Ferroelectricity due to medium-sized transition-metal cations

7 Ferroelectricity due to medium-sized transition-metal cations 

Ferroelectric properties can be attributed to the presence of medium-sized cations in many oxides with structures related to that of perovskite, for example BaTiOa and KNbOa. These contain ions 

There is no preference as to which of the original cubic axes becomes the polar direction, and so this can take one of six equivalent directions, parallel to x, y or z. On cooling, a ferroelectric domain pattern forms, reflecting that the transformation takes place along any of the allowed cubic directions in various parts of the phase. 

A ferroelectric crystal does not normally show any observable polarisation, because the domain structure leads to overall cancellation of the effect. PolycrystaUine ceramics would be expected to be similar. In order to form a material with an observable polarisation the crystals are poled. This process involves heating the crystals above the Curie point, Tc, and then cooling them in a strong electric field. The effect of this is to favourably orient dipoles so that the crystal or polycrystaUine ceramic shows a strong ferroelectric effect The majority of ferroelectric materials used are, in fact, polycrystal-Une. 

The same is true of polymer piezoelectrics. In these materials, the crystalhtes that give rise to piezoelectricity are oriented at random within the polymer matrix. Poling can give the dipoles an overall preferred orientation. Naturally, poling will not affect a crystallite that does not show a permanent dipole, and so poling applies only to pyroelectric and ferroelectric materials. 

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I I.J.7 Ferroelectricity due to medium-sized transition-metal cations... 

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