Crystal Structures and Ferroelectric Properties of (0.4-y) Bi_0.5K_0.5TiO₃-0.6BiFeO₃-y K(Nb_xTa_1-x)O₃

Abstract

INTRODUCTION Pb(Zr,Ti)O3 (PZT) have been widely commercialized as actuators and sensors because of the excellent piezoelectric and ferroelectric properties. From the demand to reduce the environmental load, the development of lead-free material is expected now. Bi0.5K0.5TiO3 (BKT) with the perovskite structure has attracted attention as a new bismuth-based ferroelectric material which has high piezoelectric properties and the Curie temperature. BiFeO3 (BF) is also known as a material with the high Curie temperature and a potential of an excellent remnant polarization. From such background, studies on (1-x)BKT-xBF (BKTBF) solid-solution ceramics have been investigated, and it has been reported that a morphotropic phase boundary (MPB) exists between the rhombohedral and tetragonal phases at around x=0.4 as in the case of PZT.1) Since the ferroelectric/piezoelectric properties of PZT are improved by doping Nb5+,2) our laboratory has also focused on partial substitution of higher-valent cations for the B site of BKTBF and has studied solid solutions between BKTBF and KNbO3 or KTaO3. As a result, we have found that there were the relationship between the ferroelectric properties and crystal structure.3) In this study, to clarify the effect of amount of solid solution and Nb/Ta ratio, we newly focused on the (0.4-y)Bi0.5K0.5TiO3-0.6BiFeO3-yK(NbxTa1-x)O3(x=1/3, 1/2, 2/3). These materials were prepared by a conventional solid-state reaction method with normal sintering and spark plasma sintering (SPS). By carrying out synchrotron X-ray diffraction measurements, we examined a relation between the ferroelectric properties, the metal composition and the crystal structure. EXPERIMENTAL We performed wet mixing of Bi2O3 and KHCO3 TiO2 Fe2O3 Nb2O5 Ta2O5 at specific ratios. After the wet mixing, the materials were calcined and then sintered. In the case of the normal sintering, the sintering processes were carried out in air at 1000 oC for 4 h as for all the samples. In the case of SPS, the products after the heat-treatment at 1000 oC were pulverized again. The resulting powder filled in a carbon die were sintered under uniaxial pressing of 50 MPa at 800 °C for 5 min in a vacuum. Identification of the phases and evaluation of the lattice parameters were carried out by XRD. The compositions of metal constituents were measured by ICP and AAS. Relative densities of the ceramics were estimated by the Archimedes method, and the morphologies of the samples were observed by SEM. P-E hysteresis loop measurements were carried out at various frequencies using a TF-2000FE device (aixACCT). The temperature dependences of the dielectric permittivities (εs) and dielectric losses (tanδ) of the samples were measured by a LCR meter (HP-4284A). The piezoelectric properties was also measured by a d33meter (PM300) and Impedance analyzer (HP4192A). In order to investigate the crystal structures in detail, we performed the Rietveld analysis (RIETAN-FP) using synchrotron X-ray diffraction data (BL19B2, SPring-8). RESULTS AND DISCUSSION It was found from the powder XRD that the main phase of the all samples have been attributed to the perovskite structure of the same space group P4mm as Bi0.5K0.5TiO3. From observations of the morphologies of the sintered bodies by SEM, the pellets were sufficiently dense in all the prepared compounds for measurements of the electrochemical properties. As an example, Fig.1 shows of temperature dependences of dielectric constant εr and dielectric loss tanδ of the samples prepared by SPS (y=0.025, x=1/ 3, 1/ 2, 2/ 3). It was demonstrated that the relative dielectric constant became higher with increasing Nb ratio. P-E hysteresis showed an increase in remanent polarization in samples with SPS. Such a tendency of the remanent polarization became significant as the Nb/Ta ratio decreased. As for effects of the amount of solid solution (y), the lattice constant was increased with increasing the solid solution amount up to y=0.05, but decreased above the y value. As the amount of solid solution increased, relative dielectric constant increased at room temperature and Tm. As described above, it was demonstrated that the metal 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, and investigated the structural changes. From the result, a good fitting was obtained by using a space group of R3c. REFERENCES 1) H. Matsuo, et al. J.Appl.Phys. 108, 104103(2010). 2) Y. Idemoto, T. Mizoguchi, N. Kitanura, T.Itoh, J. Solid State Chem., 210, 275-279(2014). 3) Kosuke Miyazaki, Naoya Ishida, Naoto Kitamura, Yasushi Idemoto, Abstract for 27th Fall Meeting of The Ceramic Society of Japan, 1P042 (2014). Figure 1