• journal_title：American Mineralogist
• Contributor：Ning Chen ; Yuanming Pan ; John A. Weil ; Mark J. Nilges
• Publisher：Mineralogical Society of America
• Date：2002-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：87
• issue：1
• firstpage：47
• section：Articles

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/g_eβ_e = 1069.2(1) G and E/g_eβ_e = 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.