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

Abstract

In a porous crystal of zeolite low-silica X (LSX), β-cages and supercages (cavities) are arrayed in a diamond structure, respectively. We loaded potassium metal into zeolite LSX which has a chemical formula of Na 7.3 K 4.7 Al 12 Si 12 O 48 per β-cage (or supercage), and generated Na-K alloy clusters in β-cages and=or supercages. We have investigated the magnetic properties, the optical ones and the electrical resistivity at various values of K-loading density n per β-cage (or supercage) up to n = 9.7. Localized magnetic moments are observed at 8.2 < n < 9.7. Almost simultaneously, nearly pure ferromagnetism is observed at 8.4 < n < 9.7. The highest Curie temperature is ≈12 K at n ≈ 9. Optical reflection spectra for 8 < n have a new band at 2.8 eV which is assigned to the optical excitation of s-electrons of clusters generated at β-cages. The origin of the magnetic moments is assigned to these β-cage clusters because of the coincidence between the growths of the 2.8 eV band and localized magnetic moments. The origin of magnetic ordering is explained by a ferromagnetic interaction between β-cage clusters. All samples are found to be insulating from the temperature dependence of electrical resistivity. A direct magnetic interaction between β-cage clusters is not expected because of the high electronic barrier between them. A ferromagnetic superexchange coupling between β-cage clusters is newly proposed via the sp 3 -like closed-shell clusters at supercages. A thermal hysteresis is observed in the electrical resistivity at intermediate temperatures, and the origin is assigned to low-density carriers generated in the Na-K eutectic alloy structures in nanospace.

Electrons of alkali metals in regular nanospaces of zeolites

“…At much higher values of x (x > 4), some Na cations are distributed in supercages as well as β-cages. In Na-rich system at x = 7.3 (namely K n /Na 7.3 K 4.7 -LSX), nearly pure ferromagnetism has been observed at 8.4 < n < 9.7 [50]. A typical result at n = 8.9 is shown by blue curve in Figure 5(a).…”

“…β-cage clusters is proposed via the sp 3 -like closed-shell clusters at supercages [50]. The superexchange coupling between magnetic ions via ligand oxygen orbitals is known to be essential in the magnetism of transition metal oxides.…”

“…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 …”

Thermally Activated Motion of Sodium Cations in Insulating Parent Low-Silica X Zeolite

“…16) They are mutually connected through "windows" with diameters ranging from 2.8 Å to 8 Å. 16) The β cage sites are expected to be fully occupied with sodium cations, leaving no space for the intra-cage sodium jumps.…”

“…16) They are mutually connected through "windows" with diameters ranging from 2.8 Å to 8 Å. 16) The β cage sites are expected to be fully occupied with sodium cations, leaving no space for the intra-cage sodium jumps. 1) Therefore, the lowtemperature atomic motion observed in Na-LSX most likely involves sodium cations occupying some of the available sites within the supercage.…”