Optical constants of an apatite single crystal in the IR range of 6–28 $\mu$m

On the basis of polarized IR-reflection spectra in the range of 5000–350 cm$^{-1}$ measured from the natural face of an optically transparent fluorapatite single crystal, components of a complex refractive index (optical constants) for radiation-vector orientations $\mathbf{E}\parallel\mathbf{c}$ and $\mathbf{E}\perp\mathbf{c}$ have been calculated by the Kramers–Kronig method. The fluorapatite single crystal has been chosen from several samples: it contains a minimum amount of impurities and has a high degree of crystallinity in accordance with the criteria of IR spectroscopy and Raman spectroscopy. Tabular data on optical constants for ordinary and extraordinary rays are given for the IR range of 6–28 $\mu$m. The obtained absorption spectra are compared with the results of quantum-chemical ab initio calculations within the B3LYP simulation.