Valence-selective local atomic structures in inorganic materials by X-ray fluorescence holography

X-ray fluorescence holography (XFH) can conduct atom-resolved structural characterization of materials around a specific element, which has been applied to various function materials. Recently, a valence-selective function has been found in this technique by employing incident X-ray energies near an absorption edge of a specified element. In this article, the principle and experimental procedure of a valence-selective XFH and subsequent data analysis procedure by using a sparse modeling approach of a L1- regression are introduced. Then, excellent XFH results of valence-selective are reviewed, such as an Y oxide thin film, YbInCu4 valence transition material, and Fe3O4 mixed valence material.