Insulator-to-metal transition and magnetism of potassium metals loaded into regular cages of zeolite LSX

Zeolite LSX (low-silica X) crystals have an aluminosilicate framework with regular supercages and β-cages. They are arrayed in a double diamond structure. The loading density of guest K atoms per supercage (or β cage), n, can be controlled from 0 to ≈ 9. At n < 2, samples are nearly nonmagnetic and insulating. The Curie constant has a clear peak at n = 3, and the electrical resistivity suddenly decreases simultaneously. The electrical resistivity suddenly decreases again at n = 6 and shows metallic phase at n > 6. These properties are explained by the polaron effect including the electron correlation. Ferrimagnetic properties are observed at n ≈ 9. A remarkable increase in the resistivity is observed at very low temperatures at n ≈ 9, and is discussed in terms of the hypothesis of a Kondo insulator.