Effect of Mo, W Substitution on Ferroelectric Characteristics, Crystal and Electronic Structure of Bi0.5K0.5TiO3-BiFeO3-KTaO3 Based Ferroelectric Ceramics
Authors: Kawamoto Tsuyoshi , Ishida Naoya , Kitamura Naoto , Idemoto Yasushi 
Published: December 2020

1.Introduction

As the realization of a society with less environmental impact is expected, lead-free ferroelectric materials have been actively developed to replace the lead zirconate titanate (PZT) -based ferroelectrics widely used in electronic devices. As a candidate for alternative materials, our laboratory has focused on (Bi0.5K0.5)TiO3-BiFeO3(BKTBF) -based solid solution that exhibits high ferroelectric and piezoelectric properties1). It has been also reported that a solid solution of 0.4BKT-0.6BF with KNbO3, KTaO3 has MPB such as PZT and then the ferroelectric properties are improved by formation of MPB 2). In this study, we focused on (Bi0.5K0.5)TiO3-BiFeO3-KTaO3-based solid solution (100Ta) and tried to suppress oxygen deficiency by substituting Mo and W, which have higher valences than Ta (97Ta03Mo, 97Ta03W). Ferroelectric properties of these materials were investigated and a crystal structure analysis using synchrotron radiation X-rays was also performed. Based on the results, we clarified the effect of substitution of the other elements with higher valence on the ferroelectric properties and discussed the relationship between the ferroelectric properties and the crystal structures.

2.
Experimental

The sample was synthesized by the solid phase method. In addition, since the solid solution targeted in this study is difficult to be sintered, the sintering by spark plasma sintering (SPS) was applied, and then the obtained sintered body was subjected to recovery annealing at 750℃ for 4 hours in an oxygen atmosphere. The phases of these samples were identified by powder X-ray diffraction measurement, and the metal composition was calculated by ICP emission spectroscopy. Moreover, in order to examine the sinterability, density measurement by true density and morphological observation by SEM were performed. Regarding the ferroelectric properties, P-E hysteresis measurement and temperature dependence of permittivity / dielectric loss were measured. To investigate the relationship between the crystal structure and the ferroelectric properties, we used the data obtained from synchrotron X-ray diffraction measurements (BL19B2, SPring-8) to analyze the crystal structure by the Rietveld method (RIETAN-FP) and the electron density distribution by MEM.

3.
Results and discussion

From the XRD results, all the samples could be assigned in the rhombohedral system (S.G.:R3c).In addition, cross-sectional morphology observation by SEM and true density measurement revealed that all samples were sufficiently dense. No change in sinterability due to substitution of the other elements was observed. By the P-E hysteresis loop and the measurement of the dielectric constant, it was confirmed that the sample in which Mo was substituted had a significant improvement in the insulating property, and the samples in which Ta and W were added showed a decrease in the coercive electric field. In order to clarify the factors of the above characteristic changes, Rietveld analysis was performed with the space group R3c and the samples were examined from the viewpoint of the crystal structure.

References

1)H. Matsuo et al., J. Appl. Phys., 108, 104103 (2010).
2) Kosuke Miyazaki, Naoya Ishida, Naoto Kitamura, Yasushi Idemoto, The Ceramic Society of Japan 2014 Annual Meeting Proceedings, 1P042 (2014).

Figure 1