Ferroelectricity and crystal structure. II

Ferroelectrics are crystals with pseudosyrmnetric structures of polar symmetry in which the direction of polarity can be reversed by an applied field. They are found when the directed bond system of certain atoms changes with temperature while the ionic character of the structure is still important. If only one kind of atom suffers such a change, the resulting displacements are likely to have non-polar symmetry; if two change together, there is a good chance that the displacements of each kind of atom will all be parallel, and the substance therefore ferroelectric. These ideas are applied to the perovskite family, where the importance of the homopolar character of the O atom is emphasized; to KH~PO a, where the significance of the K atom is deduced from the geometry of the structure, and the contrast with ND4D~PO4 is discussed; to Rochelle salt, where attention is again directed to the K atom, and the chemical implications of theories of ordering in the hydrogen bond are noted; and o Li-Nb03, where the room-temperature structure found by Bailey is shown to be that of a 'frozen ferroelectric' derived from the ideal perovskite structure.