F19andP31magic-angle spinning nuclear magnetic resonance of antimony(III)-doped fluorapatite phosphors: Dopant sites and spin diffusion

Phosphors based on calcium fluorapatite [${\mathrm{Ca}}_5$F(${\mathrm{PO}}_4$ ${)}_3$] doped with small amounts of ${\mathrm{Sb}}^{3+}$ as an activator are used in most fluorescent lamps. We have used quantitative ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ and ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ magic-angle spinning nuclear magnetic resonance (MAS-NMR) to study seven samples of calcium fluorapatite containing 0.0–3.0 wt % ${\mathrm{Sb}}^{3+}$ in order to determine the site of antimony substitution. The ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ MAS-NMR spectra of fluorapatite containing 3.0, 2.1, and 1.3 wt % antimony contain a single sharp peak at 2.8 ppm indistinguishable from undoped fluorapatite, and show no additional peaks attributable to the influence of antimony. The ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ MAS-NMR spectra of the model compounds ${\mathrm{SbPO}}_4$, ${\mathrm{Sr}}_{1.03}$ ${\mathrm{Ca}}_{8.97}$ ${\mathrm{F}}_2$(${\mathrm{PO}}_4$ ${)}_6$, ${\mathrm{Sr}}_5$F(${\mathrm{PO}}_4$ ${)}_3$, and ${\mathrm{Ba}}_5$F(${\mathrm{PO}}_4$ ${)}_3$ were also obtained. The ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ MAS-NMR spectra of the antimony-doped samples exhibit, in addition to the main peak at 64.0 ppm (downfield from ${\mathrm{C}}_6$ ${\mathrm{F}}_6$) arising from unperturbed fluorapatite, a shoulder at 65.6 ppm, and a sharp peak at 68.6 ppm. The measured spin-lattice relaxation times ${\mathit{T}}_1$ of these antimony-related peaks are equal in all cases to that of the main peak in a given sample, and vary from 129 to 378 sec, indicating that these peaks arise from apatitic fluoride ions perturbed by antimony.