Preparation of Alkaline Earth Bismuth Pyrochlores Containing Bi5+ by Low Temperature Hydrothermal Reaction

✍ Scribed by N. Kumada; M. Hosoda; N. Kinomura

• Publisher: Elsevier Science
• Year: 1993
• Tongue: English
• Weight: 277 KB
• Volume: 106
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1993.1307

✦ Synopsis

New alkaline earth bismuth oxides containing (\mathrm{Bi}^{5+}) with the pyrochlore type structure were prepared by low temperature hydrothermal reaction using a hydrated sodium bismuth oxide (\mathrm{NaBiO}{3} \cdot n \mathrm{H}{2} \mathrm{O}) as a starting material. The (\mathrm{Ca}-) and (\mathrm{Sr})-pyrochlores contain (\mathrm{CO}{3}) group and the ratios of (\mathrm{Ca} / \mathrm{Bi}) and (\mathrm{Sr} / \mathrm{Bi}) vary from 0.22 to 0.44 and from 0.24 to 0.46 , respectively. The Ba-pyrochlore contains a (\mathrm{H}{2} \mathrm{O}) or (\mathrm{OH}) group instead of a (\mathrm{CO}{3}) group and the ratio of (\mathrm{Ba} / \mathrm{Bi}) is constant. The chemical compositions of new pyrochlores are represented as (A{x} \mathrm{Bi}{y} \mathrm{O}{7-39}\left(\mathrm{CO}{3}\right) \delta) for (A=\mathrm{Ca}, \mathrm{Sr}), and (\mathrm{Ba}{0.68} \mathrm{Bi}{3,05} \mathrm{O}{6.41}\left(\mathrm{H}{2} \mathrm{O}, \mathrm{OH}\right){0.59}). The (\mathrm{X})-ray powder patterns for (\mathrm{Ca}-, \mathrm{Sr})-, and (\mathrm{Ba}-) pyrochlores are indexed in the monoclinic, cubic, and tetragonal systems, respectively. All the pyrochlores decompose above (400^{\circ} \mathrm{C}) with two steps of weight losses caused by complete reduction from (\mathrm{Bi}^{3+}) to (\mathrm{Bi}^{3+}). The Ca-pyrochlores change from the monoclinic to cubic symmetry by partial release of (\mathrm{CO}{3}) group, though the symmetry for the (\mathrm{Sr}) - and Ba-pyrochlores are not affected by release of (\mathrm{CO}{3}) group or water. The electric resistivity of the monoclinic Ca-pyrochlores is (\sim 10^{4} \mathrm{ohm} \cdot \mathrm{cm}) and the cubic Ca-, (\mathrm{Sr} \cdot), and the Ba-pyrochlores exhibit high electric resistivities (\left(\sim 10^{7} \mathrm{ohm} \cdot \mathrm{cm}\right)) at room temperature. 1993 Academic Press, Inc.