SITE PREFERENCE OF Gd IN SYNTHETIC FLUORAPATITE BY SINGLE-CRYSTAL W-BAND EPR AND X-RAY REFINEMENT OF THE STRUCTURE: A COMPARATIVE STUDY

Abstract

Fluorapatite containing 1.2(2) wt% Gd2O3 (sample AP30–1), synthesized from a CaF2-rich melt in a Pt crucible, has been investigated by single-crystal W-band (~94 GHz) electron paramagnetic resonance (EPR) spectroscopy and X-ray structure refinement. The EPR spectrum of AP30–1 measured at ~287 K, with Zeeman field B // z’ and microwave field B // x’, gave a Gd site-occupancy ratio ( Ca2 Gd/ Ca1 Gd) of 1.4(4), indicating a weak preference between the two types of Ca sites. Similarly, X-ray refinement of the structure of AP30–1 suggests that Gd is essentially equally distributed between the two Ca sites: Ca2 Gd/ Ca1 Gd = 0.8(2), although the Gd content in this sample is close to the practical limit of detection of this method. This agreement between the W-band EPR and X-ray structural data confirms that EPR is capable of determining the distribution of paramagnetic trace elements between structurally nonequivalent positions in minerals. X-ray refinement of the structure of another sample of fluorapatite [AP40–0, containing 2.9(1) wt% Gd2O3] synthesized in a sealed Pt capsule, yielded a Ca2 Gd/ Ca1 Gd value of 3.1(6), indicating a marked preference for the Ca2 site, consistent with the site preference of Gd in hydrothermally grown Gd-rich fluorapatite. The marked preference of Gd for the Ca1 site observed in previous EPR studies of flux-grown fluorapatite samples [AP30–0, 0.8(1) ppm Gd and Ca2 Gd/ Ca1 Gd = 0.13; AP30–5, 57(4) ppm Gd and Ca2 Gd/ Ca1 Gd = 0.20] is now attributed to the presence of Ca 2+ vacancies at ppm concentrations in these crystals. These point defects are unlikely to reach appreciable (weight %) concentrations in fluorapatite crystals synthesized from CaF2-rich melts, and hence the normal preference of Gd for the Ca2 site in AP30–1 and AP40–0.