NMR study in the superconducting silicon clathrate compound ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{Ba}}_{\mathit{y}}$${\mathrm{Si}}_{46}$

${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$, ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}$, and ${}_{}{}^{135}{}_{}{}^{}\mathrm{Ba}$ NMR experiments were carried out in the superconducting silicon clathrate compound ${\mathrm{Na}}_{\mathit{x}}$ ${\mathrm{Ba}}_{\mathit{y}}$ ${\mathrm{Si}}_{46}$ to study the electronic states above the superconducting transition temperature. We observed three distinct ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ signals with different Knight shift (2036, 862, and 720 ppm at 90 K with a small temperature dependence of ∼10%), which were ascribed to inequivalent Si sites in the ${\mathrm{Si}}_{46}$ structure. The results indicate that the conduction-electron spin and/or charge density is strongly site-dependent. At all the atomic sites, the Korringa relation in NMR relaxation was observed, indicative of metallic electronic structure with moderate electronic correlation. The Na and Ba atoms are partially ionized in ${\mathrm{Na}}_{\mathit{x}}$ ${\mathrm{Ba}}_{\mathit{y}}$ ${\mathrm{Si}}_{46}$, which shows the noticeable difference from the system of alkali-metal-doped fullerides. The comparison with band calculations is discussed. © 1996 The American Instituteof Physics