Collective Phase-like Mode and the Role of Lattice Distortions at TN~TC in RMn2O5 (R= Pr, Sm, Gd, Tb, Bi)

TL;DR

This study investigates collective electronic excitations and lattice distortions in RMn2O5 compounds, revealing their role in magnetic and ferroelectric properties without structural phase transitions, influenced by rare earth element substitution.

Contribution

It uncovers the origin of collective modes in RMn2O5 related to Jahn-Teller distortions and electron hybridization, highlighting the impact of rare earth substitution on phonon behavior.

Findings

Collective excitations condense below TN and TC.

No structural phase transition occurs at magnetic ordering.

Phonon hardening correlates with lanthanide contraction.

Abstract

We report on electronic collective excitations in RMn2O5 (R= Pr, Sm, Gd, Tb) showing condensation starting at and below TN\simTC\sim40-50 K. Its origin is understood as partial delocalized eg electron orbitals in the Jahn-Teller distortion of the pyramids dimmer with strong hybridized Mn3+-O bonds. Our local probes, Raman, infrared, and X-ray absorption, back the conclusion by which there is no structural phase transition at TN\simTC. Ferroelectricity is magnetically assisted by electron localization triggering lattice polarizability by unscreening. We have also found phonon hardening as the rare earth is sequentially replaced. This is understood as consequence of lanthanide contraction. It is suggested that partially f-electron screened Rare Earth nuclei might be introducing a perturbation to eg electrons prone to delocalize as the superexchange interaction takes place.