Giant oxygen isotope effect on the metal-insulator transition of RNiO3 perovskites

The metal to insulator transition displayed by all the members of the perov skite family RNiO3 (R = 4f rare earth different from La) has attracted a lo t of interest since it constitutes one of the few examples of this phenomen on in perfectly stoichiometric compounds. In spite of the great deal of wor k performed during the last 6 years, the mechanism responsible for the elec tronic localization is still a matter of controversy. The observation of un usually large O isotope shifts on the metal-insulator temperature T-MI repo rted in this study represents an important advance since it clearly proves the dominant role of the electron lattice interaction as driving force for the transition. Moreover, the good agreement between this observation and a simple model based on the existence of Jahn-Teller polarons in the metalli c state gives further qualitative and quantitative support to the polaronic picture recently suggested to account for O isotope effects in other 3-D t ransition metal oxides containing Jahn-Teller ions.