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)

The structure of Ln 0.5 Sr 0.5 CoO 3 is rhombohedral ( R 3̄ 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 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, σ 2 . It is found that T C decreases linearly with σ 2, according to the relation, T C = T ° C − p σ 2 . When σ 2 is large (>0.012 Å 2 ), the material becomes insulating, providing evidence for a metal−insulator transition caused by cation-size disorder. Thus, Gd 0.5 Ba 0.5 CoO 3 with a large σ 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.