Electron paramagnetic resonance spectroscopic study of synthetic fluorapatite: Part II. Gd3+ at the Ca1 site, with a neighboring Ca2 vacancy
-
Ning Chen
,
John A. Weil
Abstract
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 S4and S6 as well as BS3 and BS5. Agreement between the observed and simulated single-crystal spectra confirmed the validity of the spin-Hamiltonian analysis.β
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 Ca 1 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 Cal site. The data suggest that the incorporation of Gd3+ into the Cal 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.
© 2015 by Walter de Gruyter Berlin/Boston
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- Study on adsorption of submicrometer gold on kaolinite by quantum chemistry calculations
- Image distortion in scanning probe microscopy
- Multiple length scale growth spirals on metamorphic graphite {001} surfaces studied by atomic force microscopy
- 29Si MAS NMR study of diopside–Ca-Tschermak clinopyroxenes: Detecting both tetrahedral and octahedral Al substitution
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