with , J random bond variance, random fields variance, Jo random bonds coupling strength, g effectiv

1
Field Cooled-Field Heated dielectric constant of
the 6.5/65/35
hot-pressed PLZT ceramics
B. Vodopivec, Z. Kutnjak, C. Filipic, A.
Levstik, J. Holc, and M. Kosec Joef Stefan
Institute, P. O. Box 3000, 1001 Ljubljana,
Slovenia
ABSTRACT
Lanthanum doped lead zirconate-titanate ceramics
Pb1-xLax(ZryTi1-y)1-x/4O3 (PLZT), with La content
varying between 6 and 12 at. , belong to the
relaxor-ferroelectric systems. In order to
reveal the influence of the bias electric field
on the formation of the long range ferroelectric
phase from relaxor-like polar nanoclusters in
6.5/65/36 PLZT hot-pressed ceramics, a
field-cooled-field-heated (FC/FH) measurements
were done by measuring a spontaneous polarization
as a function of the bias electric field and
temperature. Saturated polarization and
calculated pyroelectric current, show increase
of the phase transition temperature TC with
increasing electrical field and a smeared first
order phase transition, as seen from hysteresis
of pyroelectric current peaks. Dielectric
susceptibility at various bias electrical fields,
obtained from polarization data, revealed a
transformation between disordered state and
ordered ferroelectric state. Results
are in a good accordance with predictions of the
SRBRF model.
Figure 2. Rescaled pyroelectric current peaks
proportional to obtained at different bias
external electric fields for (a) cooling and (b)
heating run.
Figure 1. Temperature dependence of FC/FH
effective dielectric polarization P, as a
function of bias external electric fields.
RESULTS

• From the temperature and bias electric field
  dependence of spontaneous polarization (Fig.1), a
  derivative of , ?, and ,
  pyroelectric current, were calculated (Figs.
  2,3).
• ? (Fig.2) and pyroelectric currents (Fig.3) show
  peaks at similar temperatures, i.e.
  relaxor-ferroelectric (RF) transitions
  temperatures.
• With increasing bias field the RF peaks are more
  pronounced 1,2,4.
• RF transition temperature increases with higher
  electrical fields (Fig.4).

?P/?E
SRBRF MODEL

• Spontaneous polarization at RF phase transition
  temperatures at diffe-rent bias electrical fields
  was fitted with SRBRF model (Fig.5) 2,4.

• SRBRF model has dimensionless spherical glass
  (q) and polarization (P) order parameters

(1)
(2)
with , J random bond variance, ? random
fields variance, Jo random bonds coupling
strength, g effective field constant and E bias
electrical field. In order to fit both relaxor
and ferroelectric state in the sample the ansatz
for Jo was chosen
Figure 3. Dielectric susceptibility as a function
of temperature and bias electrical field,
obtained from spontaneous polarization data.
(3)
with ? and ? as free parameters. Fits on measured
data (Figs.5,6,7) were obtained with varying
parameters ?, ?, ?, g and J.
CONCLUSIONS

• Relaxor to ferroelectric phase transition takes
  place even in a zero-field cooled run 1.
• Peaks in ? may be associated with a soft mode
  behavior 3.
• Observed thermal hysteresis (Figs. 1,3,4) could
  be a consequence either of the nanodomain
  structure, depinned effects due to impurities, or
  the smeared latent heat effect. Ferroelectric
  transition in 6.5/65/35 PLZT is probably of the
  weakly first order type 1,2.
• SRBRF fits are in a good accordance with
  measured data.

Figure 4. RF transition temperature on heating
and cooling run as a function of the bias
external electric field.
Figure 5. Spontaneous polarization as a function
of electrical fields at different transition
temperatures fitted with SRBRF model.
REFERENCES
1 Xunhu, D., Xu, Z., Viehland, D., Phyl. Mag.
B, 70 (1), 33 (1994). 2 Bobnar, V., Kutnjak,
Z., Levstik, A., Appl. Phys. Lett., 76 (19), 2773
(2000). 3 Levstik, A., Kutnjak, Z., Filipic, C.
and Pirc R., Phys. Rev. B, 57, 11 204 (1998).4
Bobnar, V., Kutnjak, Z., Pirc, R., Levstik, A.,
Phys. Rev. B, 60(9), 6420 (1999).
Figure 7. Dielectric susceptibility as a function
of temperature at different bias electric fields
(Fig.3). Solid lines represents fits to the SRBRF
model predictions (equations 1.-3. derived over
E).
Figure 6. Spontaneous polarization as a function
of temperature at different bias electric fields
(Fig.1). Solid lines represents fits to equations
1.-3.
Electronic address boris.vodopivec_at_ijs.si