Giant isotope effect and spin state transition induced by oxygen isotope exchange in (Pr₁₋ₓSmₓ)₀.₇Ca₀.₃CoO₃

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We systematically investigate effect of oxygen isotope in (Pr₁₋ₓSmₓ)₀.₇Ca₀.₃CoO₃ 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 ¹⁶O with ¹⁸O leads to a giant up-shift of the spin-state transition temperature (Tₛ) in the insulating phase, especially Tₛ shifts from 36 to 54 K with isotope component αₛ=-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 absence of them in the metallic phase.