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 thermally activated in the metallic Na_{12}/Na_{12}-LSX with an activation energy of around 0.1 eV. At the same time, a new shifted component (SC) 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 fluctuations determined by the same activation energy as obtained from the bulk magnetic susceptibility data. The timescale 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.