Structures and properties of the perovskite-type compounds Na1-xSrxNbO3 (0.1 <= x <= 0.9) - From insulating to metallic conductivity

摘要

Reduced perovskite-type compounds with compositions Na1-xSrxNbO3 (0.1less than or equal toxless than or equal to0.9) have been synthesized. X-ray powder diffraction (XRD) studies for the 0.2less than or equal toxless than or equal to0.7 phases indicate a tetragonal structure (P4/mbm) with unit-cell parameters aapproximate toroot2 x a(per), capproximate toa(per) (the subscript 'per' refers to the cubic perovskite structure), and for x = 0.8 and 0.9, the space group P4/mmm was found with aapproximate toa(per), capproximate toa(per). Structure refinements based on XRD and neutron diffraction data using the Rietveld technique confirmed this for Na1-xSrxNbO3, 0.3less than or equal toxless than or equal to0.7 and x = 0.5, 0.6, respectively. Selected-area and convergent beam electron diffraction studies show that the crystallites of x = 0.2 and 0.3 phases in fact have the GdFeO3-type structure (space group Pnma and unit-cell parameters aapproximate toroot2 x a(per), bapproximate to2 x a(per), capproximate toroot2 x a(per),) while crystallites in the 0.4less than or equal toxless than or equal to0.9 samples consist of intergrown domains between this orthorhombic structure and the tetragonal structure (P4/mbm). The reason for the appearance of two phases is due to local variations in the Na/Sr content within the crystallites. Measurements of the magnetic susceptibilities show that the itinerant behavior in the samples Na1-xSrxNbO3 increases with the carrier concentration. The resistivity measurements indicate that an insulator-metal transition takes place between x = 0.5 and 0.6. The samples with x = 0.4 and 0.5 exhibit a complex temperature dependence of the resistivity with a maximum around 70 K. (C) 2002 Elsevier Science (USA). [References: 18]