Disorder-induced orbital ordering inL0.7M0.3MnO3perovskites

A series of eight ${\mathrm{AMnO}}_3$ perovskites in which the A-cation sites are occupied by a distribution of 70% trivalent lanthanide and 30% divalent Ca, Sr, or Ba ions has been prepared. These all have a mean A-cation radius of 1.20 Å, but different values of the size variance ${\sigma }^2.$ All of the samples show resistive and magnetic transitions from a ferromagnetic metallic to a paramagnetic insulating phase. An abrupt change from orbital disorder to order (cooperative Jahn–Teller distortions) in the insulating regime at ${\sigma }^2\approx 0.005{\AA }^2$ is evidenced by anomalies in the unit cell parameters and conductivity and magnetization data. The dependence of the metal-insulator transition temperature upon A-cation distribution in these and other 30% doped compositions are compared against a simple model and displayed in a general phase diagram.