Comparative study of the core level photoemission of the ZrB2 and ZrB12

X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were used to investigate the binding energies and valence band for ZrB2 and ZrB12. The Zr 3d and B 1s core levels were identified. The Zr 3d core level shows a spin–orbit split 3d5/2 and 3d3/2 while that for B 1s core level exhibited a single symmetric peak, these being typical of zirconium and boride signals. Comparing the Zr 3d and B 1s core levels with metallic Zr, B2O3 and ZrO2 reference materials only a negative chemical shift for Zr 3d associated to ZrB2 was observed, which suggests that the charge transfer model based on the concept of electronegativity was not applicable to explain the superconductivity in the ZrB12 sample. The measured valence band using UPS is consistent with the band-structure calculations indicating a higher density of states (DOS) at EF for ZrB12 respect to ZrB2. Finally, we found that the weak mixed B-p and Zr-d states for ZrB12 is crucial for the superconductivity due to the state population increased the DOS at the EF.