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

Abstract

The charge ordering in Nd_0.5Si_0.5MnO₃(<r_A> = 1.24 Å), which occurs on cooling the ferromagnetic metallic ground state, is readily destroyed on application of a magnetic field of 6 T. For Y_0.5Ca_0.5MnO₃ (<r_A>=1.13 Å), 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_0.5Ca_0.5MnO₃ compositions (Ln = Nd, Sm, Gd and Dy) where in <r_A> varies over the range 1.17-1.13 Å. The lattice distortion index, D, and charge-ordering transition temperature, T_co, for the manganates increase with the decreasing <r_A>. The charge-ordered state is transformed to a metallic state on applying a magnetic field of 6 T in the case of Nd_0.5Ca_0.5MnO₃(<r_A>=1.17 Å), but this is not the case with the analogous Sm, Gd and Dy manganates with <r_A> less than 1.17 Å. 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 <r_A> 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 T_co.