Morphotropic composition

The isovalent A-site substituents barium, strontium and calcium lower the Curie point and have a small influence on the morphotropic composition. At the 5-10 mol.% level they enhance the permittivity and piezoelectric properties. Pb0 94Sr0 06Ti0 47Zr0 53 for instance has a relative permittivity of 1300, a kp of 0.58... 

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.)...

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],... 

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. 

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... 

The compositional perovskite series that has served as the basis for much of this research is the so-called PBZT system. The quadrilateral that joins the endmembers PbZr03-BaZr03-BaTi03-PbTi03 exhibits virtually complete solid solution (Fig. 12). Nevertheless, this system displays a number of morphotropic phase transitions that involve transformations of several kinds ferroelastic, ferroelectric, antiferroelectric, and relaxor. Understanding the nature of these isothermal transitions requires a review of the thermal distortions that occur as these perovskites are cooled. 

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... 

The similarity in behaviors of thermally and compositionally induced transitions suggests that the dependence of spontaneous strain on Li+Al content may parallel the dependence of strain on temperature. Xu et al. (2000) determine elastic strains (ei =02 = aloQ - 1 3 = c/co - 1) and volume strains (Vs = V/Fo - 1) by referencing the paraelastic cell dimensions to the P-quartz-like phases within the Lii fAli. fSii+ (04 series. They find that the data are consistent with the relation ei (or or Vg) = A (X-Xff, and with ei oc 3 oc Fs oc the morphotropic transition appears to be tricritical. This transition character conforms closely to that observed by Carpenter et al. (1998), who argued that the thermally induced a-P quartz transition is first-order but very close to tricritical. In... 

SiOj fact, additional studies are needed near the critical composition in Lii.jcAli.jcSii+ fOq to determine the precise character of this morphotropic transition. 

Finally, it is clear that many more solid solutions must be examined for minerals and mineral analogs in order to achieve a fundamental understanding of compositionally induced phase transitions. Currently it is not possible to predict how transition temperatures will change when a particular impurity substitutes in a mineral structure, nor can we predict the interaction length for that impurity in the mineral. Landau-Ginzburg analysis provides an ideal framework for comparing the character of phase transitions that are activated by different variables (temperature, pressure, composition), and future studies of this type will lay an empirical foundation from which the detailed character of morphotropic transitions in minerals may be inferred. 

E. Parthe, R.S. Roth, R.S. Berry, R. Metselaar and S.C. Abrahams, Acta Crystallogr., Sect. A, 54, 1028-1033 (1998). Nomenclature of magnetic, incommensurate, composition-changed morphotropic, polytype, transient-structural and quasicrystalline phases undergoing phase transitions, J.-C. Toledano, R.S. Berry, P.J. Brown, A.M. Glazer, R. Metselaar, D. Pandey, J.M. Perez-Mato, R.S. Roth and S.C. Abrahams, Acta Crystallogr., Sect A, 57, 614-626 (2001), and erratum in Acta Crystallogr., Sect. A, 58, 79 (2002). 

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 4.1.3 Dependence of several d constants on composition near the morphotropic phase... 

Pure PZN (as well as PMN) has a trigonal ferroelectric structirre (3/w), pure PT a tetragonal ferroelectric structure (4 i/w) at room temperature. Both components imdergo the phase transition to the paraelectric cubic (m3m) phase at Cirrie temperature. It depends on the chemical composition and varies typically from 150 to 250°C for PZN-PT (for 0-20%PT content) and 0 to 250°C for PMN-PT (0-50% PT content). Both systems PZN-PT and PMN-PT have the morphotropic phase boundary (MPB) between rhombohedral and tetragonal ferroelectric phases - at 8-10%PT in PZN-PT and at 33-35%PT in PMN-PT for the temperature range interesting for technical applications. For the phase diagrams of PZN-PT and PMN-PT see Fig. 7.16. 

The most favourable concentration for the formation of intermetallic compoimds is 33at.% of R (RM2-RGe2), particularly the composition RMGe. The compounds of equatomic composition crystallize in 20 structure types. The morphotropic transformation within the RMGe series occurs both if R is replaced by another R, and when the M component is changed. 

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