Mössbauer Studies of Natural Apatite

Apatite $\mathrm{Ca}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_{3}X$ ($X=\mathrm{F},\mathrm{C}\mathrm{l},\mathrm{o}\mathrm{r}\mathrm{O}\mathrm{H}$), which is of biological importance, has been studied using the Mössbauer technique. ${\mathrm{Fe}}^{57}$ replacing substitutionally ${\mathrm{Ca}}^{2+}$ ions in apatite showed four-line Mössbauer spectra with quadrupole splittings of 3.47 and 0.98 mm/sec at room temperature, corresponding, respectively, to two ${\mathrm{Ca}}^{2+}$ sites, one of them (I) has a threefold symmetry axis, and the other (II) has a less regular arrangement of the nearest neighbors so that it has only one reflection plane of symmetry. Using the angular dependence of the intensity ratio of each doublet for apatite single crystals, each site was identified with a quadrupole doublet. The quadrupole splitting of ${\mathrm{Fe}}^{57}$ on a Ca(I) site is large, positive, and essentially insensitive to temperature variation, which suggests the existence of an axial field which is large compared to the spin-orbit coupling. Calculation shows that the energy separation between the ground and first excited orbital states is 12 times as large as the spin-orbit coupling constant. On the other hand, the quadrupole splitting of ${\mathrm{Fe}}^{57}$ on a Ca(II) site is small and positive, and decreases rapidly with increasing temperature, suggesting that the crystal-field splitting is comparable to the spin-orbit coupling.