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

The charge ordering in Nd0.5Sr0.5MnO3 ([r(A)] = 1.24 Angstrom), 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 ([r(A)] = 1.13 Angstrom), 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 Ln(0.5)Ca(0.5)MnO(3) compositions (Ln = Nd, Sm, Gd and Dy) wherein (rA) varies over the range 1.17-1.13 Angstrom. The lattice distortion index, D, and charge-ordering transition temperature, T-CO, for the manganates increase with the decreasing [rA]. 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 ([r(A)] = 1.17 Angstrom), but this is not the case with the analogous Sm, Gd and Dy manganates with [r(A)] less than 1.17 A. In order to explain this behaviour, we have examined the [r(A])-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 (rA) 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 sigma orbital with the A-site and B-sire cation orbitals plays a crucial role. Strain effects due to size mismatch between A-site cations could also cause considerable changes in T-CO.