Collective Phase-like Mode and the Role of Lattice Distortions at TN~TC   in RMn2O5 (R= Pr, Sm, Gd, Tb, Bi)
  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. 
N\'estor E. Massa, Ali F. Garc\'ia-Flores, Domingos De Sousa Meneses,   Leire del Campo, Patrick Echegut, Gilberto F. L. Fab...
cond-mat.str-el