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 Ln0.5Sr0.5CoO3 is rhombohedral (R3̄c) when Ln = La, Pr, or Nd, but orthorhombic (Pnma) when Ln = Gd. The Ln0.5Ba0.5CoO3 compounds, except for Ln = La, are orthorhombic (Pmmm). The ferromagnetic Curie temperature, TC, of Ln0.5A0.5CoO3 increases with the average size of the A-site cation up to an 〈rA〉 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 〈rA〉 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 Å2), the material becomes insulating, providing evidence for a metal−insulator transition caused by cation-size disorder. Thus, Gd0.5Ba0.5CoO3 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.