Ferroelectric/piezoelectric perovskites

A wide array of ferroelectric, piezoelectric and pyroelectric materials have titanium, zirconium and zinc metal cations as part of their elemental composition Many electrical materials based on titanium oxide (titanates) and zirconium oxide (zirconates) are known to have structures based on perovskite-type oxide lattices Barium titanate, BaTiOs and a diverse compositional range of PZT materials (lead zirconate titanates, Pb Zr Tij-yOs) and PLZT materials (lead lanthanum zirconate titanates, PbxLai-xZryTii-yOs) are among these perovskite-type electrical materials. 

Table 27.5 lists applications of some of the most commercially important mixed metal, perovskite-t5q)e oxides, and illustrates that it is the dielectric, ferroelectric, piezoelectric (see Section 13.9) and pyroelectric properties of these materials that are exploited in the electronics industry. 

Until the late sixties the only known ferroelectrics, piezoelectrics, and pyroelectrics were certain inorganic monocrystals, or polycrystalline ceramics like lead titanate zirconate perovskites. Other known materials with macroscopic polarization were electrets, (for example mixmres of beeswax and rosin) in which the polarization was produced by application of the electric field in the melted state and then by cooling and the solidification of the polarized material. 

Perovskite compounds have the general formula ABX3 for the purposes of this study, we are concerned with ATiOa derivatives. Perovskites may be ferroelectric, piezoelectric, etc., and due to their low cost, are currently being re-explored for a variety of purposes, such as in solar cells [124] and catalysts [125,126]. In many cases, the structure—property relationships in the ATi03 compound are unclear. The use of SSNMR for... 

It is far beyond the scope of this chapter to review the electronic structures and properties of all metal oxides, or even all of the important metal oxide stmc-ture types. Instead, this section covers some featnres of one stmctural family, perovskite, in some detail. In doing so, it is hoped that the important concepts will be illnstrated in snch a way that they can be widely appUed. Of course, the choice of the perovskite stmctnre as an illnstrative example is not a random choice. The perovskite family of componnds is very extensive, encompassing most of the periodic table. Fnrthermore, perovskites exhibit nearly every type of interesting electronic or magnetic behavior seen in oxides (ferromagnetism, ferroelectricity, piezoelectricity, nonlinear optical behavior, metaUic condnctivity, snpercondnct-ivity, colossal magnetoresistance, ionic conductivity, photoluminescence, etc.). One important property that is not readily found among perovskites, transparent conductivity, is the focus of Section 6.7. 

Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

Most niobates and tantalates, however, are insoluble and may be regarded as mixed oxides in which the Nb or Ta is octahedrally coordinated and with no discrete anion present. Thus KMO3, known inaccurately (since they have no discrete MO3 anions) as metaniobates and metatantalates, have the perovskite (p. 963) stmcture. Several of these perovskites have been characterized and some have ferroelectric and piezoelectric properties (p. 57). Because of these properties, LiNb03 and LiTa03 have been found to be attractive alternatives to quartz as frequency filters in communications devices. 

An alternative structure that has also been widely investigated both for high temperature piezoelectric, as well as for ferroelectric memory applications is the bismuth layer structure family as shown in Figure 1.14 for SrBi2Ta209 (sbt), e.g. [8], The structure consists of perovskite layers of different thicknesses, separated by Bi20 + layers. It has been shown that when the perovskite block is an even number of octahedra thick, the symmetry imposes a restriction on the polarization direction, confining it to the a-b plane. In contrast, when the perovskite block is an odd number of octahedra thick, it is possible to develop a component of the polarization along the c axis (nearly perpendicular to the layers). This could be used in... 

Figure 2.1 Schematic illustrations of intrinsic and extrinsic contributions to the piezoelectric constant of perovskite ferroelectrics. (a) and (b) correspond to the intrinsic unit cell shape (a) without and (b) with applied electric field, (c) and (d) correspond to the extrinsic response associated with the change in position of a non-180° domain wall (shown as a black line) (c) before and (d) after an electric field is applied. Note that both intrinsic and extrinsic responses lead to a change in shape of the material due to application of an electric field (and hence to a piezoelectric response). In both cases, the actual distortions are significantly exaggerated to make visualization easier.

The first piezoceramic to be developed commercially was BaTi03, the model ferroelectric discussed earlier (see Section 2.7.3). By the 1950s the solid solution system Pb(Ti,Zr)03 (PZT), which also has the perovskite structure, was found to be ferroelectric and PZT compositions are now the most widely exploited of all piezoelectric ceramics. The following outline description of their properties and fabrication introduces important ideas for the following discussion of the tailoring of piezoceramics, including PZT, for specific applications. It is assumed that the reader has studied Sections 2.3 and 2.7.3. 

Ferroelectric ceramics and single crystals have found wide applications in many electronic, acoustoptic and piezoelectric devices [1,2], Perovskites represent one of the most important classes of inorganic powders that are of great interest in functional ceramics used for electronic components among them BaTiOj is a typical and most frequently used representative. 

An important group of piezoelectric ceramics are solid solutions of PbZrOj and PbTiOs represented as Pb(Zr, Ti)03 (and commonly referred to as PZT). At high temperature these compounds have the cubic perovskite structure (Fig. 1). In the ferroelectric phase, which is stable at room temperature, the lattice is distorted, and the asymmetry of the positive and negative ions results in a net dipole moment. Spontaneous polarization is the dipole moment per unit volume. 

The M M03 compounds crystallize with perovskite structures Figure 5.23), and exhibit ferroelectric and piezoelectric properties (see Section 13.9) which lead to uses in electrooptical and acoustic devices. 

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