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 [Na 12 Al 12 Si 12 O 48 (Z = 8)] with FAU-type topology are analyzed by powder X-ray diffraction. The two different Na-loaded samples, Na 9.4 /Na 12 -LSX and Na 16.7 /Na 12 -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 Na 9.4 /Na 12 -LSX and a = 2.495060(11) nm for Na 16.7 /Na 12 -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 Na 16.7 /Na 12 -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 Na 9.4 /Na 12 -LSX. It was identified that the total number of Na cations in the cavity in Na 16.7 /Na 12 -LSX is much larger than that in Na 9.4 /Na 12 -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.

Ferromagnetism of Na–K Alloy Clusters Incorporated in Zeolite Low-Silica X

“…According to the structural analysis of Na n =Na 12 -LSX at n ¼ 9:4 and 16.7, the cation sites I and IA are fully occupied by Na cations at the same time, irrespective of a Coulomb repulsion between three Na cations at D6R. 21) This cation-rich β-cage structure is stabilized at higher loading densities, and may have a deeper electronic potential with a slight narrowing of potential size, because of a large cohesion energy of Na atoms. The slight increase from 2.5 to 2.8 eV in the optical excitation energy can be assigned to the slight narrowing of potential size.…”

“…Empty sites are occupied by these guest cations, and new cation sites also appear at higher loading densities, such as sites near the center of supercage in Na-loaded Na 12 -LSX. 21) Guest selectrons are well repelled by the negatively charged framework. The s-electrons in β-cages are well localized inside, but have a finite interaction with s-electrons in supercages through S6Rs.…”

Electrons of alkali metals in regular nanospaces of zeolites

“…The direct magnetic interaction between β-cage clusters, however, is impossible (bipartite lattice), because of the large separation by D6Rs, as shown in Figure 4 Temperature dependences of (a) magnetic susceptibility [42] and (b) 23 Na-NMR spectra [46] in Na n /Na 12 -LSX at n = 10 and ≈ 16. (c) Illustration of average distributions of Na cations in Na n /Na 12 -LSX at n = 9.4 and 16.7 extracted from the data in [47]. The radius of each Na atom is drown in proportion to the square root of the site occupancy.…”

“…A spin-lattice relaxation rate T −1 1 resembles the Bloembergen-Purcell-Pound (BPP)-type mechanism [110] with the activation energy ≈ 0.11 eV. A structure analysis of Na n /Na 12 -LSX has been performed at n = 9.4 and 16.7 [47]. Average distributions of Na cations are illustrated in Figure 7(c), where the radius of each atom is drown in proportion to the square root of the site occupancy, namely a cross section of each Na sphere is graphically proportional to the site occupancy.…”

“…A new paradigm of electronic system can be provided by the alkali-metal loading into zeolites. Up to now, comprehensive researches have been carried out , especially on magnetic properties [1,2,9,11,[13][14][15][16][17][18][23][24][25][26][27][28][29][30][31][34][35][36][37]39,41,[43][44][45][46]48,50,51], optical properties [1,3,7,24,38,49,53], electrical properties [42,43,49,50,52,53], structural analyses [8,12,22,47] and theoretical calculations …”