|dc.description.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. 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+ + 02- ↔ 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 Ca2-02 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.||en_US