Electronic structure ofRNi2B2C (R=rare earth) from x-ray-absorption spectroscopy

We have measured the Ni L and B K near-edge x-ray-absorption spectra of polycrystalline R${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$C (R=Y, Sm,Tb,Ho,Er,Tm,Lu) samples. The Ni spectra were found to be very similar throughout the entire series, indicating a constant Ni 3d electron occupation number and no significant variation in the unoccupied electronic density of states close to the Fermi level. This rules out the notion that the loss of superconductivity with earlier rare-earth constituents (R=Sm and Tb) might be due to a decrease in the density of states at the Fermi level. The B spectra show significant changes between 4 and 7 eV above the Fermi level, possibly due to the increasing distortion of the ${\mathrm{NiB}}_4$ tetrahedra when going from the late to the early rare-earth constituents. Several corresponding broad absorption features were observed in the Ni and B spectra, providing evidence for the strong covalent bonding between the Ni and B atoms and indicating a significant electron-phonon interaction associated with the Ni-B bond.