New Neutron Holographic Method for Studying Materials at the Atomic Level

Holograms are generally associated with futuristic 3D display technologies, however, in reality, people are using holographic technologies to help study materials at the atomic level. X-rays referred to as a high energy form of light are frequently used to study atomic structure.

Nearest Ca2+ images are split into two parts due to the extra positive charge by Eu3+. The interstitial F- image is observed between Ca2+ images. The additional F- is needed for the compensation of the excessive charge. Dashed circles indicate original positions of Ca atoms without doping Eu. CREDIT NITech

However, X-rays are considered to be sensitive to only the number of electrons associated with an atom. This restricts the use of X-rays for studying materials developed from lighter elements. Neutron measurements are capable of frequently filling in the gaps in structures when X-ray measurements fail, but neutron beams are considered to be difficult to make and have lower intensities when compared to X-ray beams, which confines their versatility.

A recent collaboration among Japanese researchers working at national particle accelerator facilities across Japan led to the development of a new multiple-wavelength neutron holography technique capable of providing insights into earlier unknown structures. The researchers demonstrated a new neutron holographic method with the help of a Eu-doped CaF2 single crystal and then attained clear three-dimensional atomic images around trivalent Eu substituted divalent Ca, thus revealing for the very first time intensity features of the local structure that permits it to maintain charge neutrality.

We knew that neutron holography might be able to tell us more about the structure of a europium-doped calcium fluoride crystal. Europium ions add extra positive charge to the crystal structure, and our neutron holograms showed how fluorine atoms arranged in the lattice to balance this excess charge. These kinds of structural problems are often encountered by materials scientists developing new electronic materials, and our method offers an exciting new tool for these researchers.

Lead author Kouichi Hayashi.

Workings of the new holographic method take place by firing neutrons with controlled speed at a sample, which in this situationrefers to the europium-doped calcium fluoride crystals. Normally, neutrons are assumed to be particles, but also comprise of wave-like properties similar to light, based on their speed. Gamma rays are produced in a pattern controlled by the local structure when the neutrons hit europium atoms. The holograms, or gamma ray patterns, measured from neutrons travelling at different speeds are merged in order to produce a three-dimensional representation of the europium atoms in the crystal.

Neutron sources are less intense than X-ray sources, but it is essential that we work around this issue to develop more effective methods for exploring structures with light elements. Our work here represents a step towards a full toolbox of commentary X-ray and neutron techniques for materials research.



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