Effect of Cation Size and Disorder on the Structure and Properties of the Rare Earth Cobaltates, Ln0.5A0.5CoO3 (Ln = Rare Earth, A = Sr, Ba)

Abstract

The structure of Ln(0.5)Sr(0.5)CoO(3) is rhombohedral (R (3) over bar c) when Ln = La, Pr, or Nd, but orthorhombic (Pnma) when Ln = Gd. The Ln(0.5)Ba(0.5)CoO(3) compounds, except for Ln = La, are orthorhombic (Pmmm). The ferromagnetic Curie temperature, T-C, of Ln(0.5)A(0.5)CoO(3) increases with the average size of the A-site cation up to an [r(A)] of 1.40 Angstrom, and decreases thereafter due to size mismatch. Disorder due to cation-size mismatch has been investigated by studying the properties of two series of cobaltates with fixed [r(A)] and differing size variance, sigma(2). It is found that T-C decreases linearly with sigma(2), according to the relation, T-C = T-C(degrees) - p sigma(2). When sigma(2) is large (>0.012 Angstrom(2)), the material becomes insulating, providing evidence for a metal-insulator transition caused by cation-size disorder. Thus, Gd0.5Ba0.5CoO3 with a large sigma(2) is a charge-ordered insulator below 340 K. The study demonstrates that the average A-cation radius, as well as the cation-size disorder, affects the magnetic and transport properties of the rare earth cobaltates significantly.