PREPARATION OF ALKALINE-EARTH BISMUTH PYROCHLORES CONTAINING BI5+ BY LOW-TEMPERATURE HYDROTHERMAL REACTION

New alkaline earth bismuth oxides containing Bi5+ with the pyrochlore type structure were prepared by low temperature hydrothermal reaction using a hydrated sodium bismuth oxide NaBiO3 · n H2O as a starting material. The Ca- and Sr-pyrochlores contain CO3 group and the ratios of Ca/Bi and Sr/Bi vary from 0.22 to 0.44 and from 0.24 to 0.46, respectively. The Ba-pyrochlore contains a H2O or OH group instead of a CO3 group and the ratio of Ba/Bi is constant. The chemical compositions of new pyrochlores are represented as AxBiyO7-3δ(CO3)δ for A = Ca, Sr, and Ba0.68Bi3.05O6.41(H2O, OH)0.59. The X-ray powder patterns for Ca-, Sr-, and Ba-pyrochlores are indexed in the monoclinic, cubic, and tetragonal systems, respectively. All the pyrochlores decompose above 400°C with two steps of weight losses caused by complete reduction from Bi5+ to Bi3+. The Ca-pyrochlores change from the monoclinic to cubic symmetry by partial release of CO3 group, though the symmetry for the Sr- and Ba-pyrochlores are not affected by release of CO3 group or water. The electric resistivity of the monoclinic Ca-pyrochlores is ∼ 104 ohm · cm and the cubic Ca-, Sr-, and the Ba-pyrochlores exhibit high electric resistivities (∼ 107 ohm · cm) at room temperature. © 1993 Academic Press, Inc.