Structure study of Bi2.5Na0.5Ta2O9 and B2.5Nam-1.5NbmO3m+3 (m=2-4) by neutron powder diffraction and electron microscopy

The crystal structures of Bi2.5Na0.5Ta2O9 and Bi2.5Nam-1.5NbmO3m+3 (m = 3,4) have been investigated.ysis of their neutron powder diffraction by the Rietveld anal patterns (lambda = 1.470 Angstrom). These compounds belong to the Aurivillius phase family and are built up by (Bi2O2)(2+) fluorite layers and (A(m-1)BnO(3m+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 A2(1)am, 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) Angstrom, 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) Angstrom. 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 h 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. (C) 2002 Elsevier Science (USA)