Dielectric materials are used in various electronic devices, such as capacitors and actuators. At the atomic scale, the positively and negatively charged ions in these devices are displaced from their original positions. Therefore, observing these displacements is essential for improving the performance of electronic devices and developing new dielectric materials.

BaTiO₃ has a high capacity for electricity storage. Below 400 K, this material becomes ferroelectric owing to the displacement of the positively charged titanium and negatively charged oxygen. However, it is difficult to evaluate the displacements of the constituent ions, especially that of oxygen.

Holography is a technique for generating three-dimensional (3D) images from the interference patterns of waves. Holography with visible light is extensively used in applications such as the anticounterfeiting of banknotes. If we use quantum beams (e.g., electron beams, X-rays, and neutron beams) instead of visible light, a 3D atomic arrangement of materials can be obtained. Atomic-resolution holography with X-rays has been used to evaluate ionic displacement. However, the oxygen atoms cannot be detected because X-rays are not sensitive to light elements. Therefore, the understanding of the ionic displacement mechanism remains elusive.

In this study, we addressed this problem using a newly developed neutron holography. Unlike X-rays, neutrons are sensitive to oxygen atoms; therefore, in principle, it is possible to obtain atomic images of oxygen. The experiment was performed at the Japan Proton Accelerator Research Complex (J-PARC), which provides the world’s highest-intensity neutrons. After accurate data acquisition and processing, the researchers successfully reconstructed an atomic-scale image of BaTiO₃ and obtained oxygen atomic images, as expected.

Furthermore, a detailed analysis of the image intensities obtained by the neutron holography showed that the magnitude of the oxygen displacement in BaTiO₃ relative to the titanium atom was evaluated to be 0.16 Å. This value is very close to those reported in previous study [1], demonstrating the usefulness of the new analytical method.

Although these experiments were performed on standard ferroelectrics, doped materials are interesting targets. This is because dielectric performance is often enhanced by elemental doping, and neutron holography can be used to visualize the 3D atomic arrangement around doped elements. Therefore, this technique has great potential for revealing the dopant-induced displacements of cations and anions, which cannot be determined by other methods.

[1] J. Harada, T. Pedersen, and Z. Barnea, Acta Cryst. A26, 336 (1970).

(Written by K. Kimura on behalf of all authors)

Share this topic

Fields

Related Articles