Successful X-ray fluorescence holography measurement under high-pressure conditions

X-ray fluorescence holography (XFH) is a structural analysis method that can reconstruct three-dimensional atomic images around specific elements and is widely used as an experimental technique that can visualize at the atomic level which parts of materials with characteristic functions are involved in those functions. However, measurements under high pressure were not possible due to the weak signal.

A research team including Professor Naoki Ishimatsu and Professor Tetsuo Irifune from the Geodynamics Research Center (GRC) of Advanced Research Institute at Ehime University, Doctoral Student Xinhui Zhan and Associate Professor Nobuo Nakajima from Hiroshima University's Graduate School of Advanced Science and Engineering, Assistant Professor Koji Kimura and Professor Koichi Hayashi from Nagoya Institute of Technology's Department of Physical Science and Engineering, and Associate Professor Naohisa Happo from Hiroshima City University's Graduate School of Information Sciences has succeeded for the first time in measuring XFH under high-pressure conditions. Their work was published in the Journal of Synchrotron Radiation.

XFH requires extracting weak scattering signals of about 0.1% relative to the fluorescent X-ray intensity from the sample, making measurement difficult under high pressure due to interference (noise) from high-pressure generating devices. For example, conventional single-crystal diamond used as material for diamond anvil cells (DAC) generates pseudo-Kossel lines using the sample's fluorescent X-rays as a light source, which become strong noise that completely cancels out the hologram image from the sample.

When the research team attempted to eliminate pseudo-Kossel lines by using nano-polycrystalline diamond (NPD) as anvils, clear hologram images from Sr fluorescent X-rays of the single-crystal SrTiO₃ paraelectric sample were observed.

Kossel lines originating from the single-crystal nature of the SrTiO₃ sample were also observed in the hologram images. On the other hand, while NPD does not generate pseudo-Kossel lines, its polycrystalline nature causes powder diffraction patterns to be superimposed on the sample's hologram image. To remove this noise, an yttrium (Y) metal foil was placed in front of the two-dimensional detector as a filter to eliminate the powder diffraction patterns.

As a result, clear hologram images and Kossel lines from only the SrTiO₃ sample were successfully extracted under high pressure up to 13.3 GPa. At this time, it was found that reciprocating the metal foil to improve foil homogeneity was also important for noise removal.

Continuous changes accompanying crystal lattice contraction with increasing pressure were observed based on the hologram images obtained up to the maximum pressure of 13.3 GPa. The atomic arrangement around Sr atoms at 1.3 GPa was confirmed by extending the obtained holograms to full-sphere hologram images and applying Fourier transformation. In this way, the arrangement of Ti atoms and other Sr atoms around a single Sr atom was clearly observed under high pressure.

NPD, a polycrystalline superhard material, exhibits excellent characteristics in X-ray absorption spectroscopy measurements under high pressure because it does not generate noise specific to natural single-crystal diamond. For this reason, it is used at major synchrotron radiation facilities worldwide such as SPring-8 and the European Synchrotron Radiation Facility (ESRF). These results demonstrate that NPD's excellent material properties can also be utilized in XFH experiments, another X-ray spectroscopy technique.

Journal Information
Publication: Journal of Synchrotron Radiation
Title: X-ray fluorescence holography under high-pressure conditions
DOI: 10.1107/S1600577525005284