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]