Elucidation of the Local Structure of the Topological Insulator β-PdBi₂ by X-ray Fluorescence Holography

The local structures around the Bi and Pd atoms in β-PdBi2 were determined from Bi Lα and Pd Kα X-ray fluorescence holography (XFH) images measured with synchrotron radiation at room temperature, demonstrating clear atomically resolved images along the c-axis (z-axis). More precisely, the location of atoms in the atomic layer at z = 0 Å was unambiguously determined from each hologram; that is, the location of Bi atoms at z = 0 Å was determined from Bi Lα XFH, whereas the location of Pd atoms at z = 0 Å was unambiguously determined from Pd Kα XFH. Moreover, the Bi atoms in the atomic plane at z = 3.6 Å were clearly reconstructed from Bi Lα XFH with Barton's algorithm. However, clear atomic images of the planes at z = 1.8 Å and z = 2.9 Å were not obtained from the Bi Lα holograms, nor were they observed at z = 1.8 Å in the case of the Pd Kα holograms; some weak atomic images of Bi at z = 4.7 Å were observed from Pd Kα holograms. Thus, two different patterns can be distinguished for the atomic images in the plane at z ≠ 0 Å: a clear atomic image and an ambiguous atomic image, owing to the order or disorder between the atomic layers. The small fluctuation or the small disorder between the atomic layers formed through the attractive interaction of Bi-Pd2+-Bi- provides clear atomic images, whereas the atomic layers formed via weak van der Waals interactions lead to ambiguous atomic images. Thus, the atomic images of β-PdBi2 obtained by XFH clearly reproduce not only the crystal structure determined by single-crystal X-ray diffraction but also provide information on thermal fluctuation and static disorder between the atomic layers.