Bi<SUB>4</SUB>LnNb<SUB>3</SUB>O<SUB>15</SUB> (Ln = La, Pr, Nd) and Bi<SUB>4</SUB>LaTa<SUB>3</SUB>O<SUB>15</SUB>: new intergrowth Aurivillius related phases

We describe the synthesis and characterization of new intergrowth Aurivillius related phases, Bi4LnNb3O15 (Ln = La, Pr, Nd) and Bi4LaTa3O15. Both powder X-ray diffraction and electron microscopy investigations show that the compounds adopt orthorhombic structures with the cell parameters a ~ 5.5 Å, b ~ 5.5 Å and c ~ 20.9 Å, suggesting an ordered intergrowth structure that consists of n = 1 [Bi2NbO6]- and n = 2 Bi2LnNb2O9]+ Aurivillius fragments which are stacked alternately along the c-axis. The oxides do not show a second harmonic generation (SHG) response toward 1064 nm laser radiation; they do not show a ferroelectric-paraelectric transition either between 30 and 900 ° C in dielectric measurements, indicating a centrosymmetric structure. Optical absorption studies show that the intergrowth phases possess considerably smaller band gaps than the parent Nb2O5 and Ta2O5