Observation of the Superstructure in Fe_5-x GeTe₂ by X-ray Fluorescence Holography

Fe5-xGeTe2 is a two-dimensional van der Waals material that exhibits ferromagnetic order with a high Curie temperature (T-C) of around room temperature. In addition to T-C, two magnetic transitions occur with decreasing temperature, and a charge-ordered state is observed at low temperatures. We employed Ge K alpha X-ray fluorescence holography (XFH) for Fe5-xGeTe2 to directly investigate the local structure in the charge-ordered state, i.e., the root 3x root 3 superstructure. The Ge K alpha XFH results revealed local atomic structures around the Ge atom, thus clarifying the simultaneous locations and arrangements of the Te, Fe, and Ge atoms. The atomic positions relative to the Ge atom are useful for understanding the coexistence of the ideal 1 x 1 structure and root 3x root 3 superstructure found in the charge-ordered state.