Ferrimagnetic States of Na-K Alloy Clusters in Zeolite Low-Silica X

Abstract

In zeolite low-silica X (LSX), beta-cages with the inside diameter of approx 7 AA{} are arrayed in a diamond structure. Among them, supercages with the inside diameter of approx 13 AA{} are formed and arrayed in a diamond structure by the sharing of windows with the inside diameter of approx 8 AA{}. The chemical formula of zeolite LSX used in the present study is given by Na${}_x$K${}_{12-x}$Al${}_{12}$Si${}_{12}$O${}_{48}$ per supercage (or beta-cage), where Na${}_x$K${}_{12-x}$ and Al${}_{12}$Si${}_{12}$O${}_{48}$ are the exchangeable cations of zeolite LSX and the aluminosilicate framework, respectively. Na-K alloy clusters are incorporated in these cages by the loading of guest K metal at $n$K atoms per supercage (or beta-cage). A N'eel's N-type ferrimagnetism has been observed at $n=7.8$ for $x=4$. In the present paper, optical, magnetic and electrical properties are studied in detail mainly for $x=4$. Ferrimagnetic properties are observed at $6.5<n<8.5$. At the same time, the Curie constant suddenly increases. An optical reflection band of beta-cage clusters at 2.8 eV is observed at $n>6.5$ in accordance with the sudden increase in the Curie constant. An electrical resistivity indicates metallic values at $n$ gtrapprox 6, because a metallic state is realized in the energy band of supercage clusters. The ferrimagnetism is explained by the antiferromagnetic interaction between the magnetic sublattice of itinerant electron ferromagnetism at supercage clusters and that of localized moments at beta-cage clusters. The electrical resistivity in ferrimagnetic samples at $n=8.2$ for $x=4$ increases extraordinarily at very low temperatures, such as approx $10^6$ times larger than the value at higher temperatures. Observed anomalies in the electrical resistivity resembles the Kondo insulator, but itinerant electrons of narrow energy band of supercage clusters are ferromagnetic.

16 pages, 20 figures