Crystal Structure and Ferroelectric Properties of Bi0.5K0.5TiO3-BiFeO3-K(Nb, Ta)O3 Ferroelectric Oxide

INTRODUCTION

Pb(Zr,Ti)O₃ (PZT) have been widely used as actuators and sensors because of the excellent piezoelectric properties and ferroelectric properties. From the idea to reduce the environmental impact, the development of lead-free material is required now. Bi_0.5K_0.5TiO₃ (BKT) has attracted attention as a new bismuth-based ferroelectric material has a high piezoelectric properties and Curie temperature. BiFeO₃ (BF) is known as a substance having a high Curie temperature, indicating remnant polarization. Studies　on (1-x)BKT-xBF(BKTBF) solid-solution ceramics have reported that an morphotropic  phase boundary (MPB) exists between the rhombohedral and tetragonal phases at around x=0.4 as in the case of PZT.^{1) ­}Also the ferroelectric/piezoelectric properties are improved by doping Nb to PZT is known.²⁾ In this study, we prepared BKTBF (0.4BKT-0.6BF) and Nb,Ta-doped (0.4-x)Bi_0.5K_0.5TiO₃-0.6BiFeO₃-xKNbO₃ (x=0.025[KN2.5], 0.05[KN5])_, (0.4-x)Bi_0.5K_0.5TiO₃-0.6BiFeO₃-yKTaO₃ (y=0.025[KT2.5], 0.05[KT5]) ceramics by solid state reaction in order to improve ferroelectric properties. By carrying out powder neutron diffraction (iMATERIA, J-PARC) and synchrotron X-ray diffraction (BL02B2, SPring-8) measurement, we examined a relation among the ferroelectric properties and the chemical composition from the viewpoint of the crystal structure. Additionally we examined an influence of Spark Plasma Sintering (SPS) method.

EXPERIMENTAL

In the solid state reaction, we performed wet mixing of Bi₂O₃ and KHCO₃ TiO₂ Fe₂O₃ Nb₂O₅ Ta₂O₅ at specific ratios. After wet mixing, the materials were calcined and then sintered. The powder obtained by the synthesis was then molded into pellets, and the bulk ceramics were made by sintering. As for all the synthetic process, sintered processes were carried out at 1000^oC in air for 4 h.

RESULTS AND DISCUSSION

It was found by the powder XRD that the obtained all ceramics could be identified as a single phase regardless of the chemical composition. From observations of the surface morphologies of the sintered bodies by SEM, the pellets were sufficiently dense in all the prepared compounds to measure the electrochemical properties. As an example, Fig. 1 shows the applied field dependence of the remnant polarization(P_r) of BKTBF, KN2.5, KN5, KT2.5, KT5. The higher remnant P_r was obtained in KN2.5, KN5, KT2.5 than BKTBF. Figure 2 shows the applied field dependence of the coercive field
(E_c). KN2.5, KN5, KT2.5, KT5 exhibited lower E_cthan BKTBF.

As described above, it was demonstrated that the  chemical composition affected the ferroelectric and piezoelectric properties. In order to clarify the origin of these features, we carried out the Rietveld analysis using synchrotron X-ray diffraction and neutron diffractions, and investigated the structural changes. From the result, a good fitting was obtained by using a two-phase model of the tetragonal (P4mm) and rhombohedral (R3c) structures.

REFERENCES

1)  H. Matsuo et al. J.Appl.Phys. 108, 104103(2010).

2)  Y. Idemoto, T. Mizoguchi, N. Kitanura, T.Itoh, J. Jpn. Soc. Powder Powder Metallurgy, 58, 703-709(2012).