Giant 16O-18O isotope effect on the metal-insulator transition of RNiO3 perovskites (R equals rare earth)

The observation of unusually large, positive and rare-earth dependent O-16-O-18 isotope shifts in the metal-insulator transition temperature T-MI Of RNIO3 perovskites is reported (Delta T-MI less than or equal to +10.3 K). The results clearly indicate that the mechanism of the transition involves a strong electron-lattice interaction. A simple model based on the existence of Jahn-Teller polarons in a stoichiometric, metallic, charge-transfer system is used to account for the evolution of Delta T-MI along the series.