Variable-temperature X-ray diffraction of the ferroelectric transition in Bi4Ti3O12

Variable-temperature X-ray diffraction has been performed to study the ferroelectric transition with T-c = 948 K in Bi4Ti3O12, which corresponds to the orthorhombic-to-tetragonal structural change. All the lattice parameters of the orthorhombic structure expand with increasing temperature until only the b-axis exhibits a sudden decrease at T-c and approaches the a-axis of the tetragonal structure; no jumps are apparent in both the a- and c-axes at T-c whereupon they increase exhibiting different thermal expansivities from those below T-c. The present work indicates that the orthorhombic distortion bla decreases inflecting in slope at T-c with temperature while a=b is not achieved at T-c and thereafter. Interestingly enough, the (0 2 0) reflection shifts downwards whereas the (2 0 0) reflection remains relatively stationary as the temperature is increased through T-c. Resides, the linewidth/intensity of the (2 0 0) reflection inflects in slope ac T-c and decreases/increases subsequently whereas such a discontinuity is not observed for the (0 2 0) reflection. It is suggested that the a-axis displacement of Bi atoms in the perovskite A sites with respect to the chains of TiO6 octahedra, that is the main structural cause of ferroelectricity in Bi4Ti3O12, could be responsible for the present findings. (C) 1997 Elsevier Science Ltd.