Effect of Cation Size and Disorder on the Structure and Properties of the Rare Earth Cobaltates, Ln_0.5A_0.5CoO₃ (Ln = Rare Earth, A = Sr, Ba)

The structure of Ln_0.5Sr_0.5CoO₃ is rhombohedral when Ln = La, Pr, or Nd, but orthorhombic (Puma) when Ln = Gd. The Ln_0.5Ba_0.5CoO₃ compounds, except for Ln = La, are orthorhombic (Pmmm). The ferromagnetic Curie temperature, Tc, of Ln_0.5A_0.5CoO₃ increases with the average size of the A-site cation up to an 〈r_A〉 of 1. 40 Å, 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, σ². It is found that Tc decreases linearly with σ², according to the relation, Tc = T°c − pσ². When σ² is large (>0.012 Ų), the material becomes insulating , providing evidence for a metal–insulator transition caused by cation-size disorder. Thus, Gd_0.5Ba_0.5CoO₃ with a large σ² 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.