Low frequency nonlinearity in layered ferroelectrics BaBi<inf>2</inf>Nb<inf>2</inf>O<inf>9</inf> and SrBi<inf>2</inf>Ta<inf>2</inf>O<inf>9</inf>

The dielectric nonlinearity in layered ferroelectric ceramics of BaBi 2 Nb 2 O 9 and SrBi 2 Ta 2 O 9 is studied by analysing polarisation loops and measuring dielectric permittivity as a function of frequency. In case of BaBi 2 Nb 2 O 9 the system has a broad phase transition manifesting essential features of a slow relaxation of polarisation; in case of SrBi 2 Ta 2 O 9 the processes contributing to polarisation switching are driven by rearrangement of the domain structure.