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Successful Three-Dimensional Visualization of Local Structures of Relaxor Ferroelectric by X-ray Fluorescence Holography (Press Release)

Release Date
22 Apr, 2014
  • BL22XU (JAEA Quantum Structural Science)
- Breakthrough to realizing novel dielectric and piezoelectric materials with high functionality -
　

Japan Atomic Energy Agency (JAEA)
Institute for Materials Research, Tohoku University
Hiroshima City University
Kumamoto University

Key points
• World's first application of atomic-level-resolution X-ray fluorescence holography to inhomogeneous crystals
• World's first successful three-dimensional visualization of local structures of relaxor ferroelectric with high dielectric constant and piezoelectric coefficient
• Further encouragement of local structural analysis of inhomogeneous crystals and expected contribution to developing novel materials

A research group has clarified the steric (three-dimensional) local atomic arrangements (structures) of a relaxor ferroelectric*2 by X-ray fluorescence holography*1 for the first time in the world. The group was led by Wen Hu (postdoctoral researcher) of JAEA (President, Shojiro Matsuura) and Kouichi Hayashi (associate professor) of the Institute for Materials Research, Tohoku University (President, Susumu Satomi), and collaborated with researchers from Hiroshima City University (President, Nobuyuki Aoki) and Kumamoto University (President, Isao Taniguchi).
Pb-based ferroelectrics known as relaxor ferroelectrics have a very high dielectric constant and piezoelectric coefficient and can largely deform with weak electric input (known as high functionality). Hence, they are in high demand on the market as materials used in vibrators for transmitting and receiving ultrasonic waves in medical probes and driving units for ultrasonic motors. However, they contain toxic Pb and are considered to contaminate the environment upon disposal and to adversely affect the ecosystem. Therefore, the development of Pb-free materials is an urgent task. To realize such materials, the mechanism behind the expression of high functionality must be clarified in terms of atomic arrangement. In particular, the clarification of local structures is important because highly functional materials are inhomogeneous at the atomic level (that is, inhomogeneous crystals).
In this research, the group investigated Pb(Mg1/3Nb2/3)O3, a typical relaxor ferroelectric with a perovskite*3 structure, by X-ray fluorescence holography and successfully modeled the three-dimensional local structure of the inhomogeneous crystal for the first time in the world. This achievement will encourage the clarification of the origin of the high functionality of relaxor ferroelectrics, leading to a breakthrough in the realization of high-performance ferroelectric materials with excellent dielectric and piezoelectric properties without using harmful materials such as Pb. X-ray fluorescence holography is also expected to further encourage the structural analysis of inhomogeneous crystals, as well as the development of novel device materials that will support Japan's green innovation strategies and contribute to the efficient and smart use of energy.
The achievements of this research has been published online in the American scientific journal Physical Review B (Rapid Communications).

Publication：
Physical Review B 89(14) 140103 (2014).
"Acute and obtuse rhombohedra in the local structures of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3"
Authors & Affiliations: Wen Hu1, Kouichi Hayashi2, Kenji Ohwada1, Jun Chen3, Naohisa Happo4, Shinya Hosokawa5, Masamitu Takahasi1, Alexei A. Bokov6, and Zuo-Guang Ye6
1Quantum Beam Science Directorate, Japan Atomic Energy Agency, 2Institute for Materials Research, Tohoku University,3University of Science and Technology Beijing,4Hiroshima City University,5Kumamoto University,6Simon Fraser University
<<Caption>>
Fig. 1 Experimental conditions adopted in this research
Fig. 1 Experimental conditions adopted in this research

A three-dimensional X-ray fluorescence hologram is reconstructed by controlling the direction of the X-ray incident to the crystal in terms of the Θ- and Φ-axis rotations.

Fig. 2 (a) Three-dimensional atomic image reconstructed from Nb hologram
Fig. 2 (a) Three-dimensional atomic image reconstructed from Nb hologram

Oxygen (O) and lead (Pb) atoms are shown in blue and red, respectively. The niobium (Nb) atom, as the origin, is shown in green. The green lattice has sides of 0.405 nm (1 nm is approximately one-hundred-thousandth of the width of a human hair).

(b) Ideal perovskite structure
Compared with (b), Pb atoms in (a) are split (and widely distributed) in the diagonal <111> direction.

Fig. 3 Three-dimensional network model combining obtuse and acute rhombohedral structures found in relaxor ferroelectric
Fig. 3 Three-dimensional network model combining obtuse and
acute rhombohedral structures found in relaxor ferroelectric

 

《Glossary》
*1　X-ray fluorescence holography and holograms
Holography is a technique for recording and reconstructing stereoimages using the wave nature of light (including X-rays). Holography for the visible range (wavelength, ~0.6 µm) has been already practically available. Holograms are records of information for reconstructing a stereoimage and consist of stripe patterns with ultrafine intervals (equivalent to the wavelength of light). For example, a holographic pattern is found in the lower left of the front side of a 10,000 yen bill. When you look at the shiny pattern from various angles under fluorescent light, you can see the Bank of Japan mark, cherry blossom, and numbers.

Fig. 4 Principle of holograph
Fig. 4 Principle of holography

When a hologram (stripe pattern) containing information on an image is exposed to light, the observer can see the image.

X-ray fluorescence holography has an applicable wavelength range extended to X-rays (wavelength: ~0.0001 µm). When X-rays are absorbed by atoms, X-rays with atom-specific wavelengths are emitted from the atoms; this is known as X-ray fluorescence. In X-ray fluorescence holography, this phenomenon is used to reconstruct holograms. The research group led by Kouichi Hayashi of the Institute for Materials Research, Tohoku University, developed experimental methods for the technique and demonstrated its effectiveness for the analysis of three-dimensional local atomic structures of trace impurities in single crystals of shape-memory alloys and semiconductor materials.

Fig. 5 Principle of X-ray fluorescence holography
Fig. 5 Principle of X-ray fluorescence holography

 

Fig. 6 Example of X-ray fluorescence hologram
Fig. 6 Example of X-ray fluorescence hologram

An atomic-level-resolution image is reconstructed from a hologram.

*2　Ferroelectrics, relaxor ferroelectrics
When a material is placed in an electric field, its charges are attracted to either the positive or negative side. As a result, one side of the material is positively charged, whereas the other side is negatively charged, which is known as polarization. Materials spontaneously polarized without the application of electric field are referred to as ferroelectrics.
Polarized materials have the following three properties: (1) a dielectric property to store electricity, (2) a piezoelectric property to generate electricity through deformation upon applying force (an inverse piezoelectric property to deform upon applying voltage), (3) a pyroelectric property to generate electricity upon temperature change. With these properties, ferroelectrics can be used in various applications such as sensors, probes, batteries, capacitors, memories, and solar cells.
Ferroelectrics that have a very high dielectric constant and piezoelectric coefficient as well as maintain their properties stably over a wide temperature range (relaxation) are particularly called relaxor ferroelectrics and are distinguished from others. Pb(Mg1/3Nb2/3)O3 is a well-known example.

*3　Perovskite, complex perovskite
Perovskite is the mineralogical name of calcium titanate (CaTiO3). From this name, substances with the chemical formula ABO3 are generally called perovskite compounds. Perovskite compounds are known as “the treasure house of functions” since many perovskite compounds with different properties have been reported.
In a perovskite structure, many ABO3 unit cells are regularly arranged. The original perovskite structure only contains one type of B atom. In contrast, a complex perovskite structure contains different types of B atom; for example, some cells have a BI atom and other cells have a BII atom. The arrangement of BI and BII atoms is generally irregular. In addition to Pb(Mg1/3Nb2/3)O3 examined in this research, Pb(Zr1-xTix)O3 and Pb[(Mg1/3Nb2/3)1-xTix]O3 are also complex perovskites.

Fig. 7 Perovskite structure
Fig. 7 Perovskite structure

In a cubic structure comprising A atoms, B and O atoms occupy the body and face centers, respectively.


For more information, please contact:
　Quantum Beam Science Directorate, Japan Atomic Energy Agency
　　Wen Hu (Present address: Toyota Technological Institute)
　　Kenji Ohwada
　　Masamitu Takahasi　TEL：+81-(0)791-58-2639
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