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

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Abstract
The charge ordering in , which occurs on cooling the ferromagnetic metallic ground state, is readily destroyed on application of a magnetic field of 6 T. For , 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 compositions (Ln = Nd, Sm, Gd and Dy) wherein varies over the range 1.17-1.13 Å. The lattice distortion index, D, and charge-ordering transition temperature, , 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 , but this is not the case with the analogous Sm, Gd and Dy manganates with less than 1.17 Å. 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 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 .