Incommensurate and commensurate modulations of Ba5RTi3Nb7O30 (R = La, Nd) tungsten bronzes and the ferroelectric domain structures

Incommensurate and commensurate structural modulations of Ba5RTi3Nb7O30 (R=La, Nd) tungsten bronze ceramics were investigated by using a cooling holder equipped transmission electron microscopy in the temperature range from 100 K to 363 K. The incommensurate modulation was observed in both Ba5LaTi3Nb7O30 and Ba5NdTi3Nb7O30 at room temperature, while there was a transition from incommensurate tilted structure to commensurate superstructure for Ba5NdTi3Nb O-7(30) with decreasing temperature. The incommensurate and commensurate modulations were determined by the A-site occupancy of Ba and R cations. The A-site disorder resulted in larger incommensurability parameter delta and the diffusion of the satellite reflection spots. The effect of A-site disorder on the coupling between long-range dipolar order and the commensurate modulation was also discussed. The obvious ferroelectric 180 degrees domains with spike-like shape parallel to c axis were observed for Ba5NdTi3Nb7O30, while no macro ferroelectric domain was determined for Ba5LaTi3Nb7O30. (C) 2015 AIP Publishing LLC.