Raman spectroscopy of rare earth orthoferrites R FeO 3 R =La, Sm, Eu, Gd, Tb, Dy)
We report a Raman scattering study of six rare earth orthoferrites $R{\mathrm{FeO}}_{3} with $R$ La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of \mathrm{SmFeO}}_{3} and first principles calculations enable the assignment of the observed phonon modes to vibrational symmetries and atomic displacements. The assignment of the spectra and their comparison throughout the whole series allow correlating the phonon modes with the orthorhombic structural distortions of $R{\mathrm{FeO}}_{3} perovskites. In particular, the positions of two specific $A{}_{g} modes scale linearly with the two \mathrm{FeO}}_{6} octahedra tilt angles, allowing the distortion to be tracked throughout the series. At variance with literature, we find that the two octahedra tilt angles scale differently with the vibration frequencies of their respective $A{}_{g} modes. This behavior, as well as the general relations between the tilt angles, the frequencies of the associated modes, and the ionic radii are rationalized in a simple Landau model. The reported Raman spectra and associated phonon mode assignment provide reference data for structural investigations of the whole series of orthoferrites.