Ionic motion in a double-well local potential can be cast theoretically in the language of cooperative magnetism. In this modern approach, the potential minima referred to are with respect to a generalized displacement co-ordinate describing the motion of all ions in a unit cell. The more traditional approach for potassium dihydrogen phosphate (KH2PO4 or KDP) and its isomorphs has been to concentrate on the specific motion of the hydrogens (which is of largest amplitude) and to include the motion of the other ions, if at all, in terms of a specific coupling mechanism. Thus, for example, any tunnelling characteristics in the more generalized scheme involve all the atoms, whereas the tunnelling character in the more traditional approach is specifically reserved for the hydrogens. This chapter deals with order-disorder ferroelectrics, theory and experiment regarding KDP-type ferroelectrics, other KDP ferroelectric isomorphs, (NH)H2PO as a KDP-type antiferroelectric, triglycine sulphate and sodium nitrite.
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