Structure Refinement of Commensurately Modulated Bismuth Strontium Tantalate, Bi2SrTa209

The displacive ferroelectric Bi2SrTa209 [M,= 1011.47, a = 5.5306 (5), b -- 5.5344 (5), c = 24.9839 (24)/~, Z = 4, Dx = 8.785 g cm-3, Mo Ka, A = 0.7107 A, /z = 806.8 cm-1, F(000) = 1687.64], is described at room temperature in space group A21am as a commensurate modulation of an idealized Fmmm parent structure derived from an 14/mmm structure. A final value of 0.045 for R1 = ~'.hllgobs(h)l -]Fcalc(h)l[/Y.hlFobs(h)[ was obtained for 3082 unmerged data with l(h)> 3tr[I(h)]. The crystal studied showed both twinning and disorder which were successfully modelled in the refinement. The