Influence of nonstoichiometry in barium(3+3x) B(1+y) niobium oxide (B = cobalt or zinc) perovskites on the microwave properties

Near stoichiometric compositions of Ba3+3xB 1+yNb2O9 (B =Co or Zn) perovskites were studied by microstructure analysis and optical techniques. Materials considered in the present research belong to the family of perovskites exhibiting disorder-1:2 order phase transitions that are important for microwave applications. It was found that deviation from stoichiometry involving cation deficiencies on Ba- or B-positions facilitates formation of an ordered structure for small values of cation deficiencies. Excessive deviation from the nominal values as well as introduction of extra cations destabilizes the perovskite structure leading to the precipitation of secondary phases.;Appearance of Raman active modes in the considered Ba3+3xB 1+yNb2O9 materials was attributed to the formation of a 1:2 cation ordered structure. It was shown that microwave losses are influenced by the degree of 1:2 cation ordering that depends on the formation of secondary phases as well as a densification process. The appearance of an "extra" peak in Raman spectra was attributed to the formation of 1: 1 cation order described based on the "space-charge" model. Changes in the position of the mode, attributed to "breathing-type" vibrations of oxygen anions from materials having "partially" ordered 1:1 structure to those having 1:2 ordered structure, indicate formation of more rigid oxygen octahedra associated with lower microwave losses.;Structural distortion caused by 1:2 cation ordering results in changes in the mutual orientation of transition metal-ligand molecular orbitals and the appearance of two emission bands signifying formation of two different NbO6 octahedra. The first octahedron, present in the 1:2 ordered structure, gives origin to the lower energy photoluminescence band, while the second one, forming a disordered cubic structure, produces an emission peak at higher energies with the variation in the position of the maximum depending on the type of cation on the B-site. Changes in the maximum position of the high-energy peak were attributed to different structure distortions caused by off-center motion of Nb5+ and stabilization by neighboring BO6 octahedra. The stabilization power of BO6 octahedra depends on the covalency of B-O bonds and is larger for cobalt containing perovskites.;Formation of a Ba-deficient Ba6BNb9O30 (B = Co or Zn) phase influences the grain growth rate through reduction in the surface energy of grains. In combination with large strain in precursor materials caused by applied pressure during fabrication and high sintering temperature this results in increased porosity and lower density.