Gd-doped fluorapatite (1.2 ± 0.2 wt% Gd2O3), synthesized from CaF2-rich melts, has been investigated as single crystals and powder samples by using X-band (~9.4 GHz) electron paramagnetic resonance (EPR) spectroscopy at ~295 and 120 K. The well-resolved X-band EPR spectra yielded a previously unreported type of Gd3+ center “a” (S = 7/2) and also suggested the possible presence of a second and partly resolved type of Gd3+ center “b.” In particular, the single-crystal X-band EPR spectra of center “a” from three orthogonal-rotation planes allowed determination of the spin-Hamiltonian parameters, including the spin terms of type BS (matrix g) and S2 (matrix D) and the parameters associated with the high-spin terms of type S4 and S6 as well as BS3 and BS5. The validity of the parameters has been confirmed by agreement between observed and simulated EPR spectra for both single crystals and powder samples.
The principal values of the matrices g and D indicate that the local symmetry of center “a” in the X-band EPR spectra is rhombic. The principal axis directions of the D suggest that this Gd3+ center arises from a substitution of Gd3+ ion into the Ca2 type of site. This assignment is supported by the results of a pseudo-symmetry analysis using the S4 parameters, e.g., the calculated twofold pseudo-symmetry axis coincides with the twofold rotoinversion axis of the Ca2 site. The local structural environment of this Gd3+ ion suggests that the ion is incorporated via the mechanism Gd3+ + O2− ↔ Ca2+ + F−.