SYNTHESIS, CRYSTAL-STRUCTURE, AND PROPERTIES OF METALLIC PRNIO3 - COMPARISON WITH METALLIC NDNIO3 AND SEMICONDUCTING SMNIO3

The compound PrNiO3 has been prepared for the first time. Using only moderate oxygen pressure, PrNiO3, NdNiO3, and SmNiO3 were made and found to crystallize in the GdFeO3-type orthorhombically distored perovskite structure. Structural refinements for all three compounds reveal NiO6 octahedra with an average NiO distance of 1.94-1.95 Å, about the same as in the more distorted HoNiO3. The electrical conductivity of PrNiO3 is metallic at room temperature, but undergoes a transition at 130 K to an insulating state. Examination of the conductivities of the corresponding Sm, Nd, and La compounds reveals a monotonic pattern of behavior: as the rare earth radius is increased, the compounds become more conducting because the metal-insulator transition temperature decreases from 400 K(Sm) to 200 K(Nd) to 130 K(Pr) to none for La. From DSC and lattice constant measurements, this transition is shown to be first order, with a ∼0.2% contraction upon heating into the metallic state. Low temperature neutron measurements in the insulating phase reveal new diffraction peaks, probably related to magnetic ordering of Ni and/or RE moments. © 1991.