Effect of synthesis conditions on cation ordering in barium bismuth oxide (Ba/Bi = 2)

Abstract

Recent studies of superconducting oxides containing bismuth or copper show that changes in metal oxidation states and in oxygen stoichiometry typically occur during normal syntheses. Reaction of the component oxides at high temperature to form a specific structure in oxygen-deficient form, followed by low-temperature oxidation, exemplified by the preparation of YBa[sub 2]Cu[sub 3]O[sub 7], is a commonly used technique. In other cases, for example Ba[sub 0.7]K[sub 0.3]BiO[sub 3], specific compositions can only be synthesized by control of the oxygen partial pressure during the entire synthesis. The dependence of cation distributions on sample history with respect to oxygen partial pressure in addition to temperature is a similar though more subtle effect. For example, in YBa[sub 2](Cu[sub 1[minus]x]Fe[sub x])[sub 3]O[sub 7], an increased occupancy of the CuO[sub 2] planes by iron is obtained when reducing conditions are used to convert Fe[sup 3+] to Fe[sup 2+] cations in the initial synthesis. The authors have been investigating the effect of thermal history and oxygen partial pressure on the B cation distribution in the Ba-Bi-O system. Attempts to synthesize [open quotes]Ba[sub 2]BiO[sub 4][close quotes] with the La[sub 2]CuO[sub 4] structure type by changing the reaction conditions always result in compounds with the perovskite structure. The more » specific cation distribution, however, depends markedly on the sequence of temperatures and oxygen partial pressures that is used in a particular synthesis. 2 figs. « less