The phase transitions of lanthanum modified barium sodium niobate system has been investigated by the x-ray diffraction, dielectic constant measurements.
Specimens are prepared by the conventional mixed method. When lanthanum ions has been substituted for barium ions, a dielectric anomaly, which was not observed so far in the single crystalline $Ba_4Na_2Nb_{10}O_{30}$(BSN) has been found in the vicinity of 300°C.
This anomaly is associated with the ferroelastic phase transition. This dielectric anomaly originates from the ferroelastic transition below the ferroelectic transition point, X-ray diffraction of the specimens quenched from the ferroelastic temperature region reveals a superlattice structure composed of two sublattice reflections.
However these sublattice reflections exhibit an incommensurate pattern. The incommensurate phase of the specimen quenched from 280°C is a retained incommensurate phase that formed in the temperature region around 280°C. High temperature X-ray diffraction also shows a crystal structure very similar to the quench specimen. This evidence supports so called memory effect of the incommensurate BSN phase.
