Continuous structural evolution of (Bi2O2)(2)V2yO4y+2 (1<=y<=4) aurivillius phases in the Bi2O3-VO2 system

In the Bi2O3-VO2 system, the series (Bi2O2)(2)V2yO4y+2 with 1 less than or equal to y less than or equal to 4 has been examined and its structural character determined. The main phase Bi4V2O10 exhibits three varieties and transformations occur with increasing temperature: alpha(T > 690 degrees), beta (T > 850 degrees), gamma. The series of (Bi2O2)(2)V2yO4y+2 showed a typical layered structure built up by alternating (Bi2O2)(n) layers with (V2yO4y+2)(n) sheets. These structures have been derived by comparison with the series CaVnO2n+1 where single crystal structures have been established for CaV2O5, CaV3O7, and CaV4O9. The latter phases are models for the particular values y = 1, 2, 3, and 4 corresponding to (Bi2O2)(2)(VO3)(2), (Bi2O2)(2)(V2O5)(2), (Bi2O2)(2)(V3O7)(2) and (Bi2O2)(2)(V4O9)(2), but in contrast to the calcium-vanadium series, there is no definite composition for the (Bi2O2)(2)V2yO4y+2 which evolves gradually from y = 1 up to y = 4. The environment of both bismuth and vanadium atoms are more or less distorted square pyramids (SP), VO5 and BiO(4)E (E, lone pair). Correlating with the enrichment of VO2, i.e., with the increasing number of VO5 SP, the layer V2yO4y+2 shows a continuous densification, the VO5 SP sharing an increasing number of edges, compatible with the organization governed by the (Bi2O2)(n) Aurivillius layer. (C) 1996 Academic Press, Inc.