Theoretical Study of Encapsulated Alkali Metal Atoms in Nanoporous Channels of ITQ-4 Zeolite: One-Dimensional Metals and Inorganic Electrides

Electronic structure calculations within density functional theory have been carried out in a class of {\it M}-ITQ-4 zeolite ({\it M} = Na, K, Rb, Cs) to understand the competing effects of guest-guest ({\it M}-{\it M}) and guest-host ({\it M}-ITQ-4) interactions. These compounds are known as inorganic electrides because the state of the valence electron of the alkali atom is manipulated by trapping the alkali atoms inside inorganic zeolite channels $^{1,2}$. We find that the arrangements of alkali atoms in the ITQ-4 zeolite channel change dramatically in going from Cs to Na. In Na-ITQ-4, the Na atoms form a nearly perfect 1D metal undergoing Peierls distortion and concomitant dimerization. However, in Cs-ITQ-4, the Cs atoms form a zig-zag chain and couple rather strongly to the host. The calculated geometry for Cs-ITQ-4 zeolite is in very good agreement with the pair distribution function (PDF) measurement $^3$. Optical absorptions have also been calculated which are in qualitative agreement with experiment. In addition to the guest-host high energy excitations ranging from 0.54 eV to 2.10 eV, we also find an infrared peak at 3300 nm, which should be carefully tested by experiments.\\ 1. A. S. Ichimura {\it et. al.}, J. Am. Chem. Soc. {\bf 124}, 1170 (2002).\\ 2. D. P. Wernette, {\it et. al.}, Chem. Mater. {\bf 15}, 1441 (2003).\\ 3. V. Petkov {\it et. al.}, Phys. Rev. Lett. {\bf 89}, 075502 (2002).