NMR study of thermally activated paramagnetism in metallic low-silica X zeolite filled with sodium atoms

Abstract

We report a ${}^{23}$Na and ${}^{27}$Al nuclear magnetic resonance (NMR) investigation of low-silica X (LSX) zeolite with chemical formula Na${}_{12}$Al${}_{12}$Si${}_{12}$O${}_{48}$ (Na${}_{12}$-LSX) loaded with $n$ additional guest sodium atoms. Na${}_{n}$/Na${}_{12}$-LSX exhibits an insulator-to-metal transition around $n=11.6$, which is accompanied by a significant enhancement of bulk magnetic susceptibility. Paramagnetic moments are in the metallic Na${}_{12}$/Na${}_{12}$-LSX thermally activated with an activation energy of around 0.1 eV. Simultaneously a new shifted component appears in the ${}^{23}$Na NMR, whose large and positive NMR shift scales with bulk magnetic susceptibility. Its spin-lattice relaxation rate $1/{T}_{1}$ is governed by the local-field fluctuations characterized by the same activation energy as obtained from the bulk magnetic susceptibility data. The time scale of these fluctuations is typical for atomic motions, which suggest strong electron-phonon coupling, a hallmark of polaron states. The insulator-to-metal transition in Na${}_{n}$/Na${}_{12}$-LSX is thus discussed within a polaron model.