Infrared Dispersion and Lattice Vibrations of LaF3

Abstract

The infrared spectral emittance of oriented single crystals of La${\mathrm{F}}_{3}$ has been measured in the range 4-125 \ensuremath{\mu}m. 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\ifmmode^\circ\else\textdegree\fi{}K, were treated by Kramers-Kronig analysis and by classical oscillator dispersion theory. The results support a crystal structure of ${{D}_{3d}}^{4}(P\overline{3}c1)$ which is very slightly distorted from a unit cell of ${{D}_{6h}}^{3}(\frac{P{6}_{3}}{\mathrm{mcm}})$ symmetry. The infrared results are in good agreement with the results of a recent Raman investigation of La${\mathrm{F}}_{3}$.