Crystal Structures of Heavily Na-Loaded Low-Silica X (LSX) Zeolites in Insulating and Metallic States

Abstract

Crystal structures of sodium clusters incorporated in LSX zeolite [Na12Al12Si12O48 (Z = 8)] with FAU-type topology are analyzed by powder X-ray diffraction. The two different Na-loaded samples, Na9.4/Na12-LSX and Na16.7/Na12-LSX, show insulating and metallic states, respectively. The insulating state is changed to the metallic state upon increasing the loading density of Na atoms. The two samples have the space group Fd3̅ attributed to cubic symmetry and lattice constants of a = 2.49646(3) nm for Na9.4/Na12-LSX and a = 2.495060(11) nm for Na16.7/Na12-LSX. In both samples, three cation sites at the centers of two types of 6-ring of the sod-cage and the centers of the d6r prism are fully occupied by Na ions. In addition, a large number of Na cations forming Na clusters are distributed in the cavity (supercage), and Na distribution inside the cavity is disorderly, which is represented by several independent sites with low site occupancy. Electron-density distribution analysis clearly reveals the covalent bond of Si(Al)–O and disordered Na distribution in the cavity. In addition, electron densities based on the orbital hybridization between a Na cation and framework oxygen atom are observed around the 12-ring. In Na16.7/Na12-LSX, the formation of the chain-like Na cation distribution, which connects between the second neighboring cavities, is represented. However, this Na–Na connectivity is not formed in Na9.4/Na12-LSX. It was identified that the total number of Na cations in the cavity in Na16.7/Na12-LSX is much larger than that in Na9.4/Na12-LSX. The cation sites can be classified into two types of character: (a) close to the inner wall of cavity and (b) around the center of cavity and/or at the center of 12-ring pore window. In the domain (b), the number of cations remarkably increases with the increase in n. This change must be strongly related to the insulator-to-metal transition.