Soft Modes and Relaxor Ferroelectrics

Relaxor ferroelectrics are difficult to study and understand. The experiment shows that at low energy scattering there is an acoustic mode, an optic mode, dynamic quasi-elastic and strictly elastic scattering as well as Bragg peaks at the zone centre. We have studied the scattering using the TASP spectrometer at PSI and have analysed the data using a model with interactions between the different components particularly to determine the properties of the elastic scattering. The quasi-elastic scattering begins to become significant at the Burns temperature of 620 K. It steadily increases in intensity on cooling reaching a maximum at 400 K. Below this temperature the strictly elastic scattering begins to increase and shows a broadened line shape characteristic of crystals in a random applied field. We show that all the results obtained from PMN for the elastic scattering are consistent with the crystal having a random field transition at 400 K. We have obtained similar results for PMN-PT and PZN-PT suggesting that random fields also play an important role in these materials.