Morphotropic boundary

However, a giant piezoelectric effect has been found in the Pb-based complex pervoskite oxides. In particular, the morphotropic boundary relaxor and PbTio3 complex exhibits huge piezoelectric response, so that an effective piezolysis is expected. 

The replacement of zirconium by isovalent hafnium has little effect apart from shifting the morphotropic boundary (see Fig. 6.7) to 52mol.% PbTi03. Replacement by tin causes a slight loss in piezoelectric activity, shifts the morphotropic boundary to 42 mol. % PbTi03 and lowers the Curie point from 370 to 250 °C. 

The substitution of lead by barium enhances the piezoelectric properties which peak when Pb/Ba I. At this and higher barium contents the structure changes to tetragonal with the polar axis parallel to the A-site tunnels. There is a morphotropic boundary, similar to that found in PZT compositions, and peak values of piezoelectric properties are found near the Pb1/2Ba1/2Nb206 composition. d33 rises to 220 pCN-1 and d31 rises to — 90pCN 1, while Qm increases to 300 and the Curie point falls to 250 °C thus most of the features peculiar to... 

Lead zirconate titanate (PhTij Zr e03 with x 0.5), also abbreviated as PZT, is a ferroelectric mixed perovskite with a Curie temperature that depends on the composition x (Figure 4.28). It also has a very high value of k on the so-called morphotropic boundary (the value of x at the boundary between the tetragonal and rhombohedral structures). It is a material of choice for making sensors and actuators, e.g., in medical echoscopes and sonar equipment. 

Figure 4.28. Phase diagram of the mixture of lead zirconate and lead titanate (PZT). Poling is particularly easy at the morphotropic boundary of the mixed oxide.

Doping PZT with lanthanum ions (La replaces Pb ) shifts the morphotropic boundary to higher x-values and lowers the Curie temperature of the oxidic alloy. At 5% La, the Curie temperature can even drop below room temperature if X is high enough. The solid has become cubic and paraelectric and no longer has a permanent electric dipole moment at room temperature but is very polarizable. This material is called PLZT (short for lead lanthanum zirconate titanate) and is used for its nonlinear optical properties, e.g., in doubling the frequency of laser radiation. 

Fig. 9.4. Dependence of piezoelectric properties of PbZrOj-PbTiOj on composition. The zirconate-rich phase is rhombohedral, whereas the titanate-rich phase is tetrahedral. The piezoelectric coefficients reach a maximum near the morphotropic phase boundary, approximately 45% PbZrOj and 55% PbTiOj. (After Jaffe et al., 1954.)...

Figure 1.13 Phase diagram for PZT showing the morphotropic phase boundary between rhom-bohedral and tetragonal phases.

Analogous C(V) curves were recorded on pzt bulk ceramics with compositions around the morphotropic phase boundary (mpb). Figure 1.25 displays the relative permittivity as a function of DC-bias for a tetragonal (x = 0.48), a morphotropic (x = 0.52) and a rhombohedral (.x = 0.58) sample. In contrast to thin films additional humps observed in the e E) curves. This could be explained by different coercive fields for 180° and non-180° domains [31]. Their absence in ferroelectric thin films could be taken as evidence for suppressed non-180° domain switching in thin films [30],... 

Figure 2.4 Strain-field curves for < 001 > oriented 0.91PbZn1/3Nb2/303-0.09PbTi03 single crystals. The sample in (a) was poled at room temperature, where the resulting domain state is unstable (due to induction of tetragonal material associated with the curved morphotropic phase boundary), yielding substantial hysteresis. In (b) the crystal was poled at low temperatures to keep it in the rhombohedral phase. When measured at room temperature, the piezoelectric response is much more linear and non-hysteretic, due to the improved stability of the ferroelectric domain state. Data courtesy of S. E. Park.

The system now shows two morphotropic phase boundaries, the one already discussed on the PZ-PT join, and the other on the PT-Pb(B B")03 join. Compositions close to that of the MPB (II) have attracted considerable interest because of their potential for electromechanical applications. The properties of some selected systems are given in Table 6.2 together with those for the PZT system for comparison. 

Eitel, R.E. et al. (2001) New high temperature morphotropic phase boundary piezoelectrics based on Bi(Me)03-PbTi03 ceramics, Jpn. J. Appl. Phys., 40, 5999-6002. 

Zhang H, Leppavuori S, Karjalainen P (1995) Raman spectra in laser ablated lead zirconate titanate thin films near the morphotropic phase boundary. J Appl Phys 77 2691 Ching-Prado E, Cordero J, Katiyar RS, Bhalla AS (1996) Temperature-dependent Raman scattering in PT and PMN-PT thin films. J Vac Sci Technol A 14 762... 

As a consequence, the joins for (Pbi. (Bajc)Ti03 at low temperature and for Pb(Zri cTy03 at room temperature are interrupted by a morphotropic phase boundary (MPB), which separates tetragonal and rhombohedral phases (Fig. 14). The structural state of the oxides in the vicinity of the MPB is a subject of active inquiry, because many of the physical properties of PBZT ferroelectrics are maximized at the MPB. These include the dielectric constant, the piezoelectric constant, and the electromechanical coupling coefficients (Jaffe 1971, Thomann and Wersing 1982, Heywang and Thomann 1984). For industrial purposes, this behavior is exploited by annealing PBZT ferroelectrics with compositions near the MPB close to the Curie temperature in an... 

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).

Carl K, Hardtl KH (1971) On the origin of the maximum in the electromechaiucal activity in Pb(Zr,tTii.J03 ceramics near the morphotropic phase boundary. Phys Stat Sol A8 87-98 Carpenter MA (1980) Mechaiusms of ordering and exsolution in omphacites. Contrib Mineral Petrol 71 289-300... 

Mishra SK, Pandey D (1997) Thermodynamic nature of phase transitions in Pb(Zr Tii.,c)03 ceramics near the morphotropic phase boundary. 11. Dielectric and piezoelectric studies. Phil Mag B 76 227-240 Mishra SK, Singh AP, Pandey D (1997) Thermodynamic nature of phase transitions in Pb(Zr Tii.,c)03 ceramics near the morphotropic phase boundary. I. Structural Studies. Phil Mag B 76 213-226 Mitscherlich E (1819) Liber die Kristallisation der Salze, in denen das Metall der Basis mit zwei Proportionen Sauerstof verbunden ist. Abh Koniglichen Akad Wiss Berlin 5 427-437 Mitscherlich E (1822) Sur la relation qui existe entre la forme cristalline et les proportions chimiques. 1. 

Figure 6.23 The PbZrOfPbTiO phase diagram showing the morphotropic phase boundary (MPB) separating the rhombohedral and tetragonal phases. Region A is antiferroelectric orthorhombic space group Pbam.

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