Soft X-ray emission spectroscopy study of electronic structure of sodium borosilicide Na8B74.5Si17.5

Chemical bonding state of sodium borosilicide Na8B74.5Si17.5, which is a new member of B-12-cluster materials, is investigated by soft X-ray emission spectroscopy. The material is composed of B-12 cluster network and characteristic silicon chains of [-Si-(Si-Si)(3)-Si-] connected by sp(3) bonding, in which bonding distances and bonding angles are close to those in cubic Si crystal. B K-emission spectrum of the material showed a similar but a broader intensity distribution with those of B-12 cluster materials of alpha-r-B, B4C and ss-r-B. The broader intensity distribution can be due to a variation of B-B bond length in B-12 cluster. The density of states (DOS) of silicon chains of [-Si-(Si-Si)(3)-Si-] was experimentally derived. It shows a similar energy width, and peak or shoulder structures in intensity distribution with those of L-emission spectrum of cubic Si. From comparisons between experimental spectra and corresponding calculated DOS, covalent bonding between Si chain and B-12 cluster network is suggested. Those are discussed by using a theoretically calculated density of state of Na8B74.5Si17.5 by using WIEN2k code.