Simultaneous Metal–Insulator and Spin-State Transition in (Pr1-yREy)1-xCaxCoO3 ($\mathrm{RE}=\text{Nd}$, Sm, Gd, and Y)

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We have studied a Pr-site substitution effect using various RE ions ($\mathrm{RE}=\text{Nd}$, Sm, Gd, and Y) on a simultaneous metal–insulator (MI) and spin-state (SS) transition in (Pr1-yREy)1-xCaxCoO3 using measurements of electrical resistivity, magnetization, and thermal dilatation. The MI–SS transition took place at the appropriate combination of $x$ and $y$ for samples of $\mathrm{RE}=\text{Sm}$, Gd, and Y. The MI–SS transition temperatures $T_{\text{MI--SS}}$ can be scaled universally by the average ionic radius $\langle r_{A} \rangle$ of the $A$-site in the perovskite $A$CoO3, which is independent of $x$, $y$, and the RE ion species. The atomic randomness of the $A$-site, which is defined as the mean square deviation $\sigma^{2}$, larger than the critical value $\sigma_{\text{cr}}^{2}$ is also necessary for the occurrence of the MI–SS transition and $T_{\text{MI--SS}}$ increases with increasing $\sigma^{2}$. In contrast, no MI–SS transition was observed in the $\mathrm{RE}=\text{Nd}$ samples ($x=0.2--0.4$), which can be inferred from the small $\sigma^{2}$ value because of the small difference in ionic radius between Pr3+ and Nd3+. The atomic randomness of the $A$-site might be an important parameter that dominates the MI–SS transition through the difference in electronic energy $\delta E$ between the spin states of Co3+ ions.
2010-03-15