8.9 GHz EPR results for high spin Fe3+ in CaWO4 at around 10K, including W-183 and Fe-57 hyperfine interactions, are presented. The special features of the main Fe3+ spectrum confirm, as discussed in an earlier room temperature X-band study, the site of the paramagnetic ion as Laue class (1) over bar with near 'complete rhombic' symmetry in the second degree zero held splitting terms. A complete spin-Hamiltonian analysis of the 10K results is presented which includes (g) over bar and (D) over bar parameter matrices and high spin Zeeman terms of dimension (SB)-B-3 and (SB)-B-5. The sizes of the parameters arising from these latter terms are several orders of magnitude greater than have previously been found in transition metal ion EPR spectra. The magnitudes of the effects which they produce, and which are correctly predicted by the spin-Hamiltonian containing them, are also surprisingly large. In particular, the very complicated angular dependence of the g(eff) approximate to 4.3, middle Kramers' doublet 'isotropic' lines, and the positions of outer Kramers' doublet high held lines are predicted to within the inherent error in their measurement, similar to 0.01 mT in each case. In the absence of (SB)-B-3 and (SB)-B-5 terms the predicted positions of some high field lines at their turning points differ from those measured by around 2T, i.e. by five orders of magnitude. (C) 1997 Elsevier Science Ltd.
