Structural aspects of the giant oxygen isotope effect in perovskite manganese oxides

Abstract

The structure of manganites R 1−x A x MnO3 (R = La, Sm, or a mixture of rare earth cations; A = Ca, Sr; x = 0.3, 0.5) has been studied by neutron diffraction to find out the reasons for the giant oxygen isotope effect—the transition from a low-temperature metallic state to an insulating state as a result of replacement of 16O with 18O. It is shown that this effect is observed in compositions in which the two phases P1 and P2 coexist at low temperatures. They have the same crystal symmetry (sp. gr. Pnma) but different types of Jahn-Teller distortions of oxygen octahedra and different magnetic structure. Phase P1 has a conductivity of the metallic type, weakly distorted MnO6 octahedra, and a ferromagnetic structure. Phase P2 is insulating, with MnO6 octahedra extended or compressed in the apical direction and the moments of Mn ions forming an antiferromagnetic structure. The relative volume of phases P1 and P2 in samples depends on the average radius of the A cation and changes occurring upon replacement of 16O with 18O. The percolation transition from the metallic to insulating state upon substitution of 16O with 18O is caused by the sharp decrease in the volume of the ferromagnetic metallic phase in favor of the insulating antiferromagnetic phase. An effect of the sample microstructure on the formation of the two-phase state is found.