Charge ordering in the rare-earth manganates: the origin of the extraordinary sensitivity to the average radius of the A-site cations,

The charge ordering in Nd0.5Sr0.5MnO3 ( = 1.24 A, 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 ( = 1.13 A), 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) wherein varies over the range 1.17-1.13 A. The lattice distortion index, D, and charge-ordering transition temperature, TCO, for the manganates increase with the decreasing . 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 (=1.17 A), but this is not the case with the analogous Sm, Gd and Dy manganates with less than 1.17 A. In order to explain this behaviour, we have examined the -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 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: 2pσ 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. (author)