Theoretical studies of Electronic Structures and Optical Absorptions of Encapsulated Guest Alkali Metal Atoms in ITQ-4 Z
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Theoretical studies of Electronic Structures and Optical Absorptions of Encapsulated Guest Alkali Metal Atoms in ITQ-4 Zeolite Hong Li and S. D. Mahanti Department of Physics and Astronomy, Michigan State University East Lansing, MI 48823 ABSTRACT 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. INTRODUCTION Experimental investigations [1,2,3] on alkali metal atoms absorbed inside pure silica zeolite channels have revealed many interesting magnetic and optical properties of these materials, which are called inorganic electrides. In addition, they might have wide range of commercial applications as reducing agents. NMR, optical absorption and pair distribution function (PDF) measurements on ITQ-4 [1,2,3] zeolite encapsulating alkali metal atoms such as Cs, Na and Rb, have provided insights into their electronic properties as well as the structures of the alkali metal atoms inside the channels. In order to understand the observed electronic, magnetic and optical properties and geometries of these systems, a careful ab-initio electronic structure calculation is necessary. A previous density functional theory (DFT) calculation [4] using local orbital functional method has given the geometry and band structure of Cs doped ITQ-4 zeolite, but its optical absorption and the electronic structures of other alkali metal guest atoms have not been studied yet. In the present work, a DFT calculation using plane wave basis sets and projector-augmented wave (PAW) [5,6] method has been carried out in the (ITQ-4 + 4Cs) and (ITQ-4 + 4Na) systems, and the electronic structures and optical absorption spectrum have been investigated and compared with each other. Crystalline ITQ-4 is an all-silica zeolite, which has a monoclinic I12/m1 space group and includes 32 silicon and 64 oxygen atoms in a unit cell, with lattice parameters of a=18.652Å, b=13.496Å, c=7.631Å, and α=90.0, β=101.98, γ=90.0 [7]. Each unit cell has 2 pseudo 1dimensional sinusoidal channels with pore diameter of ~7 Å, running parallel to the c-axis. The Cs or Na atoms are intercalated into the channels of the crystalline ITQ-4, with each c
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