**2.1. Sample preparation**

The remanent polarization (*Pr*) can be obtained from the pyroelectric current *ip*(*T*) using equation 3, where *A* is the area of the samples. The integration is made from the operation temperature *T* (usually room temperature) until *Tm* (or a higher temperature in the case of

**Figure 2.** Theoretical decomposition of the *i-T* dependence using the Gaussian method.

1 *mT r p T P i dT*

Other parameters can be evaluated from the *ip*(*T*) dependence, such as the pyroelectric coefficient (*p*) and several merit figures. The pyroelectric coefficient is related to the variation of *Pr* (equation 4). The current response parameter (*Rv*) is one of the important merit figures which are associated with pyroelectric behaviour, and can be obtained using equation 5.

*<sup>r</sup> dP*

´ *<sup>v</sup> <sup>p</sup> <sup>R</sup>* <sup>=</sup> <sup>e</sup>

There are not many reports concerning the pyroelectric behaviour of ferroelectric systems from the Aurivillius family. Most of the studies have been carried out on pure and modified bismuth

*<sup>A</sup>* = - <sup>b</sup> ò (3)

*<sup>p</sup> dT* <sup>=</sup> (4)

(5)

relaxor ferroelectrics).

90 Ferroelectric Materials – Synthesis and Characterization

Sr1-xBaxBi2Nb2O9 (x = 0, 15, 30, 50, 70, 85, 100 at%) ferroelectric ceramic samples were prepared by solid-state reaction method (Figure 3).

The powders of the starting materials SrO, BaO, Bi2O3 and Nb2O5 were mixed with a desired weight ratio. The mixture of oxides was milled with alcohol for two hours, dried and pressed by applying 100 MPa. The pressed samples were calcined in air atmosphere at 950 °C for two hours. After calcination the samples were milled again for one hour, dried and pressed by applying 200 MPa. The sintering process was made in a sealed alumina crucible at 1100 °C for one hour. Samples with density values higher than 90 % of the theoretical density values were obtained. Silver electrodes were deposited on the opposite faces of the disk-like samples by using a heat treatment at 590 °C. The samples were named SBN (x=0), SBBN-x (x=15-85 at%) and BBN (x=100 at%), respectively.

### **2.2. Ferroelectric measurements and thermally stimulated discharge current experiments**

Polarization-electric field (*P*–*E*) loops were obtained at room temperature for 10 Hz by using a precision ferroelectric analyser (Premier II, Radiant Technologies Inc.), which is combined with a high-voltage power supply (TReK Model 663A). The highest applied electric field was 90 kV/cm for the studied samples.

The study of thermally stimulated depolarization currents was carried out in sequential thermal cycles as follows: (*i*) zero-field heating - heating from room temperature to 60 °C under zero electrical field; (*ii*) field cooling – cooling to room temperature while a polarizing electrical field is applied (*EP* = 2 kV/mm); (*iii*) zero-field heating – heating from room temperature to temperatures higher than *Tm* under zero electrical field. The thermal discharge current was

**Figure 3.** Solid-state reaction method for the sample preparation.

measured (during the third step) using a Keithley 6485 Electrometer, while keeping a temper‐ ature rate of about 5 K/min.
