Ferroelectric/piezoelectric antiferroelectrics

Figure 11.6 Schematic relationship between dielectric solids (E is an applied electric field, and a is an applied stress), (a) Dielectric (i) E — 0, (ii) E is finite a dielectric, normally unpolarised, becomes polarised is an electric field), (b) Piezoelectric (i) cr = 0, (ii) a is finite (a piezoelectric, normally unpolarised, developes a polarisation when subjected to stress, even is no electric field), (c) Pyroelectric and ferroelectric E — 0, a — 0. (d) Anti-ferroelectric E = 0,

Detailed hst of the material properties for the piezoelectric crystals can be foimd in I andolt-Bomstein Tables and in other sources. Also the temperature coefficients for mai of these crystals were reported. Revised and up-dated Tables of material properties for piezoelectric rrraterials were published in recent volitmes of Landolt-Bomstein Tables. Special volumes are devoted to ferroelectric and antiferroelectric substances (Group 111, Volrrmes 3, 9,16,28 and 36). 

Concepts like piezoelectric, pyroelectric, ferroelectric, ferrielectric, antiferroelectric, paraelectric, electrostrictive, and several more, relate to distinct phenomena and are themselves interrelated. They are bound to appear in the description of liquid crystals and liquid crystal polymers, as they do in normal polymers and crystalline solids. Presently, great confusion is created by the uncritical use of these terms. For example, in the latest edition of the Encyclopedia Britannica [4] it is stated that pyroelectric-... 

Although the nematic phase is nonpolar, there are very interesting and important polar effects in this phase, in a sense analogous to piezoelectric effects in solid crystals. This was recognized by Meyer [18] in 1969. These so-called flexoelectric effects are discussed in Sec. 2.4 of this Chapter. Meyer also recognized [61] in 1974 that all chiral tilted smectics would be truly polar and the first example of this kind, the helielectric smectic C, was presented [62] in 1975. Out of Meyer s discovery grew the whole research area of ferroelectric and antiferroelectric liquid crystals, which is today a major part of liquid crystal physics and chemistry. 

The unit cells in the lattice of the lower-symmetry phases that are stable below the Curie temperature have ea permanent electrical dipole moment. In analogy with the magnetic phases, these are called ferroelectric if the moments are coupled in parallel and antiferroelectric if they are antiparallel. When the temperature is decreased to below the Curie temperature the symmetry becomes lower than cubic, the inversion center disappears, and the compound becomes piezoelectric. Figure 4.24 shows the change in properties at the phase transition of BaTi03. Further down... 

Ionic crystals are sometimes ferroelectric, antiferroelectric, or paraelectric. Ferroelectric crystals belong to one of the ten polar classes (1, 2, m, mm2, 3, 3m, 4, 4mm, 6, 6mm) and are therefore pyroelectric and piezoelectric. The spontaneous polarization of a ferro-... 

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