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

The structure of Ln0.5Sr0.5CoO3 is rhombohedral (R3c) 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 &lt;rA&gt; of 1.40 &#197;, 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 &lt;rA&gt; and differing size variance, &#963;2. It is found that TC decreases linearly with &#963;2, according to the relation, TC=T&#176;C-p&#963;2. When &#963;2 is large (&#62;0.012 &#197;2), the material becomes insulating, providing evidence for a metal-insulator transition caused by cation-size disorder. Thus, Gd0.5Ba0.5CoO3 with a large &#963;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