The dielectric behavior of the Bi-layered perovskite oxides Sr2Bi4Ti5O18 and Pb2Bi4Ti5O18 with five perovskite-like units (m=5) was studied. A new anomaly was found at T 1=428 K for Sr2Bi4Ti5O18 and 480 K for Pb2Bi4Ti5O18, in addition to the previously reported phase transition at T c. The dielectric constant does not show the Curie–Weiss behavior in both Sr2Bi4Ti5O18 and Pb2Bi4Ti5O18 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.

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