Barium titanate - the prototype ferroelectric ceramic

Barium titanate, the first ceramic material in which ferroelectric behaviour was observed, is the ideal model for a discussion of the phenomenon from the point of view of crystal structure and microstructure see also [10] and [11]. 

BaTi03 is isostructural with the mineral perovskite (CaTiO ) and so is referred to as a perovskite . The generalized perovskite structure AB03, is visualized as based on a cubic close-packed assembly (see Fig. 2.1) of composition A03 with the A-ion cordinated with 12 oxygen ions and the B-ion in the octahedral interstices (see Fig. 2.39). A consideration of the geometry shows that for a prefect fit the following relationship between the ionic radii holds [12]. 

For the many compounds having the perovskite structure the relationship will not hold exactly because of small variations in the sizes of the A and B ions. Therefore, to allow for this Eq. (2.121) is written 

Above its Curie point (approximately 130 °C) the unit cell is cubic with the ions arranged as in Fig. 2.39. Below the Curie point the structure is slightly distorted to the tetragonal form with a dipole moment along the c direction. Other transformations occur at temperatures close to 0°C and — 80 °C below 0 °C the unit cell is orthorhombic with the polar axis parallel to a face diagonal and below —80 °C it is rhombohedral with the polar axis along a body diagonal. 

The transformations are illustrated in Fig. 2.40(a), and the corresponding changes in the values of the lattice parameters, the spontaneous polarization and the relative permittivity are shown in Fig. 2.40(b-d). 

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