STRUCTURE AND GIANT MAGNETORESISTANCE OF La_1/3R_1/3Ca_1/3MnO₃ ; by Guanghui Rao, Jingkui Liang, Jirong Sun and Yezhou Sun; Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China

The crystal structure of La_1/3R_1/3Ca_1/3MnO₃ was refined by conventional Rietveld method with x-ray powder diffraction data. The structure belongs to an orhorhombically distorted perovskite-type, S.G. Pnma. Under our synthesis conditions (sintering the mixture of R₂O₃, CaCO₃ and MnO₂ at 1373K for 96hr with two intermediate grindings), single-phase samples with the orthorhombic structure were obtained for R=La, Pr, Nd, Sm, Eu, Ga and Tb), while the samples with R=Dy, Er, Tm and Y consist of the orthorhombic phase (S.G. Pnma) and a hexagonal phase (S.G. P6₃cm). The stability of the orthorhombic structure seems to be determined by a geometrical tolerance criterion for the perovskite: t 0.945. The magnetoresistance in a field up to 1T and the magnetization as well as ac susceptibility of the samples were measured. A negative giant magnetoresistance effect as high as -96% was observed in bulk La_1/3Nd_1/3Ca_1/3MnO₃ at a filed of 0.67T. An insulator-metal transition occurs far below Curie temperature. The conductivity of the compounds exhibits a metallic behavior at low temperature, and a semiconductor behavior at high temperature dominated by magnetic polaron hopping. The correlation between the distortion of perovskite structure, magnetic structure and magnetoresistance was investigated.