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

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.

Figure 1

Schematic structure of the FAU zeolite framework. The small circles with a plus sign indicate possible cation sites.

Figure 2

Temperature dependence of bulk magnetic susceptibility $\chi$ of Na${}_n$/Na${}_{12}$-LSX powder samples with (a) $n=10$ and (b) $n=12$. Solid lines represent fits to the experimental data using Eqs. (1) and (2). The $\chi$ values are corrected by subtracting the diamagnetic contributions ${\chi }_0=-1.13\times {10}^{-6}$ emu/cm${}^3$ and $-0.75\times {10}^{-6}$ emu/cm${}^3$, respectively. The major contributions to ${\chi }_0$'s are from the quartz sample holders.

Figure 3

Temperature dependence of ${}^{27}$Al (left) and ${}^{23}$Na (right) NMR spectra in metallic Na${}_{12}$/Na${}_{12}$-LSX. Strongly shifted and residual components of the ${}^{23}$Na NMR spectra are marked with SC and RC, respectively. The ${}^{23}$Na peak heights are at each temperature normalized according to the intensity of the RC component. For comparison room temperature ${}^{27}$Al and ${}^{23}$Na NMR spectra of the nonmagnetic insulating Na${}_{10}$/Na${}_{12}$-LSX sample are shown at the top. Please note the complete absence of the SC component in the ${}^{23}$Na NMR spectrum in this case. (Inset) The normalized ${}^{23}$Na nuclear magnetization versus pulse length at 300 K for SC (red circles) and RC (green squares).

Figure 4

Temperature dependence of (a) peak shift and (b) the full width at half maximum (FWHM), evaluated for ${}^{23}$Na SC (red circles), ${}^{23}$Na RC (green squares), and ${}^{27}$Al line (blue triangles). The SC appears only above 150 K.

Figure 5

(a) ${}^{23}$Na NMR shift of SC versus bulk magnetic susceptibility corrected for the diamagnetic contribution, $\chi -{\chi }_0$ in Na${}_{12}$/Na${}_{12}$-LSX. The solid line is a fit to $K=A\chi$ yielding $A=13.7$ kOe/${\mu }_B$. (b) Temperature dependence of the spectral intensity corrected by the Boltzmann factor for ${}^{23}$Na SC (red circles), ${}^{23}$Na RC (green squares), and ${}^{27}$Al line (blue triangles) in Na${}_{12}$/Na${}_{12}$-LSX.

Figure 6

Temperature dependence of the ${}^{23}$Na $T_1^{-1}$ for SC (open symbols) and RC (solid symbols) measured in different magnetic fields: 2.35 T (circles), 4.7 T (diamonds), 6.34 T (triangles), and 9.4 T (squares). Solid lines represent a global fit with the BPP model, Eqs. (4) and (5). (Inset) Time dependence of ${}^{23}$Na magnetization recovery measured for the SC line at 270 K. The solid line is a fit to a single exponential recovery function $R\left(t\right)=1-M\left(t\right)/M\left(\infty \right)\propto \exp (-t/T_1)$ where $M\left(\infty \right)$ is the corresponding saturated ${}^{23}$Na magnetization and $T_1=0.25\pm 0.01$ ms.