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A d ' Ti3+ centre formed by x-irradiation of a synthetic zircon single crystal at 77 K has been studied by electton paramagneticpsonance (EPR) at 10 K. The Ti3t ion occupies n substitutional site of point group symmetry 42m (Dd. Absence of both site splitting in genenl crystal orientations and of superhyperhne structue from a +I ion indicated that a nearby charge compensator was not involved. Comparison with EPR measurements on Ti3t far quartz centres, point charge calculations and 29Si hyperfine spliuings indicated that the Ti3'' centre results from electron czpNre by Ti4+ occupying a &+ site in the crystal: the designation [TiOBI-is suggested. Precise spin-Hamiltonian parameters. including "Ti (I = z, 7.4%) and 49Ti (I = 1 2 ' 5.4%) hyperfine. nuclear electric quadrupole and nuclear Zeeman parameter matrices. together with high-spin nuclear term of dimension 14, 13S, IsS. Bf' and B f s were determined. The set of term B I , Bf' and B I s were found to be crucial in obtaining a good fit to the many 'forbidden' hyperfine lines observed. This resulted in determination of nuclear quadrupole and n u c l w Zeeman panmeters with precisions not hitherto attained in conventional single-crystal EPR measurements.

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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