Infrared Dispersion and Lattice Vibrations of LaF3

The infrared spectral emittance of oriented single crystals of La${\mathrm{F}}_3$ has been measured in the range 4-125 μm. Eleven transitions have been identified whose net moments are perpendicular to the optic axis ($E_u$, $x$, $y$), and five additional bands were observed having net moments parallel to the optic axis (${\mathrm{A}}_{2u}$, $z$). The data, taken near 4.2, 77, and 373°K, were treated by Kramers-Kronig analysis and by classical oscillator dispersion theory. The results support a crystal structure of ${D_{3d}}^4\left(P\overline{3}c1\right)$ which is very slightly distorted from a unit cell of ${D_{6h}}^3\left(\frac{P{6}_3}{\mathrm{mcm}}\right)$ symmetry. The infrared results are in good agreement with the results of a recent Raman investigation of La${\mathrm{F}}_3$.