Abstract
We report a systematic nuclear magnetic resonance investigation of the 23Na spin-lattice relaxation rate, 1/T1, in sodium loaded low-silica X (LSX) zeolite, Nan/Na12-LSX, for various loading levels of sodium atoms n across the metal-to-insulator crossover. For high loading levels of n ≥ 14.2, 1/T1T shows nearly temperature-independent behaviour between 10 K and 25 K consistent with the Korringa relaxation mechanism and the metallic ground state. As the loading levels decrease below n ≤ 11.6, the extracted density of states (DOS) at the Fermi level sharply decreases, although a residual DOS at Fermi level is still observed even in the samples that lack the metallic Drude-peak in the optical reflectance. The observed crossover is a result of a complex loading-level dependence of electric potential felt by the electrons confined to zeolite cages, where the electronic correlations and disorder both play an important role.
Highlights
We report a systematic nuclear magnetic resonance investigation of the 23Na spin-lattice relaxation rate, 1/T1, in sodium loaded low-silica X (LSX) zeolite, Nan/Na12-LSX, for various loading levels of sodium atoms n across the metal-to-insulator crossover
The structure of 23Na Nuclear magnetic resonance (NMR) lineshape reflects the multitude of Na sites in the β cages and supercages of the LSX structure[12,21]
The predominately diamagnetic susceptibility of n = 11.6 sample suggests that the lineshape and the shift of the 23Na NMR spectrum are almost entirely determined by the nuclear chemical shift and quadrupole interactions
Summary
We report a systematic nuclear magnetic resonance investigation of the 23Na spin-lattice relaxation rate, 1/T1, in sodium loaded low-silica X (LSX) zeolite, Nan/Na12-LSX, for various loading levels of sodium atoms n across the metal-to-insulator crossover. In alkali-doped zeolites, the spin-lattice relaxation rate, 1/T1, is dominated by strong fluctuations of local magnetic fields and electric field gradients originating from large amplitude atomic motion of alkali metals[17,18] masking the conventional Korringa-like behaviour expected in the metallic state. A small portion of density of states (DOS) at the Fermi level persists deep into the insulating state This important finding that was not possible before with bulk-property measurements, holds important clues about the metal-to-insulator crossover in Nan/Na12-LSX, which is here discussed within the correlation-driven and disorder-driven aspects of metal-to-insulator transition (MIT)[19,20]
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