Phase relationships and structural and physical properties of alkaline earth-bismuth oxides

Phase equilibria of Bi₂O₃–CaO were examined at temperatures of 650°–1050°C in oxygen at 1 atm. Electron microprobe analysis determined phase compositions, and phase identification was by powder X-ray diffraction. The system contains six phases with fixed composition—Bi₂O₃, Bi₁₄Ca₅O ₂₆, Bi₂CaO₄, Bi₆Ca₄O ₁₃, Bi₂Ca₂O₅ CaO—and three solid solutions—face-centered-cubic solid solution (FCC_ss), rhombohedral solid-solution (Rh_ss) and body-centered solid solution (BCC _ss). Only three of those phases—FCC_ss, BCC_ss, CaO—coexist with liquid.; In the Ba-Sr-Bi-O system, several polycrystalline specimens of rhombohedral phase with different [Ba]-[Sr] ratios and constant [Alkaline Earth:AE]-[Bi] ratio of 1:4 were synthesized and electrical conductivities of these materials measured using the 2-probe AC impedance technique at 320°–730°C. In each case, a sudden increase in conductivity indicates the β ₂ ↔ β₁ polymorphic transition. Exchange of the smaller Sr for Ba increases the transition temperature almost linearly. However, the Sr-Ba exchange has little influence on conductivities of these phases. The lattice parameter a increases linearly with increasing Ba. The c parameter also increases with increasing Ba, but the data shows deviations from linearity.; The conductivity of each of four rhombohedral single crystals (Ba _1.64Bi_8.36O_14.18, BaSrBi₈O₁₄, Sr₂Bi₈O₁₄, Sr_2.1Bi_7.9O _13.95) was measured along and perpendicular to the conduction planes. On average, the conductivity of a crystal along the conduction plane is two orders of magnitude higher than that of a polycrystalline sample of the same composition. For most single-crystal experiments, the β₂ ↔ β ₁ transition is only poorly indicated by any change in conductivity.; The effect of changing Ba:Sr on the total conductivity of polycrystalline composites of rhombohedral (R) and BaBiO₃ perovskite (P) biphasic samples was investigated by 4-probe DC resistivity measurements. Pure P phase has conductivities of 100–102 S/cm at 350–800°C in air. R-P composites are mixed ionic-electronic conductors. The 80%R–20%P sample has a conductivity profile almost identical to that of the pure Ba-Sr-Bi R phase. The 65%R–35%P sample has higher conductivities than pure R and activation energies between those of pure P and pure R. The conductivity profile of 50%R–50%P is sub-parallel to that of pure P with conductivities of 10−1 to 4 × 10−1 S/cm at the same temperatures. The percolation threshold for these composites is near 35%P.