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Journal Articles

Nanoscale structural analysis of Bi$$_{0.5}$$Na$$_{0.5}$$TiO$$_3$$

Yoneda, Yasuhiro; Noguchi, Yuji*

Japanese Journal of Applied Physics, 59(SP), p.SPPA01_1 - SPPA01_7, 2020/11

 Times Cited Count:0 Percentile:100(Physics, Applied)

We investigate A-site cation ordering in the ferroelectric perovskite Bi$$_{0.5}$$Na$$_{0.5}$$TiO$$_3$$ (BNT) by synchrotron X-ray total scattering. Although BNT has a problem of a low depolarization temperature, it is promising a lead-free piezoelectric material. Since the depolarization temperature is presumed to correspond to a relaxer like gradual order-disorder phase transition, local structure analysis is necessary to understand the structure of the diorder phase. Through this approach, the elusive connection between chemical heterogeneity and structural heterogeneity is revealed. Because of the large randomness, the Ti off-center shift is averaged out beyond the unit cell and the structure becomes very close to the average structure beyond the unit cell.

Journal Articles

Local structure and phase transitions of KNbO$$_3$$

Yoneda, Yasuhiro; Ohara, Koji*; Nagata, Hajime*

Japanese Journal of Applied Physics, 57(11S), p.11UB07_1 - 11UB07_6, 2018/11

 Times Cited Count:5 Percentile:53.96(Physics, Applied)

Local structure analysis of KNbO$$_3$$, which is the parent compound for lead-free piezoelectric materials, have been performed by X-ray pair-distribution functions (PDF). The refinements of local structure in wide temperature ranges indicates that only the rhombohedral structure can describe the observed bond distributions within the unit cell. The rhombohedral distortion maintained locally in all four phases.

Oral presentation

Local structure analysis of (Na,K,Li)NbO$$_3$$

Yoneda, Yasuhiro; Takada, Eri*; Nagai, Haruka*; Kikuchi, Takeyuki*; Kobune, Masafumi*

no journal, , 

(Na,K)NbO$$_3$$ has been a candidate for lead-free piezo electric material, since it shows a novel piezoelectric properties by making a mixture between (Na,K)NbO$$_3$$ and LiNbO$$_3$$. (Na,K,Li)NbO$$_3$$ lies around a morphotropic phase boundary at room temperature, and it has been reported some co-existing phases of a tetragonal and a orthorhombic structures. We re-examined the crystal structure of (Na,K,Li)NbO$$_3$$ by using high-energy synchrotron X-ray. It was found that (Na,K,Li)NbO$$_3$$ has a monoclinic structure at room temperature. Moreover, we found a phase transition at approximately 200 K, in which the intensity of Bragg peaks changed drastically. The novel piezoelectric response of (Na,K,Li)NbO$$_3$$ is caused by modifying the phase transition below room temperature.

Oral presentation

Growth mechanism of KNbO$$_3$$ nano crystals

Yoneda, Yasuhiro; Kunisada, Ryoichi*; Ueno, Shintaro*; Fujii, Ichiro*; Wada, Satoshi*

no journal, , 

The high-energy X-ray diffraction experiments of KNbO$$_3$$ nanocrystals were carried out. Although Scherrer's equation may be used in estimating the particle diameters, in practical use there are various restrictions. In this study, we used KNbO$$_3$$ nanocrystals synthesized by a solvothermal method with a cubic shape. The crystal structure were a cubic structure because of the small particle size, it can be said that the KNbO$$_3$$ sample is an ideal to apply the Scherrer's equation. The crystal growth proceeds with the reaction time was investigated the kinetics of crystal growth. The short-range order structure was also observed, which was directly determined in the real space from the atomic pair-distribution distribution function method. It was found that the initial growth of KNbO$$_3$$ nano crystal was suppressed because of lattice defects.

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