Effect of partial La filling on the local electronic properties of La x Co 4 Sb 12 studied using 59 Co NMR

We present a systematic investigation of the LaxCo4Sb12 skutterudites with x 6 0.2 using 59Co nuclear magnetic resonance (NMR) spectroscopy. The NMR line shape, the quadrupole splitting, the Knight shift and the spin–lattice relation rate of each compound have been identified. For LaxCo4Sb12, the central transition and the quadrupole linewidths increase with the La concentration because of the effect of the inhomogeneous electric field gradient. In addition, each linewidth exhibits a temperature-independent behavior, confirming the non-magnetic characteristic of these materials. The results of the isotropic Knight shift and the spin-lattice relaxation rate provide further information on the electronic structure around the Fermi surfaces of these compounds. In La0.1Co4Sb12 and La0.2Co4Sb12, the low-temperature NMR relaxation rates follow the Korringa behavior, indicating a finite density of carriers at the Fermi level. The high-temperature relaxation rates go over to a semiconductor-like activated form, being consistent with a semimetallic response. A detailed analysis of the spin-lattice relaxation rate established that the partial Co 3d Fermi-level density of states (DOS) increases with the La content. The NMR observations were interpreted in terms of a two-band scenario that consists of predominant d-electron features at low temperatures and exotic behavior of s-character electrons at high temperatures. Moreover, the obtained Fermi-level DOS show good agreement with those observed from the electrical conductivity and the Seebeck coefficient measurements in these La-filled skutterudites. 3 Author to whom any correspondence should be addressed. New Journal of Physics 10 (2008) 083029 1367-2630/08/083029+12$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft