Theory of zeolite supralattices: Se in zeolite Linde type A

We study theoretically properties of Se clusters in zeolites, and choose zeolite Linde type A (LTA) as a prototype system. The geometries of free-space Se clusters are first determined, and we report the energetics and electronic and vibrational properties of these clusters. The work on clusters includes an investigation of the energetics Of C-3-C-1 defect formation in Se rings and chains. The electronic properties of two Se crystalline polymorphs, trigonal Se and alpha -monoclinic Se, are also determined. Electronic and vibrational properties of the zeolite LTA are investigated. Next we investigate the electronic and optical properties of ring-like Se clusters inside the large alpha -cages of LTA. We find that Se clusters inside cages of silaceous LTA have very little interaction with the zeolite, and that the HOMO-LUMO gaps (HOMO standing for highest occupied molecular orbital and LUMO for lowest unoccupied molecular orbital) are nearly those of the isolated cluster. The HOMO-LUMO gaps of Se-6, Se-8, and Se-12 are found to be similar, which makes it difficult to identify them experimentally by absorption spectroscopy. We find that the zeolite/Se-8 nanocomposite is lower in energy than the two separated systems. We also investigate two types of infinite chain encapsulated in LTA. Finally, we carry out finite-temperature molecular dynamics simulations for an encapsulated Se12 cluster, which shows cluster melting and formation of nanoscale Se droplets in the alpha -cages of LTA.