration. The Hamiltonian eonnid&d includes thc eleamstatic term, the Trees correction, the rpin-orbit interachon and the crystal-field interaction. The role of the additional Iowsymmetry rrystal-field term ET3Oi3 (in the Stevens operator notation). neglected in he C% approximation used so far in the literahlre, is studied. The superposition model is developed for 3d3 ions at C3 ~ymmelly sites and applied to study the site occupancy d C?+ in L i m . Analysis of the optical data i n d i e s that C? ions substitute at Nb sites and Li sites simultaneously. The present considerations offer an improvement avertheearlierapproximarians using C3" symmetry wly. The zero-field splitting predicted by the crystal-field calculations for C ? ' at the Nb site matches the experimental value from EPR mdies very well. This is contrary to the earlier prediction by the superposition model analysis of the spin-Hamiltonian parameters indicating that the 2ero-Beld splilting for C?' ions at Li rites matches the experimental zerofield splitling better than that for C?+ ai Nb sites. Since the present calculations involve fitting not only the zero-field splitting but also the energies of the : E state. the present predictions may be more reliablc than the previous predidionns.
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