Electron paramagnetic resonance spectroscopic study of synthetic fluorapatite: Part I. Local structural environment and substitution mechanism of Gd3+at the Ca2 site

Abstract

Gd-doped fluorapatite (1.2 ′ 0.2 wt% Gd 2 O 3 ), synthesized from CaF 2 -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 previously unreported type of Gd 3 + center a (S = 7/2) and also suggested the possible presence of second and partly resolved type of Gd 3 + 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 S 2 (matrix D) and the parameters associated with the high-spin terms of type S 4 and S 6 as well as BS 3 and BS 5 . 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 Gd 3 + center arises from substitution of Gd 3 + ion into the Ca2 type of site. This assignment is supported by the results of pseudo-symmetry analysis using the S 4 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 Gd 3 + ion suggests that the ion is incorporated via the mechanism Gd 3 + + O 2 - ↔ Ca 2 + + F-.