Raman spectroscopy of rare-earth orthoferrites RFeO3 (R = La, Sm, Eu, Gd, Tb, Dy)

TL;DR

This study uses Raman spectroscopy and first-principles calculations to analyze phonon modes in RFeO3 orthoferrites, revealing how structural distortions relate to vibrational properties across different rare-earth elements.

Contribution

It provides a detailed phonon mode assignment for RFeO3 orthoferrites and establishes a correlation between vibrational frequencies and structural tilt angles, with novel insights into their scaling behavior.

Findings

Two Ag phonon modes scale linearly with FeO6 octahedra tilt angles.

Different tilt angles scale differently with their respective mode frequencies.

The results are rationalized using a simple Landau model.

Abstract

We report a Raman scattering study of six rare earth orthoferrites RFeO3, with R = La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of SmFeO3 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 allows correlating the phonon modes with the orthorhombic structural distortions of RFeO3 perovskites. In particular, the positions of two specific Ag modes scale linearly with the two FeO6 octahedra tilt angles, allowing the distortion throughout the series. At variance with literature, we find that the two octahedra tilt angles scale differently with the vibration frequencies of their respective Ag 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.