Charge ordering in the rare-earth manganates: the origin of the extraordinary sensitivity to the average radius of the A-site cations,(r<SUB>A</SUB>)

The charge ordering in Nd0.5Si0.5MnO3(&lt;rA&gt; = 1.24 &#197;), which occurs on cooling the ferromagnetic metallic ground state, is readily destroyed on application of a magnetic field of 6 T. For Y0.5Ca0.5MnO3 (&lt;rA&gt;=1.13 &#197;), for which the ground state is charge ordered, on the other hand, magnetic fields have no effect on the charge ordering. In order to understand such a marked difference in charge-ordering behaviour of the manganates, we have investigated the structure as well as the electrical and magnetic properties of Ln0.5Ca0.5MnO3 compositions (Ln = Nd, Sm, Gd and Dy) where in &lt;rA&gt; varies over the range 1.17-1.13 &#197;. The lattice distortion index, D, and charge-ordering transition temperature, Tco, for the manganates increase with the decreasing &lt;rA&gt;. The charge-ordered state is transformed to a metallic state on applying a magnetic field of 6 T in the case of Nd0.5Ca0.5MnO3(&lt;rA&gt;=1.17 &#197;), but this is not the case with the analogous Sm, Gd and Dy manganates with &lt;rA&gt; less than 1.17 &#197;. In order to explain this behaviour, we have examined the &lt;rA&gt;-dependence of the Mn-O-Mn bond angle, the average Mn-O distance and the apparent one-electron bandwidth, obtained from these structural parameters. It is suggested that the extraordinary sensitivity of the charge ordering to &lt;rA&gt; arises from factors other than those based on the Mn-O-Mn bond angle and average Mn-O distances alone. It is possible that the competition between the covalent mixing of the oxygen O: 2p&#963; orbital with the A-site and B-site cation orbitals plays a crucial role. Strain effects due to size mismatch between A-site cations could also cause considerable changes in Tco