An X-ray powder and electron diffraction study of reduced tantalates with the perovskite structure, Na1-xSrxTaO3, 0 < x <= 0.4

Single-phase perovskite-type reduced tantalates, Na1-x SrxTaO3 with 0 < x < less than or equal to> 0.4, have been synthesized by heating mixtures of Na TaO3, Sr5Ta4O15 and Ta in sealed tantalum ampules at 1400 degreesC for 20-2 4 h. At Sr contents of 0.5 less than or equal to x less than or equal to 0. 8 and above 1400 degreesC, polyphasic samples were obtained. X-ray powder d iffraction (XRD) and transmission electron microscopy studies (selected are a (SAED) and convergent beam electron diffraction (CBED)) showed an orthorh ombic distortion of the x = 0.1 sample (GdFeO3 type, space group Pnma). The interpretation of the XRD, SAED, and CBED studies indicates the crystallit es found in the x = 0.2 and 0.3 samples to consist of domains with tetragon al and orthorhombic symmetry having the unit Cell parameters of a = b appro ximate to root (2a(per)) and c approximate to 2a(per) and a approximate to b approximate to root (2a(per)) and c approximate to a(per) (a(per) cell ax is in the ideal perovskite structure), respectively. The x 0.4 sample was c ubic (space group Pm3m) with a = a(per). Resistivity measurements showed th at 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 . (C) 2000 Academic Press.