Preparation and characterization of the silicon clathrate NaxSi136 (x -> 0) A. Ammar, C. Cros, M. Pouchard, N. Jaussaud, J.-M. Bassat, G. Villeneuve and E. Reny J. Phys. IV France, 123 (2005) 29-34 DOI: 10.1051/jp4:2005123004