AN INVESTIGATION OF THE ELECTRONIC AND OPTICAL-PROPERTIES OF DEHYDRATED SODALITE FULLY DOPED WITH NA

Prolonged exposure of colorless dry sodalite to alkali vapor causes th e material to gradually turn blue, dark blue, and finally black. The b lue color observed at low sodium uptake appears because the absorbed s odium atoms are spontaneously ionized. The electron produced by ioniza tion is shared by the four sodium ions present in the sodalite cage (t hree initially there and the fourth originating from the absorbed atom ). The color center created in this way is represented by the formula (Na+)(4)eF(3-). Here, e stands for the electron and F3- for the negati vely charged frame surrounding a zeolite cage. At the highest loading, when each cage contains an absorbed alkali atom, the color centers ar e arranged in a body-centered cubic lattice, allowing the electrons as sociated with the centers to form bands. This may explain the black co lor observed at high concentration. In this paper we present measureme nts of the absorption coefficient of the black sodalite for photon ene rgies between 0 and 3 eV, and interpret them by performing one-electro n band structure calculations for a fully loaded compound. These calcu lations deal only with the ''solvated'' electrons. The effect of the o ther electrons is taken into account through an empirical potential en ergy representing the interaction of a solvated electron with the zeol ite frame. Because of this we study only the bands formed by the elect rons of the color centers. Since the gap in the electron energy bands of the dry sodalite is over 6 eV, the color of the black sodalite is c ontrolled by the solvated-electron bands formed in this gap. The measu red spectrum has a threshold of about 0.6 eV which seems to suggest th at the system has a gap in the electronic structure and is therefore a semiconductor. The calculations indicate, however, that, if the one-e lectron picture is valid, the fully doped black sodalite is a narrow-b and metal. The threshold in the spectrum appears because the transitio n matrix element is zero for transitions responsible for photon absorp tion, and not because of a gap in the density of states. The calculate d spectrum is in reasonable agreement with the measured one. Conclusio ns based on one-electron calculations can be altered by electron-elect ron interactions, which could turn a metal into an insulator. Two simp le criteria, proposed by Mott and Hubbard, were used to test whether t his transition might occur in our system. Unfortunately the results in dicate that the system is close to the transition region which means t hat predictions made by these simple criteria are not reliable. (C) 19 96 American Institute of Physics.