Simultaneous Metal-Insulator and Spin-State Transition in (Pr1-yREy)1-xCaxCoO3 (RE = Nd, Sm, Gd, and Y)

We have studied a Pr-site substitution effect using various RE ions (RE = Nd, Sm, Gd, and Y) on a simultaneous metal-insulator (MI) and spin-state (SS) transition in (Pr1-yREy)(1-x)CaxCoO3 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 RE = Sm, Gd, and Y. The MI-SS transition temperatures TMI-SS can be scaled universally by the average ionic radius < r(A)> of the A-site in the perovskite ACoO(3), 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(2)(cr) is also necessary for the occurrence of the MI-SS transition and TMI-SS increases with increasing sigma(2). In contrast, no MI-SS transition was observed in the RE = 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.