PbZrO

Simple ABO compounds in addition to BaTiO are cadmium titanate [12014-14-17, CdTiO lead titanate [12060-00-3] PbTiO potassium niobate [12030-85-2] KNbO sodium niobate [12034-09-2], NaNbO silver niobate [12309-96-5], AgNbO potassium iodate [7758-05-6], KIO bismuth ferrate [12010-42-3], BiFeO sodium tantalate, NaTaO and lead zirconate [12060-01 -4], PbZrO. The perovskite stmcture is also tolerant of a very wide range of multiple cation substitution on both A and B sites. Thus many more complex compounds have been found (16,17), eg, (K 2 i/2) 3 ... 

Lead zirconate [12060-01 -4] PbZrO, mol wt 346.41, has two colorless crystal stmctures a cubic perovskite form above 230°C (Curie point) and a pseudotetragonal or orthorhombic form below 230°C. It is insoluble in water and aqueous alkaUes, but soluble in strong mineral acids. Lead zirconate is usually prepared by heating together the oxides of lead and zirconium in the proper proportion. It readily forms soHd solutions with other compounds with the ABO stmcture, such as barium zirconate or lead titanate. Mixed lead titanate-zirconates have particularly high piezoelectric properties. They are used in high power acoustic-radiating transducers, hydrophones, and specialty instmments (146). 

In lead zh conate, PbZrOs, the larger lead ions are displaced alternately from the cube corner sites to produce an antifeiToelectric. This can readily be converted to a feiToelectric by dre substitution of Ti" + ions for some of the Zr + ions, the maximum value of permittivity occumirg at about the 50 50 mixture of PbZrOs and PbTiOs. The resulting PZT ceramics are used in a number of capacitance and electro-optic applicahons. The major problem in dre preparation of these solid soluhons is the volatility of PbO. This is overcome by... 

It is easy to determine the environment of Pb in PbZrOs accurately, since PbZrOs is a well ordered compound. But many useful ferroelectric oxides, for instance well-known ferroelectric oxides, Pb(Zr,Ti)03 (PZT), are mixed ion systems in which the crystal sites are occupied by two or more different ions. In such systems it is more difficult to assess the state of Pb off-centering with accuracy using conventional crystallographic analysis, such as the Rietveld refinement. Local Pb displacements are often non-collinear, displaced in different directions from site to site. These local displacements will be observed indirectly only as artificially large thermal factors, and the average displace-... 

In PbZrOs, which also has a perovskite structure, the offset atoms are arranged alternately in opposite directions. This produces an antiferroelectric state. PbZrOs with some zirconium replaced by titanium gives the widely used ferroelectric materials, PZT (PbZr, Ji/93). 

Figure 1.28 Current response of undoped and 1% Nb-doped PbZro.3Tio.7O3 after different voltage steps.

Fig. 12. Strain introduced into PZT crystallites of PbZro.5Tio.5O3 by cavitational processing in the double-orifice processor (CaviPro 300) as a function of different sets of orifices, shown on the left side of the figure. Similar data using the single orifice processor (CaviMax) are shown on the right side of the figure.

After a mixture of oxides is produced by the thermal decomposition of the freeze dried and calcined powder, fiorther solid-solid reactions can take place. Solid—solid reactions operate by the mechanisms discussed in Chapter 5, which include solid state diffusion and chemical reaction. Diffusion in ceramic solids is always ionic in nature and depends on defect or hole diffusivity, as well as electron conductivity. Once the ionic reactants are in close association, chemical reactions can take place, giving mixed oxides like BaTiOs and PbZrOs. 

Materials of particular interest are the perovskite oxides BaTiOs-SrTiOs (BST) and PbZrOs-PbTiOs (PZT) solid solutions as well as the layered perovskites based upon SrBi2Ta209 (SBT). Since the ferroelectric effect requires... 

Figure 937. A. Static and MAS NMR spectra of PbTiOs, B. Static and MAS "Pb NMR spectra of PbZrOs. From Zhao et al. (1999), by permission of the American Chemical Society.

Reaction of the PbO solid solution with the remaining PbTiOs and Zr02 to form Pb (Zr, Ti)03 solid solution with wide range of Zr/Ti ratios. It has been suggested that a PbZrOs-rich solid solution is initially formed, along with a PbTiOs-rich solid solution. ... 

In the case of coprecipitated precursors, or if the Zr02 is of ultrafine particle size, the reaction sequence changes. In the latter case, formation of PbZrOs is observed. ... 

Figure 14. Phase diagram for the PZT (PbTiOs -PbZrOs) system. Tetragonal and rhombohedral phase fields are separated by a nearly vertical morphotropic phase boundary (MPB). Adapted from Figure 1 in Oh and Jang (1999).

In some perovskite ceramics, the instability that occurs at the Curie temperature is not ferroelectric but rather antiferroelectric. In antiferroelectric crystals, the neighboring lines of ions are displaced in opposite senses which creates two alternating dipole sublattices of equivalent but opposite polarization. Consequently, the net polarization is zero, and the dielectric constant does change at the transition temperature. Examples of antiferroelectric crystals are WO3, NaNbO, PbZrO, and PbHfOj. 

Pronounced off-center distortions of a rather different kind occur in tetragonal PbTi03 (124) and orthorhombic PbZrOs (70). Both crystals have deformed perovskite structures. In the ideal cubic structure (Fig. 23) each Pb++ ion would be in cubic closest packing with 12 oxygen neigh-... 

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