Theoretical studies of electronic structures and optical absorptions of encapsulated guest alkali metal atoms in ITQ-4 zeolite

An ab-initio study of the electronic structures and optical absorption spectrum of the guest Cs and Na atoms absorbed in the pure silica ITQ-4 zeolite channels was carried out using density functional theory (DFT) and plane wave basis. Completely different geometries were found for doped Cs and Na in the channels. For Cs doping, a zig-zag chain was formed along the channel and a metallic band structure was obtained. However, a dimer structure around the channel center was found for Na doping and a narrow band gap semiconductor was observed for this system. Correspondingly, the calculated optical absorption spectrum also showed distinctive characters for Cs and Na dopings. Our theoretical results for Cs doping are compared with experimental measurements and previous DFT calculation with local orbital basis; better agreement with experiments has been reached than the previous calculation. For the Na doping, our theoretical predictions need to be confirmed by future experiments.