Relaxor phases

Figure 12. Phase diagram for the PBZT system at room temperature. Shaded area represents the relaxor phase region. Modified from Figure 1 in Li and Haerthng (1995).

Figures 1 and 2 respectively show the temperature dependence of the relative permittivity and loss tangent of relaxor ferroelectric PLZT (9.5/65/35). As the temperature increases from -60°C to 100°C, the relative permittivity generally increased due to the unfreezing of domains. Between 0°C and 10°C, a broad peak can be seen in the lower frequency curves. This peak corresponds to the diffuse phase transition in this relaxor ceramic from the ferroelectric to the paraelectric state (also called the relaxor phase). Further heating continued to increase the relative dielectric permittivity until a maximum was achieved, at which point, the crystal s structure became cubic. This maximum in the permittivity, which is frequency dependent, occurs at the Curie temperature. Evidence of these phase transitions can also be seen in the loss tangent graph in figure 2.

The increased strain with increasing dc bias in figure 7 can be explained by previous dielectric measurements of PLZT (9.0/65/35) ceramics as a function of both tempierature and dc bias (Bobnar et al. 1999) They have observed a sharp increase in the dielectric permittivity with increasing dc bias fields at temperatures dose to the ferroelectric-relaxor phase transition, indicating that the dc bias is inducing the creation of electric dipoles at this transition, and hence increasing the overall piezoelectric response. 

The various K33 coefficients were calculated using equation (14) and reported in table 3. It was found that K33 seems to decrease with increasing temperature for both PLZT (9.5/65/35) and (9/65/35). This is due to the ferroelectric to relaxor phase change occurring around the tested temperature range. 

Classical relaxors [22,23] are perovskite soUd solutions like PbMgi/3Nb2/303 (PMN), which exhibit both site and charge disorder resulting in random fields in addition to random bonds. In contrast to dipolar glasses where the elementary dipole moments exist on the atomic scale, the relaxor state is characterized by the presence of polar clusters of nanometric size. The dynamical properties of relaxor ferroelectrics are determined by the presence of these polar nanoclusters [24]. PMN remains cubic to the lowest temperatures measured. One expects that the disorder -type dynamics found in the cubic phase of BaTiOs, characterized by two timescales, is somehow translated into the... 

The present results demonstrate that the basic difference between relaxors and dipolar glasses is their response to applied electric fields polar nanoclusters, corresponding to the frozen anisotropic component in the NMR spectra, can be oriented in a strong enough applied electric field and a ferroelectric phase can be induced. This is not the case in dipolar glasses, where the response is due to single dipoles which cannot be ordered by applied electric fields. 

In addition, many of the ferroelectric solids are mixed ions systems, or alloys, for which local disorder influences the properties. The effect of disorder is most pronounced in the relaxor ferroelectrics, which show glassy ferroelectric behavior with diffuse phase transition [1]. In this chapter we focus on the effect of local disorder on the ferroelectric solids including the relaxor ferroelectrics. As the means of studying the local structure and dynamics we rely mainly on neutron scattering methods coupled with the real-space pair-density function (PDF) analysis. 

Relaxor materials as Pb(Mgi/3Nb2/3)03 (pmn), Pb(Zni/3Nb2/3)03 (PZN), and (Pbo.92 La0.o8)(Zro.7Ti0.3)03 (plzt) are a subgroup of ferroelectrics with diffuse phase transistions. Characteristic behavior of this class are the strong dielectric dispersion related with high dielectric losses, see Figure 1.18. 

Figure 1.18 (a) Normalized polarization for first-order, second-order and diffuse phase transition in ferroelectric and relaxor materials and (b) dielectric behavior of relaxor-type... 

This article shows up the possibilities to study ferroelectric media by photo-induced light scattering. In particular the optical characterization of sbn by photo-induced light scattering is reviewed including the determination of optical parameters [7], the investigations of the relaxor-kind phase transition [8] and the analysis of the polar structure [9,10],... 

These properties demonstrate that the study of photo-induced light scattering can be used to investigate the relaxor-kind phase transition by an optical method. Hereby, various measures can be used, like the temperature dependence of the intensity of the directly transmitted laser beam, of the integrated scattered light and of the intensity ratio R, respectively, as shown in Figure 9.9. 

These results clearly demonstrate that the relaxor-kind phase transition determines the appearance of photo-induced light scattering, and therefore its temperature dependence can be used as a method for optical investigation of phase transitions. E.g., the transition temperature can be determined by the asymmetry parameter as a measure for the symmetry of the scattering pattern or by the comparably simple measurement of the directly transmitted laser beam. 

However, the temperature, at which the maximum of the initial scattered light occurs, seems to be related to the scattering angle 9S and thus to the period Ag , respectively. Figure 9.14(b) shows the correspondence between the temperature Tm of maximum intensity Ig and the spatial period Agn. A spatial disorder of the smallest polar structures occurs at Tm = 45 °C, while the spatial orientation of the largest structures remains stable up to Tm = 60 °C. Such big dispersion of the thermal decay of polar structures over Agn unambiguously illustrates the relaxor behavior of sbn. At the same time it is a key point to understand the bandwidth in the determination of the phase transition temperature Tm in sbn from different methods. For example, in sbn doped with 0.66 mol% Cerium, Tm detected from the maximum of the dielectric permittivity e at 100 Hz (e-method) equals Tm = 67 °C [20], Determination of Tm from the inflection point of the spontaneous electric polarization P3... 

In conclusion, detailed information on changes in the polar structure of sisniCc after application of external electric fields as well as during the ferroelectric-paraelectric phase transition is received from the study of the initially scattered light. Strong hints are revealed that the relaxor-kind phase transition in sbn is due to a dispersion of the thermal decay for different spatial scales in the polar structure. 

Fig. 6.14 Room temperature ternary showing structural phase-fields in the PZ-PT-relaxor system.

Tinte et al.54 have carried out molecular dynamic simulations of first-principles based effective Hamiltonian for PSN under pressure and of PMN at ambient pressure that clearly exhibit a relaxor state in the paraelectric phase. Analysis of the short-to-medium range polar order allows them to locate Burns temperature Tb. Burns temperature is identified as the temperature below which dynamic nanoscale polar clusters form. Below TB, the relaxor state characterized by enhanced short-to-medium range polar order (PNR) pinned to nanoscale chemically ordered regions. The calculated temperature-pressure phase diagram of PSN demonstrates that the stability of the relaxor state depends on a delicate balance between the energetics that stabilize normal ferroelectricity and the average strength of quenched "random" local fields. 

Fig. 7.9 shows the temperature dependence of the dielectric constant and dielectric loss at 1 kHz for the PMN-PT ceramics obtained by sintering the calcined powders from a soft-mechanochemical route at 1200°C for 2 h. A diffuse phase transition, being typical for a relaxor, is observed for each ceramics. As x increases from 0 to 0.2, the maximum dielectric constant, K, , increases from 13000 to 27000. The temperature correspondent to K ,... 

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. 

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