Stress dependence of the zone-center optical phonons of LaF3

Abstract

The Raman spectrum of La${\mathrm{F}}_{3}$ was studied both with and without uniaxial stress. Spectra of unstressed samples taken at room temperature and also at 77 and 2.2 K allow identification of all observed structure, thereby correcting some inaccurate assignments in previously published works. Most of the Raman lines were studied under uniaxial stress at 77 K. This study allows the determination of anharmonic coefficients important in the lattice-dynamical description of the material. Also, the stress dependence of twofold degenerate modes permits the distinguishing between the two proposed point-group symmetries, ${D}_{6h}$ and ${D}_{3d}$, for the unit cell of La${\mathrm{F}}_{3}$. Thus quantitative comparison between the splittings produced in these modes by forces applied along two well-chosen crystallographic directions establishes that the correct point-group symmetry is ${D}_{3d}$.