Dielectric spectra of a new relaxor ferroelectric system Ba 2LnTi 2Nb 3O 15 (Ln = La, Nd)

Abstract

Ba 2LaTi 2Nb 3O 15 exhibits a smeared maximum of permittivity, characteristic of classic relaxor behaviour, with a peak shift from 185 K at 100 Hz to 300 K at 1 GHz. Ba 2NdTi 2Nb 3O 15 undergoes a first order ferroelectric phase transition at 389 K and Ba 2La 0.5Nd 0.5Ti 2Nb 3O 15 exhibits both a ferroelectric phase transition at 274 K and relaxor behaviour at higher temperatures. All three ceramic systems were investigated across a broad frequency (100 Hz–1 THz) and temperature (10–500 K) range. Slowing down of the dielectric relaxation across the whole investigated temperature range from the THz and microwave regions to below 100 Hz was observed on cooling in the relaxor ferroelectric Ba 2LaTi 2Nb 3O 15. The mean relaxation frequency obeys the Vogel–Fulcher law with freezing temperature of 40 K. In Ba 2NdTi 2Nb 3O 15 and Ba 2La 0.5Nd 0.5Ti 2Nb 3O 15 the relaxations soften only in the paraelectric phase down to the 1 GHz and 100 MHz range, respectively. Below T c the relaxations vanish. The origin of the dielectric relaxations is discussed on the basis of structural data.