Barium titanate domains

Barium titanate [12047-27-7] has five crystaUine modifications. Of these, the tetragonal form is the most important. The stmcture is based on corner-linked oxygen octahedra, within which are located the Ti" " ions. These can be moved from their central positions either spontaneously or in an apphed electric field. Each TiO octahedron may then be regarded as an electric dipole. If dipoles within a local region, ie, a domain, are oriented parallel to one another and the orientation of all the dipoles within a domain can be changed by the appHcation of an electric field, the material is said to be ferroelectric. At ca 130°C, the Curie temperature, the barium titanate stmcture changes to cubic. The dipoles now behave independentiy, and the material is paraelectric (see Ferroelectrics). 

A domain can be of the order of m or even more. Several methods can be used to obtain pictures of the domains. Figure 9.18 is a photograph taken of a thin slice of barium titanate under a polarising microscope in which different domains can clearly be seen. Note the sharpness of the domain boundaries. 

FIGURE 9.17 Sketch of domains in barium titanate, illustrating 90° and 180° boundaries. 

FIGURE 9.18 A photograph of a thin slice of barium titanate taken under the polarizing microscope, picturing domains of different polarisation. (From A.Guinier and R.Julien (1989) The Solid State from Superconductors to Superalloys, Oxford University Press/International Union of Crystallography, Oxford, Figure 2.9, p. 67. Reproduced by permission of Oxford University Press.)... 

Fig. 2.45 (a) Polished and etched surface of unpoled ceramic (b) schematic diagram of 180° and 90° domains in barium titanate. 

Since the polar axes in barium titanate and PZT (see Fig. 2.40(b) and Fig. 2.44) are longer than the perpendicular axes, ceramics expand in the polar direction during poling. The application of a high compressive stress in the polar direction to a poled ceramic causes depoling since the 90° domains switch direction as a result of the ferroelastic effect and the polar directions of the crystallites become randomized. 

Barium titanate is cubic with a perovskite structure. However, at room temperature (actually below the Curie temperature of 120°C) it is tetragonal with a spontaneous electric polarization in the direction of the c-axis (only the higher temperatures form is shown in Figure 7.2). In this ferroelectric condition a crystal of BaTiOs has a domain structure. 

Berlincourt, D. and Kmeger, H.H.A. (1959) Domain processes in lead titanate zirconate and barium titanate ceramics. /. Appl. Phys., 30 (11), 1804-1810. 

Little, E.A. (1955) Dynamic behavior of domain walls in barium titanate. Phys. Rev., 98 (4), 978-984. 

Domain. In a ferroelectric or ferromagnetic crystal, e.g. barium titanate, a domain is a small area within which the polarization is uniform. If the crystal is exposed to a high electric or magnetic field, those domains in which the polarization is in a favourable direction will grow at the expense of other domains. A domain structure gives rise to hysteresis (q.v.). 

Figure 8.5 Hysteresis loop for a single-domain single crystal of barium titanate (Moulson and Herbert, 2003). Reprinted with permission from Wiley-Blackwell Ltd, Oxford, UK.

Ferroelectric domains have also been observed by electron microscopy, in barium titanate as the domain forms, the lattice constant changes, giving a contrast difference between domains. Moreover, the electron beam can charge the surface and switch domains, making switching processes apparent. 

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