Direct Measurement of Polaron Binding Energy in AMnO3 as a Function of the A Site Ionic Size by Photoinduced IR Absorption

Photoinduced IR absorption was measured in undoped (LaMn)_1?δO₃ and (NdMn)_1?δO₃. We observe broadening and a ～44% increase of the midinfrared anti-Jahn–Teller polaron peak energy when La³⁺ is replaced with smaller Nd³⁺. The absence of any concurent large frequency shifts of the observed PI phonon bleaching peaks and the Brillouin-zone-center internal perovskite phonon modes measured by Raman and infrared spectroscopy indicate that the polaron peak energy shift is mainly a consequence of an increase of the electron phonon coupling constant with decreasing ionic radius 〈r _A〉 on the perovskite A site. This indicates that the dynamical lattice effects strongly contribute to the electronic band narrowing with decreasing 〈r _A〉 in doped giant magnetoresistance manganites.