EPR of high-spin Fe3+in calcium tungstate, CaWO4

The g eff =30/7 isotropic signal in the EPR of high-spin Fe 3+ compounds is discussed for both tetragonal rhombic crystal fields by reference to the resonance in calcium tungstate. The Fe 3+ /CaWO 4 EPR signals are shown to arise from Fe 3+ in two slightly different interstitial sites in the crystal; the crystal field in both sites is predominantly rhombic. Highly anisotropic signals arising from transitions within the other two Kramers' doublets were positively identified leading to an unambiguous calculation of the spin Hamiltonian parameters. The results are likely to be of great value in analysing similar Fe 3+ spectra in crystals of biological importance.