Axially Symmetric Paramagnetic Color Centers in Fluorapatite

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 ${\mathrm{F}}^{19}$ and ${\mathrm{P}}^{31}$ hyperfine interactions. The models proposed for these three axially symmetric defects consist of an electron trapped at (a) an isolated ${\mathrm{F}}^{-}$ ion vacancy, (b) a ${\mathrm{F}}^{-}$ ion divacancy associated with a charge-compensating ${\mathrm{O}}^{=}$ 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 μ have been observed for the hole centers.