Effect of cation size and disorder on the structure and properties of the rare earth cobaltates, Ln(0.5)A(0.5)CoO(3) (Ln = rare earth, A = Sr, Ba)

The structure of Ln(0.5)Sr(0.5)CoO(3) is rhombohedral (R (3) over bar c) wh en 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 ferr omagnetic 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 de creases thereafter due to size mismatch. Disorder due to cation-size mismat ch has been investigated by studying the properties of two series of cobalt ates with fixed [r(A)] and differing size variance, sigma(2). It is found t hat 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 tran sition caused by cation-size disorder. Thus, Gd0.5Ba0.5CoO3 with a large si gma(2) is a charge-ordered insulator below 340 K. The study demonstrates th at the average A-cation radius, as well as the cation-size disorder, affect s the magnetic and transport properties of the rare earth cobaltates signif icantly.