Perovskite-related Ca(Nb,Ti)O3.33

Crystals of nominal composition Ca(Nb0.8Ti0.2)O3.4, an n = 5 member of the homologous series AnBnO3n+2, have been synthesized by floating-zone melting. The material was found to be multiphase consisting also of the n = 6 type. A good single crystal of this n = 6 type was found and the crystal structure at room temperature was determined by X-ray diffraction using synchrotron radiation. Structure refinements indicate a composition of Ca(Nb0.76Ti0.24)O3.33. The crystal structure is monoclinic P21 and consists of slabs of corner-sharing (Nb,Ti)O6 octahedra interrupted by layers of oxygen atoms. Ca atoms within the slabs are 12-fold coordinated whereas those at the borders show irregular coordination geometries. The octahedral distortion is greatest near the borders of the slabs and least near the middle of the slabs. Ti4+ preferentially occupy octahedral sites in the middle of slabs. The non-stoichiometry in the refined composition and the insulating character of the material may be explained in terms of extra oxygen, vacancies on the cation sites, or by localization of the electrons in the 4d orbitals of Nb at the borders of slabs. (orig.)