The crystal structures of Bi 2.5 Na 0.5 Ta 2 O 9 and Bi 2:5 Na m21:5 Nb m O 3mþ3 ðm ¼ 3; 4Þ have been investigated by the Rietveld analysis of their neutron powder diffraction patterns ðk¼ 1:470 ( AÞ: These compounds belong to the Aurivillius phase family and are built up by ðBi 2 O 2 Þ 2þ fluorite layers and ðA m21 B m O 3mþ1 Þ 22 ðm ¼ 224Þ pseudo-perovskite slabs. Bi 2.5 Na 0.5 Ta 2 O 9 ðm ¼ 2Þ and Bi 2.5 Na 2.5 Nb 4 O 15 ðm ¼ 4Þ crystallize in the orthorhombic space group A2 1 am; Z ¼ 4; with lattice constants of a ¼ 5:4763ð4Þ; b ¼ 5:4478ð4Þ; c ¼ 24:9710 (15) and a ¼ 5:5095ð5Þ; b ¼ 5:4783ð5Þ; c ¼ 40:553ð3Þ ( A, respectively. Bi 2.5 Na 1.5 Nb 3 O 12 ðm ¼ 3Þ has been refined in the orthorhombic space group B2cb; Z ¼ 4; with the unit-cell parameters a ¼ 5:5024ð7Þ; b ¼ 5:4622ð7Þ; and c ¼ 32:735ð4Þ ( A. In comparison with its isostructural Nb analogue, the structure of Bi 2.5 Na 0.5 Ta 2 O 9 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 Bi 2:5 Na m21:5 Nb m O 3mþ3 (m ¼ 224), 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: # 2002 Elsevier Science (USA)