Insulating state and metallic phase transition of heavily sodium-doped low-silica X (LSX) zeolites

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 approximate to 12. Nonmagnetic and insulating properties are observed for n less than or similar to 11. A drastic decrease in the resistivity is observed at n approximate to 12, 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. (C) 2012 Elsevier Ltd. All rights reserved.