In the system Bi2O3-WO3, a phase Bi7WO13.5 (22.22 mol% WO3) with high oxide-ion conduction exists stably above 670 degreesC, although this phase was thought to be the central composition of a solid solution and stable even in room temperature. Below 670 degreesC, Bi7WO13.5 decomposes sluggishly to yield a mixture of a bismuth-rich limit of the solid solution and a small quantity of Bi14WO24 with far lower electrical conduction. The stable solid solubility region expands from 20 to 27 mol% WO3 at 850 degreesC and from 23 to 25.5 mol% WO3 below 500 degreesC. (C) 2004 Elsevier B.V. All rights reserved.