Abstract

NaInO₂ and NaInS₂ with α-NaFeO₂ layer type structure have been prepared and characterized by X-ray photoelectron spectroscopy to clarify the role of the anions in the electronic structures of the compounds. The In 3d_5/2 binding energy for both compounds was the same, though the Na 1s binding energy of NaInO₂ was larger than that of NaInS₂ by 0.4 eV. The change in the Na Auger parameter, defined as the sum of Na 1s binding energy and KLL Auger kinetic energy, suggests that the difference in extra-atomic relaxation energy between the compounds causes the Na 1s binding energy shift. Discrete variational-Xα model cluster calculations for NaInO₂ and NaInS₂ mirrored the experimental valence band spectra and the difference of 0.5 eV in optical absorption energy of the compounds. The cluster calculation revealed that in NaInO₂ the highest occupied level is formed by an antibonding Na–O bond, whereas that in NaInS₂ is formed by a bonding Na–S bond. The covalent Na–S bond decreases the extra-atomic relaxation energy for Na 1s electron emission from NaInS₂.