Electron paramagnetic resonance spectroscopic study of two gadolinium centres at calcium sites in synthetic fluorapatite
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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. Gd203 with natural abundances of isotopes and 157Gd-enriched Gd203 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 Cal 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.