Search

Temperature dependences of NMR spectra have been observed for 23Na and 27Al in the metallic phase of Na–K form low silica X (LSX) zeolite loaded with potassium, where the condition of saturation is achieved with a loading level of 9.0 atoms per supercage and the paramagnetic moment contributes to the magnetism of the system beyond simple isolated spin. Two separated peaks have been recognized for 23Na, where the shift values show a quite linear relationship with susceptibility, and the so-called K–χ plot works quite well to give values of 0.32 kOe μB−1 and 0.40 kOe μB−1 for hyperfine coupling constants. Although no separated peak is seen in the 27Al NMR spectrum, the spectral centroid deviates to the positive side. The shoulder of the spectrum scales to susceptibility and the K–χ plot also works well to give a value of 0.15 kOe μB−1 for the hyperfine coupling constant. The orbital of potassium-originated electrons confined in the cage of LSX is understood as seeping out over the framework of zeolite, which is wider than that of the sodium-originated case. [ABSTRACT FROM AUTHOR]

Abstract: Na concentration (x) dependence of ferrimagnetic properties is investigated for Na–K alloy clusters incorporated in low-silica X (LSX) zeolite. In the LSX zeolite, β-cages of inner diameter ≈7Å are arranged in a diamond structure, and supercages of inner diameter ≈13Å are formed among them. The used LSX zeolite contains xNa+ and (12−x)K+ cations per β-cage or supercage. Guest nK atoms are loaded into the zeolite, namely the loading density is given by n per β-cage. The samples at x=4 have been reported to show Néel’s N-type ferrimagnetism in the specific region of n. This ferrimagnetism is explained by the model of antiferromagnetic coupling between two non-equivalent magnetic sublattices of clusters, the ones in β-cages and the others in supercages. In the present study, the value of x is changed from 4 to 0. Ferrimagnetic properties are found to show strong x-dependence. A systematic increase in loading densities of ferrimagnetic region is clearly observed with decreasing x. A remarkable change in temperature dependence of spontaneous magnetization is observed depending on x. Na+ cations are known to be mainly distributed in β-cages. Hence, the decrease in Na concentration is proposed to change the electronic potential depth for clusters in β-cage, which leads to important differences in the interaction between electrons localized in β-cages and those in supercages. [Copyright &y& Elsevier]

Abstract: Optical and magnetic properties are investigated for low-silica X (LSX) zeolite loaded with Na metals at various loading densities. In LSX, β-cages with the inside diameter of ∼7Å are arrayed in a diamond structure and the supercages with that of ∼13Å are formed among them. When the average number of guest Na atoms per β cage, n, is smaller than ∼2, optical reflectance shows a peak at ∼2.5eV. This corresponds to the 1s–1p transition of the cluster formed in β cage. At n>2, this peak disappears and several strong peaks are seen at 1–3eV. The oscillator strength increases with n. They can be attributed to surface-plasmon-like excitations of the Na clusters formed in supercages. At n≤10, clear absorption tails with energy gaps are observed at near IR region indicating insulating states. However, when n is increased up to ∼12, a clear Drude reflection suddenly appears in the IR region, indicating that an insulator-to-metal transition abruptly occurs. This dramatic change of the electronic state may be caused by the successive connection between adjacent clusters in supercages due to some special arrangement of Na+ ions as well as the delocalized wave function of electrons at high Na-loading density. [Copyright &y& Elsevier]

Abstract: Electrical resistivity was measured for Na-doped low-silica X (LSX) zeolite at various Na-loading densities. Many s-electrons are introduced into the regular array of zeolite nanocages by the loading of guest Na atoms. The average Na-loading density per unite cage, n, was changed from 0 to . Nonmagnetic and insulating properties are observed for . A drastic decrease in the resistivity is observed at , indicating that the insulating phase is changed to metallic one by the addition of less than one Na-atom per unite cage. At the same time, a remarkable increase in paramagnetic susceptibility is observed at room temperature. Insulating states are explained by spin-singlet clusters which are assigned to the self-trapped s-electron pairs (small bipolarons) due to the strong electron–phonon interaction against the Coulomb repulsive interaction between two s-electrons. Pairs of s-electrons occupy successively the quantum states of clusters in zeolite cages with increasing the Na-loading density. The metallic phase transition is explained by the formation of large polarons which are mobile over many cages through the zeolite windows. The increase in the paramagnetic susceptibility is explained by the thermal excitation of small polarons expected in the adiabatic potential. [Copyright &y& Elsevier]