Structure refinement of commensurately modulated bismuth strontium tantalate, Bi2SrTa2O9

The displacive ferroelectric Bi2SrTa2O9 [Mr = 1011.47, a = 5.5306 (5), b = 5.5344 (5), c = 24.9839 (24) A, Z = 4, Dx = 8.785 g cm-3, Mo Kα, λ = 0.7107 A, μ = 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 I4/mmm structure. A final value of 0.045 for R1 = Σhvertical strokevertical strokeFobs(h)vertical stroke- vertical strokeFcalc(h)vertical strokevertical stroke/Σhvertical strokeFobs(h)vertical stroke was obtained for 3082 unmerged data with I(h) > 3 σ[I(h)]. The crystal studied showed both twinning and disorder which were successfully modelled in the refinement. The final structure suggests that these features are almost inevitable. The structure is contrasted with that of Bi3TiNbO9 with particular interest in the effects of a Bi3+ ion being replaced by an Sr2+ ion. The value of anomalous dispersion for detecting false minima centrosymmetric twinning and disorder is detailed. An apparent valence interpretation of the crystal chemistry is included. (orig.)