Metal-to-insulator crossover in alkali doped zeolite.

Zeolites are an important family of materials with periodic arrays of aluminosilicate cages that are widely used in different industrial processes1. Moreover, they also show interesting electronic phenomena, when intercalated with alkali metals, associated with the electronic states localized within individual cages. For example, exotic magnetism with different magnetically ordered states has been reported in alkali-doped zeolites2,3,4,5,6,7. With more than 200 possible zeolite frameworks known today8, alkali-doped zeolites thus represent a unique playground to control and study the effects of geometry and dopant concentration on the electronic potential depth, electron-electron repulsion and electron-phonon coupling at different length-scales. Since electronic states are associated to the alkali s-electrons, they remain confined to cages, just like alkali-metals that form clusters or superatoms. It is thus not surprising that the metallic zeolites have been elusive for many years with only one documented exception in rubidium doped zeolite rho, where the microwave conductivity measurements indicated the metallic ground state9. Very recently, the insulator-to-metal transition has been also reported in sodium loaded low-silica X (LSX) zeolite, Nan/Na12-LSX10,11. So far, experimental evidence for the metallic state was mainly limited to the observation of Drude reflection appearing in the infrared region10 and a drastic decrease in the resistivity11 for the heavily loaded samples. We stress that the measured resistivity is still very high and atypical of simple metals as it does not decrease with decreasing temperature. Additional hint of metallic ground state was provided by a precise x-ray diffraction analysis12, where it was shown that Na atoms make bonding network through the tunnel windows that connect zeolite cages and thus establish a precondition for a narrow conduction band. However, firm direct experimental evidence for the metallic state in Nan/Na12-LSX is still lacking. Nuclear magnetic resonance (NMR) is a powerful local-probe experimental tool to investigate a state of matter even in powder and highly air-sensitive samples. By measuring the temperature dependence of the NMR shift and spin-lattice relaxation time it is in principle possible to distinguish between insulating, metallic and superconducting states13,14,15,16. Unfortunately, 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 metals17,18 thus masking the conventional Korringa-like behaviour expected in the metallic state. Here we show for Nan/Na12-LSX that at room temperature the values of 23Na 1/T1 due to the Na motion indeed typically exceed by four orders of magnitude contributions from the coupling of nuclear magnetic moments to itinerant electrons in the metallic state. Cooling sample to cryogenic temperatures freezes out the atomic motions on the NMR time scale and 23Na 1/T1 finally discloses Korringa behaviour below 25 K thus proving the metallic ground state for n ≥ 14.2. Surprisingly, 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.