General properties of polycrystalline LnNiO3 (Ln = Pr, Nd, Sm) compounds prepared through different precursors

We report on the synthesis and general physical properties of polycrystalline samples of LnNiO(3) (Ln = Pr, Nd, Sm), These compounds crystallize in a GdFeO3-type orthorhombically distorted perovskite structure, Pbnm space group symmetry, and reveal a first-order metal-insulator transition in a large range of temperatures varying from similar to 130 to 400 K. The samples were prepared through three different methods: (1) the conventional ceramic process by mixing and reacting simple oxides; (2) through precipitation of hydroxides of Ln(3+) and Ni3+ and (3) through sol-gel precursors. The samples were subjected to heat treatments at temperatures up to 1000 degrees C and under oxygen pressures as high as 70 bar. Observations of the microstructure and measurements of X-ray powder diffraction XRD, electrical resistivity rho(T), and differential scanning calorimetric DSC, indicated important effects that precursors from different methods have upon the final properties of these compounds. A comparison between these characterizations revealed that sol-gel precursors resulted in better quality samples of LnNiO(3) compounds. (C) 2000 Academic Press.