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

Reduced niobates Ca1-xSrxNbO3 (0 ≤ x ≤ 1) with perovskite-type structures have been synthesized at 1500 C in niobium ampoules sealed under argon gas. The prepared compounds were characterized by X-ray powder diffraction, electron diffraction, high-resolution electron microscopy, and energy-dispersive X-ray analysis. The structure of CaNbO3 (GdFeO3 type) was refined using X-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 ∼ 2 x √2 x aper, b ∼ 4 x aper, and c ∼ 2 x √2 x aper (per = ideal perovskite) were observed, (ii) for x = 0.5, 0.6, and 0.7 a cubic supercell was found with a = 4 x aper whereas (iii) for x = 0.8 and 0.9 the supercell is probably orthorhombic with a ∼ c ∼ √2 x aper and b ∼ 2 x aper. For x = 1.0 a new orthorhombic modification of SrNbO3 with a ∼ c ∼ √2 x aper and b ∼ 2 x aper was found. Resistivity measurements showed CaNbO3 to have a nonmetallic type of conductivity, whereas the Sr-containing samples were metallic