Axially Symmetric Paramagnetic Color Centers in Fluorapatite

Abstract

The x-ray coloration of synthetic calcium fluorophosphate (fluorapatite) crystals is shown to result from the presence of fluorine vacancy defects. Three of these defects have been studied by correlating their optical absorption bands with their electron-paramagnetic-resonance spectra, and by application of the electron-nuclear-double-resonance technique to measure the F 19 and P 31 hyperfine interactions. The models proposed for these three axially symmetric defects consist of an electron trapped at (a) an isolated F~ ion vacancy, (b) a F~ ion divacancy associated with a charge-compensating 0 = ion, and (c) a second configuration of defect (b). Several paramagnetic hole-excess centers are also described. No optical absorption bands between 0.3 and 1.5 n have been observed for the hole centers.

Multifrequency Epr Study of Radiation-Induced Defects in Chlorapatite

“…The electron-like centers are associated with F -vacancies or substituting O 2-ions in the anion column: F E(I) is an electron trapped by one of two F -vacancies separated by a substitutional O 2-ion, F E(II) is an electron trapped by an isolated F -vacancy, F E(III) is an electron trapped by two F -vacancies adjacent to a substitutional O 2-ion, and F E(IV) an electron trapped by a F -vacancy adjacent to a substitutional O 2-ion. Piper et al (1965) also reported a diamagnetic center ( F B) with two electrons trapped by two F -vacancies adjacent to a substitutional O 2-ion. Center F H(I) involves a hole trapped by a substitutional O 2-ion associated with a neighboring F -vacancy (Segall et al 1962), whereas F H(II) consists of a hole trapped by an isolated substitutional O 2-ion.…”

“…Center F H(I) involves a hole trapped by a substitutional O 2-ion associated with a neighboring F -vacancy (Segall et al 1962), whereas F H(II) consists of a hole trapped by an isolated substitutional O 2-ion. Piper et al (1965) noted that F H(III) and F H(IV) form during the thermal conversion from F H(I) to F H(II). They interpreted F H(III) and F H(IV) to represent stable intermediate confi gurations during the progressive loss of the F -vacancy from the neighboring O 2-ion.…”

“…For example, Piper et al (1965) and Warren (1972) reported four electron-like centers [ F E(I), F E(II), F E(III) and F E(IV)] and four hole-like centers [ F H(I), F H(II), F H(III) and F H(IV)] in X-irradiated fl uorapatite; the presuperscript denotes radiation-induced defect centers in fl uorapatite (Fig. 1).…”

“…Electron paramagnetic resonance (EPR) spectroscopy has been proven to be particularly powerful in providing detailed structural information about dilute concentrations of paramagnetic defects in the anion column in fl uorapatite (Segall et al 1962, Piper et al 1965, Warren 1972, Pan & Fleet 2002. For example, Piper et al (1965) and Warren (1972) reported four electron-like centers [ F E(I), F E(II), F E(III) and F E(IV)] and four hole-like centers [ F H(I), F H(II), F H(III) and F H(IV)] in X-irradiated fl uorapatite; the presuperscript denotes radiation-induced defect centers in fl uorapatite (Fig.…”

Apatite Glass-Ceramics: A Review

“…Clifford and Hill (1996) further suggested that the apatite formed in A-W systems is therefore more likely to be a mixture of fluor-and oxyapatite. Based on the pioneering electron spin resonance (ESP) studies on fluor/oxyapatites, it has been long known that O − can occupy F − sites (Segall et al, 1962;Piper et al, 1965). Nonetheless, at present, the availability of high-resolution solid-state characterization techniques such as 17 O MAS-NMR coupled with dynamic nuclear polarization (DNP) and high field 19 F MAS-NMR could provide fast and accurate elucidation of oxygen and fluorine environments in the A-W GC; however, such work is yet to be published.…”