We systematically investigate the effect of the oxygen isotope in $({\mathrm{Pr}}_{1-x}{\mathrm{Sm}}_x{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ which shows a crossover with $x$ from ferromagnetic metal to the insulator with spin-state transition. A striking feature is that effect of oxygen isotope on the ferromagnetic transition is negligible in the metallic phase, while replacing ${}^{16}\mathrm{O}$ with ${}^{18}\mathrm{O}$ leads to a giant up shift of the spin-state transition temperature $\left(T_s\right)$ in the insulating phase, especially $T_s$ shifts from 36 to $54\mathrm{K}$ with isotope component ${\alpha }_S=-4.7$ for the sample with $x=0.175$. A metal-insulator transition is induced by oxygen isotope exchange in the sample $x=0.172$ being close to the insulating phase. The contrasting behaviors observed in the two phases can be well explained by occurrence of static Jahn-Teller distortions in the insulating phase, while the absence of them in the metallic phase.

• Received 6 July 2006

DOI:https://doi.org/10.1103/PhysRevB.74.165113