Oxygen isotope effect on the spin-state transition in (Pr0.7Sm0.3)0.7Ca0.3CoO3

Oxygen isotope substitution is performed in the perovskite cobalt oxide (Pr0.7Sm0.3)(0.7)Ca0.3CoO3 with a sharp spin state transition from the intermediate spin (IS) state to the low spin (LS) state at a certain temperature. The transition temperature (T-s) of the spin state up-shifts with the substitution of O-16 by O-18 from the resistivity and magnetic susceptibility measurements. The up-shift value is 6.8 K and an oxygen isotope exponent (alpha(S)) is about -0.8. The large oxygen isotope effect indicates strong electron-phonon coupling in this material. The substitution of O-16 by O-18 leads to a decrease in the phonon frequency and an increase in the effective mass of electron (m(*)), so that the bandwidth W decreases and the energy difference between the different spin states increases. This is the reason why the T-s is shifted to high temperature with oxygen isotopic exchange.