Electron paramagnetic resonance spectroscopic study of synthetic fluorapatite; Part II, Gd3+ at the Ca1 site, with a neighboring Ca2 vacancy

• Published in: The American mineralogist Vol. 87; no. 1; pp. 47 - 55
• Main Authors: Chen, Ning, Pan, Yuanming, Weil, John A, Nilges, Mark J
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.01.2002; Walter de Gruyter GmbH
• Subjects: chemical composition; concentration; coordination; crystal chemistry; crystal structure; Crystals; EPR spectra; fluorapatite; Magnetism; Mineralogy; Minerals; nonsilicates; phosphates; polyhedra; refinement; Resonance; single-crystal method; spectra; symmetry; synthetic materials
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2002-0106

A W-band (94 GHz) electron paramagnetic resonance (EPR) study of synthetic fluorapatite with 57±4 ppm Gd has been made on single crystals at ∼287 K. The spectra disclosed the presence of a previously unreported type of Gd3+ center denoted by "b" herein (S = 7/2), in addition to the Gd3+ center "a" assigned to the Ca2 site using the results of a previous X-band (9.5 GHz) EPR study (Chen et al. 2002). In particular, the single-crystal W-band EPR spectra from three orthogonal-rotation planes allowed determination of an appropriate spin-Hamiltonian for center "b," 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. Agreement between the observed and simulated single-crystal spectra confirmed the validity of the spin-Hamiltonian analysis. The principal values of the matrices g and D [e.g., D/geβe = 1069.2(1) G and E/geβe = 52.4(3) G] suggest a considerably distorted rhombic local environment for the Gd3+ ions in center "b." The principal directions of D suggest that "b" corresponds to Gd3+ at the Ca1 site. This site assignment is supported by a pseudo-symmetry analysis of the term S4, i.e., approximate matching of the directions of the calculated pseudo-symmetry axes to the bond directions and face normals of the coordination polyhedron of the ideal Ca1 site. The data suggest that the incorporation of Gd3+ into the Ca1 site is achieved by a coupled substitution (2Gd3++[]⇌3Ca2+) involving a Ca2+ vacancy [] and that the vacancy is located at a next-nearest-neighbor Ca2 site, resulting in a Gd3+ - - [] - - - Gd3+ arrangement, with the cations well separated.