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p* = Z*(S/d) = p'-j p" and e*=1/ M* = (j co Co Z*)(1) where S is the electrode surface area, d is the thickness of the sample, co is the angular frequency (w = 21rf) and Co is the vacuum capacitance. When both measurements (using silver or graphite electrodes) are very similar, we show the data without indicating the electrode type. 195

Mat. Res. Soc. Symp. Proc. Vol. 500 0 1998 Materials Research Society

RESULTS AND DISCUSSION The temperature dependence of the permittivity s' is shown in Fig. 1. A strong dielectric anomaly appears at 600'C, the value of the maximum of E' is about four orders of magnitude higher than the room-temperature value. It indicates a ferroelectric Curie temperature T, equal to 600'C. It is consistent with the composition Lio.9 s2Ta1 .0040 3 [3-5]. The permittivity &'present s a strong low-frequency dispersion, suggesting the existence of significant ionic conduction in this crystal. 1.2E+5

8.OE+4

4.OE+4

0.OE+0 500

550

600

650

T(°C) Fig. 1 Variation of the permittivity s' with the temperature at different frequencies. The resistivity plane plots and complex modulus plane plots are shown in Fig. 2 for three temperatures around the T,. In the resistivity plane plot only one semicircle is apparent for each temperature. But the complex modulus plots show two semicircles clearly at each temperature. These results are also confirmed in the frequency explicit plots (Fig. 3). At each temperature, the resistivity vs. frequency plots show only one peak, the position of the peak changes with the temperature. In contrast to this, two peaks can be seen in the modulus vs. frequency plots. The position and the magnitude of modulus change significantly with the ferroelectric phase transition. The modulus plots clearly suggest that two polarization processes exist in this single crystal sample.

196

2.OE+3

1.5E+3

1.OE+3

5.OE+2

O.OE+O O.OE+O

5.OE+2

1.OE+3

1.5E+3 p'(

2

2.OE+3

2.5E+3

3.OE+3

m)

2.OE-4

1.5E-4

1.OE-4

600'C

;610c

5.OE-5

O.OE+O O.OE+O

5.OE-5

1.OE-4

1.5E-4

2.OE-4

2.5E-4

3.OE-4

3.5E-4

Fig. 2 Resistivity plane plots (up) and Complex modulus plots (down) for three temperatures. Since long-range Li' ion diffusion occurs in these ferroelectric crystals, two polarization mechanisms are possible, both polarizations are related to Li+ ion motions through an octahedron face: 1) the dielectric relaxation (or local response) due to the short-range lithium displacements along the polar axis across an oxygen triangle common to two octahedra. These backward and forward motions are partially responsible for the inversion of the ferroelectric spontaneous polarization. 2) the conductivity relaxation (or charge carrier response) due to Li+ ionic conductivity. Such a long-range effect is intensified by the lithium vacancies in the LiTaO3 -type crystal network. 197

1.2E+3

8.OE+2

C:

4.OE+2

O.OE+0 1.0E+I

1.OE+2

1.OE+3

1.OE+4

1.OE+5

1.OE+6

1.OE+4

I.OE+5

L.OE+6

f (Hz) 1.2E-4

8.OE-5 M11

4.OE-5

0.OE+0 I.OE+I

LOE+2

1.OE+3

f(4z) Fig.3 Frequency explicit plots of p" (up