Insulating state of Na clusters and their metallic transition in low-silica X zeolite

Nakano, T.; Mizukane, Takahiro; Nozue, Yasuo

doi:10.1016/j.jpcs.2009.12.059

Cited by 22 publications

(29 citation statements)

References 15 publications

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“…The temperature dependence of electrical resistivity is measured at various values of n [42], and revised n-dependences at 300 and 200 K are indicated in Figure 8(a). The resistivity suddenly decreases with n around n ≈ 11.5 by several orders of magnitude, and a metallic phase has been observed at n 12 in accordance with the optical spectra in [38].…”

Section: Imt and Thermally Excited Paramagnetism In Na-loaded Zeolitesupporting

confidence: 78%

“…A sudden change to the metallic state, however, has been observed in the optical reflection spectra with the increase in n, and a Drude-type optical reflection spectrum has been observed at IR-region for n ≈ 16 [38] (the value n ≈ 12 in [38] is revised here). A paramagnetic susceptibility is found to increase with temperature at n ≈ 16, as shown in Figure 7(a), where the temperature-independent susceptibility χ 0 is subtracted [42].…”

Section: Imt and Thermally Excited Paramagnetism In Na-loaded Zeolitementioning

confidence: 81%

“…As the deformation potential interaction in Na system is expected to be larger than that in K system, small bipolarons and successive small multiple-bipolarons can be stabilized up to higher electron densities. An insulating and non-magnetic phase in Na n /Na 12 -LSX at n 11 is explained by small multiple-bipolarons stabilized in supercages [38,42]. The stability of multiple-bipolaronic states is assigned to the strong electron-phonon interaction conditions U < S and 2B < 2S − U in the t-U-S triangle.…”

Section: Imt and Thermally Excited Paramagnetism In Na-loaded Zeolitementioning

confidence: 92%

“…A new paradigm of electronic system can be provided by the alkali-metal loading into zeolites. Up to now, comprehensive researches have been carried out , especially on magnetic properties [1,2,9,11,[13][14][15][16][17][18][23][24][25][26][27][28][29][30][31][34][35][36][37]39,41,[43][44][45][46]48,50,51], optical properties [1,3,7,24,38,49,53], electrical properties [42,43,49,50,52,53], structural analyses [8,12,22,47] and theoretical calculations …”

Section: Introductionmentioning

confidence: 99%

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Electrons of alkali metals in regular nanospaces of zeolites

Nakano

Nozue

2017

Advances in Physics: X

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The s-electrons of alkali metals loaded into regular nanospaces (nanocages) of zeolite crystals display novel electronic properties, such as a ferrimagnetism, a ferromagnetism, an antiferromagnetism, an insulator-to-metal transition, etc., depending on the kind of alkali metals, their loading density, and the structure type of zeolite frameworks. These properties are entirely different from those in bulk alkali metals of freeelectrons. Alkali-metal clusters are stabilized in cages of zeolites, and new quantum states of s-electrons, such as 1s, 1p, and 1d states in the spherical quantum-well model, are formed. An electron correlation, a polaron effect, and an orbital degeneracy in the quantum states of s-electrons play critical roles in taking on the novel electronic properties. Electronic properties can be overviewed systematically by a coarse-grained model of localized s-electron states in cages based on the tightbinding approximation, followed by the t-U-S-n diagram of the correlated polaron system given by the so-called HolsteinHubbard Hamiltonian: an electron transfer energy through windows of cages (t), a Coulomb repulsion energy between two s-electrons in the same cage (U), a short-range electron-phonon interaction energy due to the cation displacements (S), and an average number of s-electrons per cage (n). Beyond the jellium background model of alkali-metal clusters, a huge spin-orbit interaction has been observed in the 1p degenerate orbitals of clusters, similarly to the Rashba mechanism. ARTICLE HISTORY

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Section: Imt and Thermally Excited Paramagnetism In Na-loaded Zeolitesupporting

confidence: 78%

Section: Imt and Thermally Excited Paramagnetism In Na-loaded Zeolitementioning

confidence: 81%

Section: Imt and Thermally Excited Paramagnetism In Na-loaded Zeolitementioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

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Electrons of alkali metals in regular nanospaces of zeolites

Nakano

Nozue

2017

Advances in Physics: X

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“…1,2) The emergence of a Drude peak in optical reflectivity and finite resistivity at low temperature were demonstrated on the macroscopic scale only at high loading levels of guest sodium atoms (n > 14.2). 1) On the other hand, a continuous (crossover like) evolution from insulating to metallic ground state has been observed with a local-probe nuclear magnetic resonance (NMR) spectroscopy by extracting the electron density of states (DOS) from low-temperature spin-lattice relaxation data. 2) The observed crossover in alkali-doped zeolites is a result of a complex loading-level dependence of electronic correlations, disorder and electron-phonon coupling.…”

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confidence: 99%

Thermally Activated Motion of Sodium Cations in Insulating Parent Low-Silica X Zeolite

et al. 2017

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We report a 23 Na spin-lattice relaxation rate, T −1 1 , in low-silica X zeolite. T −1 1 follows multiple BPP-type behavior as a result of thermal motion of sodium cations in insulating material. The estimated lowest activation energy of 15 meV is much lower than 100 meV observed previously for sodium motion in heavily Na-loaded samples and is most likely attributed to short-distance jumps of sodium cations between sites within the same supercage.Na-type low-silica X (LSX) zeolite loaded with guest Na atoms represents one of the first examples of an elusive metallic behavior in the zeolite systems. 1, 2) The emergence of a Drude peak in optical reflectivity and finite resistivity at low temperature were demonstrated on the macroscopic scale only at high loading levels of guest sodium atoms (n > 14.2). 1) On the other hand, a continuous (crossover like) evolution from insulating to metallic ground state has been observed with a local-probe nuclear magnetic resonance (NMR) spectroscopy by extracting the electron density of states (DOS) from low-temperature spin-lattice relaxation data.2) The observed crossover in alkali-doped zeolites is a result of a complex loading-level dependence of electronic correlations, disorder and electron-phonon coupling.In alkali-doped zeolites the spin-lattice relaxation rate, T −1 1 , is dominated by strong fluctuations of local magnetic fields and electric field gradients originating from large amplitude atomic motion, 3, 4) which can be modeled within the socalled Bloembergen-Purcell-Pound (BPP) relaxation theory. 5) In sodium doped LSX zeolites the atomic motion is frozen on the NMR time scale at very low temperatures and thus in this temperature range other relaxation mechanisms begin to dominate. For instance, below 25 K, 23 Na T −1 1 shows Korringa behavior for n ≥ 14.2, thus proving the metallic ground state. 2) Surprisingly, a small portion of DOS at the Fermi level persists deep into the insulating state (9.4 ≤ n ≤ 11.6), where the optical measurements show no Drude peak. In order to elucidate the nature of atomic motion, this work focuses on parent (n = 0) sample of sodium LSX zeolite.The parent or non-loaded sodium LSX zeolite has a chemical formula Na 12 Al 12 Si 12 O 48 (abbreviated Na 12 -LSX) with twelve sodium cations in the structure, which are required for charge compensation of the aluminosilicate framework. When Na 12 -LSX is exposed to Na vapour, a controlled amount of n * igarashi@elc.gunma-ct.ac.jp † peter.jeglic@ijs.si sodium atoms per zeolite supercage 6) can be loaded, yielding a composition Na n /Na 12 -LSX. As shown below, the thermal motion of charge compensating sodium cations fully explains the observed 23 Na spin-lattice relaxation in the nonloaded compound without any hints of residual conduction electron contribution. Present result thus sets the benchmark for the 23 Na T 1 against which all other relaxation mechanisms in Na n /Na 12 -LSX samples have to be evaluated. For the purpose of NMR measurements the non-loaded, just dehydrated, Na-LSX sample ...

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