Pressure as a Probe of the Glassy State of Ferroelectrics with Random Site Disorder
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Mat. Res. Soc. Symp. Proc. Vol. 499 ©1998 Materials Research Society
For the present study we selected PLZT compositions near the ferroelectric-relaxor boundary. The substitution of La 3÷ for Pb2+ on the A sites in lead zirconate-titanate (PZT) is known to lead to a well-known family of ceramics having the chemical formula (Pbl-3x/2Lax) (Zr1-yTiy)O 3 , or PLZT, with unusual dielectric and electro-optic properties. 9 The La 3, ions and accompanying vacancies (one vacancy for every two La ions) which are randomly distributed on the A sites represent a type of disorder which significantly modifies the properties of these materials. One manifestation of this disorder is the condensation of local dipolar nanodomains leading to local, randomly oriented polarization at a temperature much higher than the transition temperature of the sample. 10 These polar nanodomains increase in size with decreasing temperature, and for relatively low La concentrations the disorder is not sufficient to prevent the formation of a macroscopic ferroelectric state. The peak in the dielectric constant at Tc is relatively sharp and independent of frequency below the GHz range. For higher La concentrations, on the other hand, the increased fluctuations in La content and disorder broaden the transition and induce a relaxor state. For the particular system La-modified PbZr.6 5 Ti.35 0 3 (PZT 65/35), relaxor behavior becomes experimentally discernible 7 for La concentrations above -6-7 at %. Thus, for the present study we chose compositions with 6 and 8 at % La, or PLZT 6/65/35 and PLZT 8/65/35. We also chose a composition x = 12 at % La in PLZT x/40/60, where it is known 6 that relaxor behavior sets in at about this composition. EXPERIMENTAL DETAILS The samples were prepared by conventional mixed oxide synthesis followed by hotpress annealing. 9 They were thin plates whose large faces were sputter coated by Cr followed by Au. Information about the phase transitions and relaxational properties were obtained from dielectric constant (E') and dissipation (tan 8) measurements. These measurements were performed without a dc biasing field as functions of frequency (102 - 106 Hz), temperature (290600K) and hydrostatic pressure (0-20 kbar). A 50/50 mixture of normal- and iso-pentanes served as the pressure transmitting fluid. RESULTS AND DISCUSSION Space limitations do not allow us to discuss all of the results on the three PLZT compositions. Thus, we restrict the presentation to a summary of the highlights of the work with the objective of emphasizing the generality of the pressure-induced effects which get to the heart of the physics. We begin with PLZT 6/65/35 which is very close to the nFE-R boundary for PLZT x/65/35. In the absence of relaxor behavior, the expected response at 1 bar is for the material to transform on cooling from the high temperature cubic PE phase to a rhombohedral nFE phase at -440 K, and the dielectric response should be essentially independent of frequency below -1 GHz. Figure 1 shows the temperature dependence of E' at 1 bar.
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