Soft X-ray Photoelectron Spectroscopy in Silicon Clathrate Superconductors

Silicon clathrate compounds consist of fullerenelike polyhedral Si-cages, which share their faces to form a three-dimensional Si-sp 3 covalent network. Guest elements such as an alkali atom or an alkaliearth one can be encapsulated in the cage. Recently, these compounds have attracted strong interest due to the discovery of superconductivity in a metal-doped silicon clathrate, Ba6Na2Si46 [1], as well as the potential for thermoelectric applications. Barium encapsulated type-I silicon clathrate, Ba8Si46, has been reported to show superconductivity below T c ~ 8 K [2], which is the highest Tc observed in the silicon clathrate family. As schematically shown in Fig. 1, the crystal structure of Ba8Si46 consists of two types of polyhedral cages, i .e . dodecahedral (Si20) and tetrakaidecahedral (Si24) cages. In an ideal-composition phase, Ba atoms occupy all of the cages resulting in the chemical formula of Ba8Si46 per cubic unit cell. In Ba8Si46, we can substitute Ag atoms for Si atoms in ‘Si (1)’ sites shown in Fig. 1, where the chemical formula becomes Ba8AgxSi46-x (0 x 6). The solid solution of Si and Ge clathrate, Ba8GeySi46-y (0 y 40), and pure germanium clathrate Ba8Ge43 also have b e e n syn th e s i ze d . I n b o th ca se s , t h e superconducting transition temperature T c has been found to systematical ly decrease with increasing Ag or Ge concentration. Photoelectron spectroscopy (PES) allows us to directly observe the electronic structures in the valence band in addition to the core electrons in solid substances. Recently, it has been recognized that high-resolution PES using relatively highenergy soft X-ray is quite a powerful tool for investigating the bulk-states of solids. In the present study [3], PES was carried out by using a synchrotron radiation soft X-ray to investigate the electronic structure of silicon clathrate family. The relationship between the electronic structure and the systematic change of superconducting nature of silicon clathrate family is discussed. Polycrystalline samples of clathrates Ba8AgxSi46-x (0 x 6), Ba8Gey Si46-y (0 y 40) and Ba8Ge43 (pure germanium clathrate) were prepared by using a high-pressure synthesis technique as reported in previous works [1, 2]. Photoelectron spectra were measured at beamline BL25SU with various photon energies hν between ~ 240 and ~ 1200 eV. The total energy resolution was ~ 150 meV at h ν ~ 800 eV . Samples were cooled down to 20 K. T obtain clean surfaces, samples were fractured under the ultrahigh vacuum of ~ 1 × 10-10 Torr just before the measurements were taken. The valence-band photoelectron spectra of Ba8Si46 are shown in Fig. 2. The horizontal axis indicates the binding energy E B, where 0 eV corresponds to the Fermi energy E F. Red and blue dots are measured by using excitation photon energy of hν ~ 785 and ~ 776 eV, respectively. The former corresponds to the Ba 3d to 4f transition Fig. 1. Schematic illustration of crystal structure of silicon clathrate Ba8Si46. Two types of fullerenelike polyhedral cages, i.e. Si20 and Si24 cages, share their faces to form the sp 3 covalent network. A Ba atom is encapsulated in each cage. Si(1) Si(2) Si(3)