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The crystal structures of Bi2.5Na0.5Ta2O9 and Bi2.5Nam–1.5NbmO3m+3 (m=3,4) have been investigated by the Rietveld analysis of their neutron powder diffraction patterns (λ=1.470 A˚). These compounds belong to the Aurivillius phase family and are built up by (Bi2O2)2+ fluorite layers and (Am–1BmO3m+1)2– (m=2–4) pseudo-perovskite slabs. Bi2.5Na0.5Ta2O9 (m=2) and Bi2.5Na2.5Nb4O15 (m=4) crystallize in the orthorhombic space group A21am, Z=4, with lattice constants of a=5.4763(4), b=5.4478(4), c=24.9710 (15) and a=5.5095(5), b=5.4783(5), c=40.553(3) A˚, respectively. Bi2.5Na1.5Nb3O12 (m=3) has been refined in the orthorhombic space group B2cb, Z=4, with the unit-cell parameters a=5.5024(7), b=5.4622(7), and c=32.735(4) A˚. In comparison with its isostructural Nb analogue, the structure of Bi2.5Na0.5Ta2O9 is less distorted and bond valence sum calculations indicate that the Ta–O bonds are somewhat stronger than the Nb–O bonds. The cell parameters a and b increase with increasing m for the compounds Bi2.5Nam–1.5NbmO3m+3 (m=2–4), causing a greater strain in the structure. Electron microscopy studies verify that the intergrowth of mixed perovskite layers, caused by stacking faults, also increases with increasing m. [Copyright &y& Elsevier]