Oxygen isotope effect on the spin-state transition in ( Pr 0.7 Sm 0.3 ) 0.7 Ca 0.3 CoO 3

Oxygen isotope substitution is performed in the perovskite cobalt oxide $({\mathrm{Pr}}_{0.7}{\mathrm{Sm}}_{0.3}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_{3}$ 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 $^{16}\mathrm{O}$ by $^{18}\mathrm{O}$ from the resistivity and magnetic susceptibility measurements. The up-shift value is $6.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and an oxygen isotope exponent $({\ensuremath{\alpha}}_{S})$ is about $\ensuremath{-}0.8$. The large oxygen isotope effect indicates strong electron-phonon coupling in this material. The substitution of $^{16}\mathrm{O}$ by $^{18}\mathrm{O}$ 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.