REVISED STRUCTURE OF BI3TINBO9

The displacive ferroelectric Bi3TiNbO9 [M(r) = 911.7, orthorhombic, A2(1)am, a = 5.4398 (7), b = 5.3941 (7), c = 25.099 (5) angstrom, D(x) = 8.223 g cm-3, Z = 4, Mo K-alpha, lambda = 0.7107 angstrom, mu = 737.6 cm-1, F(000) = 1535.3] is described at room temperature as a commensurate modulation of an Fmmm parent structure. Displacive modes of F2mm, Amam and Abam symmetry are all substantial and reduce the symmetry to A2(1)am. A final value of 0.0295 for R1 = SIGMA-h\\F(obs)(h)\ - \F(calc)(h)\\/SIGMA-h\F(obs)(h)\ was obtained for the 1386 unmerged data used for refinement. The modes of Amam and Abam symmetry were essentially the same as in a previous refinement [Wolfe, Newnham, Smith & Kay (1971). Ferroelectrics, 3, 1-7], but the F2mm displacive mode associated with the ferroelectricity of the compound was substantially different and agreed with features of a recent re-refinement of Bi4Ti3O12[Rae, Thompson, Withers & Willis (1990). Acta Cryst. B46, 474-487].