Incommensurate and commensurate modulations of Ba5RTi3Nb7O30 (R5La, 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 100K 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 Ba5NdTi3Nb7O30 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 d 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° domains with spike-like shape parallel to c axis were observed for Ba5NdTi3Nb7O30, while no macroferroelectric domain was determined for Ba5LaTi3Nb7O30. [ABSTRACT FROM AUTHOR]