Monoclinic spin hamiltonian analysis of EPR spectra of Mn2+ in BiVO4 single crystals

X-band EPR spectra of Mn2+ in ferroelastic BiVO4 single crystals at room temperature taken in three orthogonal planes are analyzed using a two-stage 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 degrees 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 de termined 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 fixed 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.