An X-Ray Powder and Electron Diffraction Study of Reduced Tantalates with the Perovskite Structure, Na1−xSrxTaO3, 0

Abstract

Single-phase perovskite-type reduced tantalates, Na1−xSrxTaO3 with 0<x≤0.4, have been synthesized by heating mixtures of NaTaO3, Sr5Ta4O15 and Ta in sealed tantalum ampules at 1400°C for 20–24 h. At Sr contents of 0.5≤x≤0.8 and above 1400°C, polyphasic samples were obtained. X-ray powder diffraction (XRD) and transmission electron microscopy studies (selected area (SAED) and convergent beam electron diffraction (CBED)) showed an orthorhombic distortion of the x=0.1 sample (GdFeO3 type, space group Pnma). The interpretation of the XRD, SAED, and CBED studies indicates the crystallites found in the x=0.2 and 0.3 samples to consist of domains with tetragonal and orthorhombic symmetry having the unit cell parameters of a=b≈√2aper and c≈2aper and a≈b≈√2aper and c≈aper (aper cell axis in the ideal perovskite structure), respectively. The x=0.4 sample was cubic (space group Pm3m) with a=aper. Resistivity measurements showed that all samples are semiconducting and that the conductivity increases with Sr content. Magnetic measurements of Na0.6Sr0.4TaO3 revealed a diamagnetic behavior down to 20 K. Below this temperature a slight paramagnetism arises.