Perovskite-type Ca1-xSrxNbO3 (0≤x≤1) phases:: A synthesis, structure, and electron microscopy study

Reduced niobates Ca1-xSrxNbO3 (0 less than or equal to x less than or equal to 1) with perovskite-type structures have been synthesized at 1500 degrees C in niobium ampoules sealed under argon gas. The prepared compounds were characterized by ii-ray powder diffraction, electron diffraction, high-resolution electron microscopy, and energy-dispersive X-ray analysis. The structure of CaNbO3 (GdFeO3 type) was refined using ii-ray powder diffraction data. Electron diffraction studies showed that complex superstructures of the perovskite-type structure occur for x>0 in Ca1-xSrxNbO3: (i) for x = 0.2 and 0.3 reflections corresponding to a supercell with a approximate to 2x root 2 x a(per), b approximate to 4 x a(per), and c approximate to 2 x root 2 x a(per) (per = ideal perovskite) were observed, (ii) for x = 0.5, 0.6, and 0.7 a cubic supercell was found with a = 4 x a(per), whereas (iii) for x = 0.8 and 0.9 the supercell is probably orthorhombic with a approximate to c approximate to root 2 x a(per) and b approximate to 2 x a(per). For x = 1.0 a new orthorhombic modification of SrNbO3 with a approximate to c approximate to root 2 x a(per) and b approximate to 2 x a(per) was found. Resistivity measurements showed CaNbO3 to have a nonmetallic type of conductivity, whereas the Sr-containing samples were metallic. (C) 1998 Academic Press.