Preparation and photophysical properties of some oxides in Ca–Bi–O system

Four stable phases with stoichiometric molar ratio, Ca6Bi6O15, Ca4Bi6O13, CaBi2O4 and Ca5Bi14O26 have been prepared by conventional solid-state reaction method. The details of the reaction processes were investigated and the different intermediate products were identified by X-ray diffraction before the pure phases were obtained. In the four phases the valence of Bi was +3 and the lone-pair electrons of Bi3+ led to the formation of the Bi–O sheet structures. Bi–O polyhedrons with different coordination numbers and bond lengths were discussed. The optical properties of the four samples were also investigated. The band gaps of Ca6Bi6O15, CaBi2O4, Ca4Bi6O13 and Ca5Bi14O26 were estimated from the absorption spectra as 3.34, 3.08, 2.76 and 2.60 eV, respectively, and they were all direct transition semiconductors. Moreover, the Raman spectra of Ca6Bi6O15, Ca4Bi6O13 and CaBi2O4 were also investigated and the results suggested that some high frequency phonon modes (Raman shift more than 450 cm−1) appeared in the crystals with Ca being introduced into the three complex oxides.