Simultaneous Metal–Insulator and Spin-State Transition in (Pr_1-<i>y</i>RE_<i>y</i>)_1-<i>x</i>Ca_<i>x</i>CoO₃ (RE=Nd, Sm, Gd, and Y)

Abstract

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 (Pr 1- y RE y ) 1- x Ca x CoO 3 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 T MI–SS can be scaled universally by the average ionic radius < r A > of the A -site in the perovskite A CoO 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 σ 2 , larger than the critical value σ cr 2 is also necessary for the occurrence of the MI–SS transition and T MI–SS increases with increasing σ 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 σ 2 value because of the small difference in ionic radius between Pr 3+ and Nd 3+ . The atomic randomness of the A -site might be an important parameter that dominates the MI–SS transition through the difference in electronic energy δ E between the spin states of Co 3+ ions.