Insulating state of Na clusters and their metallic transition in low-silica X zeolite

Optical and magnetic properties are investigated for low-silica X (LSX) zeolite. loaded with Na metals at various loading densities. In LSX, beta-cages with the inside diameter of similar to 7 angstrom are arrayed in a diamond structure and the supercages with that of similar to 13 angstrom are formed among them. When the average number of guest Na atoms per beta cage, n, is smaller than similar to 2, optical reflectance shows a peak at similar to 2.5 eV. This corresponds to the 1s-1p transition of the cluster formed in beta cage. At n > 2, this peak disappears and several strong peaks are seen at 1-3 eV. The oscillator strength increases with n. They can be attributed to surface-plasmon-like excitations of the Na clusters formed in supercages. At n <= 10, clear absorption tails with energy gaps are observed at near IR region indicating insulating states. However, when n is increased up to similar to 12, a clear Drude reflection suddenly appears in the IR region, indicating that an insulator-to-metal transition abruptly occurs. This dramatic change of the electronic state may be caused by the successive connection between adjacent clusters in supercages due to some special arrangement of Na+ ions as well as the delocalized wave function of electrons at high Na-loading density. (C) 2009 Elsevier Ltd. All rights reserved.