Phase transition in the family La xBi 4- xTi 3O 12: In relation to lattice symmetry and distortion

The ferroelectric phase transition in the family La _xBi _{4- x}Ti ₃O ₁₂ was investigated by X-ray diffractometry and differential scanning calorimetry. The crystal symmetry is orthorhombic in the range of 1 > x ≥ 0 and tetragonal in x ≳ 1. It was found that the shift of the phase-transition temperature is closely related to crystallographic symmetry and lattice distortion. (1) As the value of x increases, the ratio of lattice parameters, {a}/{b}, decreases toward 1. (2) In the range of 1 > x ≥ 0, the lattice distortion increases with increasing x; just before x exceeds about 1, the distortion reaches a maximum. (3) As x exceeds about 1, the distortion decreases distinctively. (4) In the orthorhombic region, the phase-transition temperature decreases with increasing x. (5) In the tetragonal region, the transition does not occur. (6) The relation of transition temperature T_c to x is represented by a nonlinear function.