Electron paramagnetic resonance spectroscopic study of two gadolinium centres at calcium sites in synthetic fluorapatite

Abstract

Gd-doped fluorapatite, synthesized from CaF₂-rich melts, has been investigated as single crystals and powder samples by using X-band (9.5 GHz) and W-band (95 GHz) electron paramagnetic resonance (EPR) spectroscopy. Gd₂O₃ with natural abundances of isotopes and ¹⁵⁷Gd-enriched Gd₂O₃ were used in the crystal synthesis. The X-band spectra obtained for the Gd-doped fluorapatite displayed a well-resolved type of Gd³⁺ centre (the centre 'a') caused by the Gd even isotopes (electron spin: S = 7/2; nuclear spin: I = 0), and suggested the possible presence of a second partly-resolved type of Gd³⁺ centre (the centre 'b') also caused by the Gd even isotopes. The latter was thoroughly disclosed in the W-band spectra. The single-crystal X- and W- band EPR spectra from three orthogonal rotation planes obtained from the Gd-doped fluorapatite allowed determination of the general spin-hamiltonian parameters for Gd³⁺ centres 'a' and 'b', including the spin terms of type BS (g matrix) and S²(D matrix), and the parameters associated with the high-spin terms of type S⁴ and S⁶, as well as BS³ and BS⁵. The validity of the spin-hamiltonian parameters was confirmed by agreement between the observed and simulated EPR spectra for both single-crystal and powder samples. The principal values of the matrices g and D for the centres 'a' and 'b' indicate that the two Gd³⁺-occupied sites in the synthetic fluorapatite have rhombic local symmetry. The principal directions of the D matrices suggest that the centres 'a' and 'b' correspond to substitutions of Gd³⁺ into Ca2 and Ca1 sites, respectively. These site assignments are supported by the results of pseudo-symmetry analyses using the S⁴ parameters. For example, the calculated pseudo-symmetry axes of the centre 'a' coincide with the local rotoinversion axis, site coordinations, as well as the faces of the coordination polyhedra of the Ca2 sites. The local structural environments of the centres 'a' and 'b' also suggest that the Gd³⁺ ions are incorporated into the Ca2 and Cal via Gd³⁺ + O²­­­­- ↔ Ca²­­­⁺ + F- and 2Gd³⁺ + □ ↔ 3Ca²­­­­⁺, respectively. The vacancy (□) associated the centre 'b' has been shown to be located at a nearest-neighbor Ca2 site, resulting in a Gd³⁺ --□---Gd³⁺ arrangement, with the cations well separated. The single-crystal X- and W-band EPR spectra of the ¹⁵⁷Gd-doped fluorapatite revealed a well-resolved ¹⁵⁷Gd (nuclear spin: I=3/2) hyperfine structure (HFS) of the centre 'a' and a partly-resolved ¹⁵⁷Gd HFS of the centre 'b'. The calculated spin-hamiltonian parameters for the hyperfine, nuclear quadrupole, and nuclear Zeeman effects (i.e., matrices A,P and gn)provide further evidence for the site assignment of the centres 'a' to ¹⁵⁷Gd nuclides at the Ca2 sites, with rhombic local symmetry. The P matrix also suggests that the electric-field gradient at the ¹⁵⁷Gd nuclides of the centre 'a' is close to uniaxial, with the largest value along the direction of the Ca2-O2 bond and almost isotropic in the horizontal plane. Moreover, single-crystal spectrum simulations have shown that the hyperfine anisotropy of the centre 'a' arises not only from A, P and gₙ but is also affected by terms BS (g), S² (D), S⁴, S⁶, BS³ and BS⁵.