Phase Transition in Bi-Layered Oxides with Five Perovskite Layers

The dielectric behavior of the Bi-layered perovskite oxides Sr 2 Bi 4 Ti 5 O 18 and Pb 2 Bi 4 Ti 5 O 18 with five perovskite-like units ( m =5) was studied. A new anomaly was found at T 1 =428 K for Sr 2 Bi 4 Ti 5 O 18 and 480 K for Pb 2 Bi 4 Ti 5 O 18 , in addition to the previously reported phase transition at T c . The dielectric constant does not show the Curie–Weiss behavior in both Sr 2 Bi 4 Ti 5 O 18 and Pb 2 Bi 4 Ti 5 O 18 crystals: the anomaly around T c is diffuse and broad, whereas those at T 1 are clearly shoulderlike. These two isomorphous layered-oxide ferroelectrics undergo a new intermediate phase transition at T 1 . The transition temperatures show a linear dependence on the ionic radii of A -site ions in these layered perovskites.