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)

Abstract

The structure of Ln_0.5Sr_0.5CoO₃ is rhombohedral (R₃c) when Ln=La, Pr, or Nd, but orthorhombic (Pnma) when Ln=Gd. The Ln_0.5Ba_0.5CoO₃ compounds, except for Ln=La, are orthorhombic (Pmmm). The ferromagnetic Curie temperature, T_C, 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 T_C decreases linearly with σ², according to the relation, T_C=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.