Infrared study of lattice dynamics and spin-phonon and electron-phonon interactions in multiferroic TbFe3(BO3)4 and GdFe3(BO3)4

TL;DR

This study investigates lattice vibrations, phase transitions, and spin-phonon and electron-phonon interactions in multiferroic GdFe3(BO3)4 and TbFe3(BO3)4 using infrared spectroscopy, revealing structural changes, magnetic effects, and electronic interactions.

Contribution

It provides a comparative analysis of phonon behavior, phase transitions, and electron-phonon coupling in two isostructural multiferroic compounds, highlighting new coupled modes in TbFe3(BO3)4.

Findings

Weak first-order structural phase transitions at 143 K and 200 K.

Evidence of spin-phonon interaction at magnetic ordering temperatures.

Observation of electron-phonon coupling and crystal-field excitations in TbFe3(BO3)4.

Abstract

We present a comparative far-infrared reflection spectroscopy study of phonons, phase transitions, spin-phonon and electron-phonon interactions in isostructural multiferroic iron borates of gadolinium and terbium. The behavior of phonon modes registered in a wide temperature range is consistent with a weak first-order structural phase transition (Ts = 143 for GdFe3(BO3)4 and 200 K for TbFe3(BO3)4) from high-symmetry high-temperature R32 structure into low-symmetry low-temperature P3121 one. The temperature dependences of frequencies, oscillator strengths, and damping constants of some low-frequency modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon mode behavior in both compounds at the temperature of an antiferromagnetic ordering (TN = 32 K for GdFe3(BO3)4 and 40 K for TbFe3(BO3)4) evidence the spin-phonon interaction. In the energy range of phonons, GdFe3(BO3)4 has no electronic levels but TbFe3(BO3)4 possesses several ones. We observe an onset of new bands in the excitation spectrum of TbFe3(BO3)4, due to a resonance interaction between a lattice phonon and 4f electronic crystal-field excitations of Tb3+. This interaction causes delocalization of the CF excitations, their Davydov splitting, and formation of coupled electron-phonon modes.