Cation disorder and size effects in magnetoresistive manganese oxide perovskites

Large disorder effects due to size differences between A-site R(3+) (R=La,PT,Nd,Sm) and M(2+) (M = Ca,Sr,Ba) cations have been found in magnetoresistive (R(0.7)M(0.3))MnO3 perovskites. The ferromagnetic-metal-paramagnetic-insulator transition temperature T-m varies as T-m = T-m(0)-pQ(2) due to strain fields resulting from ordered or disordered oxygen displacements Q that are parametrized by the statistical mean and variance of the A cation radius, respectively. The value of p is related to the Mn-O force constant showing that Mn3+ Jahn-Teller distortions assist electron localization at T-m. The maximum possible T-m is estimated to be similar to 530 K although experimentally observable values are less than or equal to 360 K. A large suppression of magnetoresistance due to cation disorder is also evidenced.