19F and 31P magic-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.Quantitative studies reveal that the 68.6-ppm peak arises from two fluoride ions and the 65.6-ppm shoulder from one fluoride ion per ${\mathrm{Sb}}^{3+}$ ion incorporated into the lattice. The selective population anti-z and rate of transfer to adjacent nuclei (SPARTAN) pulse sequence used to measure spin diffusion by selectively inverting the 68.6-ppm peak reveals the presence of cross-relaxation to the main peak at 64.0 ppm, but not to the shoulder at 65.6 ppm. Each ${\mathrm{Sb}}^{3+}$ ion thus appears to be perturbing fluoride ions in at least two different chains. An additional peak at 73.1 ppm observed in some samples is assigned to a second type of antimony(III) substitution, with a single fluoride ion perturbed by each antimony ion. The results in total provide detailed support for a substitution model in which antimony(III) occupies a phosphate site in the apatite lattice, with a ${\mathrm{SbO}}_{3}^{3\mathrm{\ensuremath{-}}}$ group replacing a ${\mathrm{PO}}_{4}^{3\mathrm{\ensuremath{-}}}$ group. Two types of substitution at this site appear to occur, depending upon which oxygen atom is replaced by the antimony lone electron pair.