Splitting of triggered phase transition in Bi4-xLaxTi3O12 mixed crystals

Phase transitions in Bi4-xLaxTi3O12 (BLT-x) mixed crystals with a low La concentration (x < 0.66) were investigated by measuring the dielectric constant and hysteresis loop, and carrying out differential thermal analysis (DTA) and domain wall observations. It was found in BLT-x that the triggered transition, i.e., the tetragonal-monoclinic transition observed in Bi4Ti3O12 (BiT, BLT-0), splits into two transitions, i.e., the tetragonal-orthorhombic and orthorhombic-monoclinic transitions, with the addition of La ions. The temperature of the tetragonal-orthorhombic transition remains unchanged at about 673 degrees C, whereas that of the orthorhombic-monoclinic transition strongly depends upon the La concentration in the range of 0 < x < 0.66. The phase transitions and their symmetry changes were discussed on the basis of the Landau theory.