Journal of the Korean Physical Society, Vol 63, No 3, August 2013, pp 512∼516 Insulator-to-metal Transition and Magnetism of Potassium Metals Loaded into Regular Cages of Zeolite LSX Takehito Nakano, Duong Thi Hanh, Akihiro Owaki and Yasuo Nozue∗ Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, Japan Nguyen Hoang Nam Center for Materials Science, Faculty of Physics, Hanoi University of Science, Vietnam National University, Hanoi, Viet Nam Shingo Araki Graduate School of Natural Science and Technology, Okayama University, Okayama 700-0082, Japan (Received June 2012) Zeolite LSX (low-silica X) crystals have an aluminosilicate framework with regular supercages and β-cages They are arrayed in a double diamond structure The loading density of guest K atoms per supercage (or β cage), n, can be controlled from to ≈ At n < 2, samples are nearly nonmagnetic and insulating The Curie constant has a clear peak at n = 3, and the electrical resistivity suddenly decreases simultaneously The electrical resistivity suddenly decreases again at n = and shows metallic phase at n > These properties are explained by the polaron effect including the electron correlation Ferrimagnetic properties are observed at n ≈ A remarkable increase in the resistivity is observed at very low temperatures at n ≈ 9, and is discussed in terms of the hypothesis of a Kondo insulator PACS numbers: 75.50.Gg, 75.50.Ee, 75.30.Mb, 71.38.-k, 75.20.Hr, 82.75.Vx Keywords: Alkali metal, Cluster, Ferromagnetism, Ferrimagnetism, Polaron, Kondo lattice, Zeolite DOI: 10.3938/jkps.63.512 I INTRODUCTION LSX [5] and sodalite [6–8], respectively Zeolite LSX has the FAU framework structure with Si/Al = as shown in Fig 1(a) The chemical formula of framework is given as Al12 Si12 O48 per supercage (or β-cage) In LSX, β cages having the inside diameter of ≈7˚ A are arrayed in a diamond structure by the sharing of double 6-memberred-rings (D6MRs) Among them, the supercages of FAU with the inside diameter of ≈ 13 ˚ A are formed and arrayed in a diamond structure by the sharing of 12-membered-rings (12MRs) having the inside diameter of ≈ ˚ A The distance between adjacent supercages is 10.8 ˚ A which is shorter than the inside diameter of supercage Each β cage shares 6-memberedrings (6MRs) with adjacent four supercages The chemical formula of zeolite LSX containing K cations is given as K12 Al12 Si12 O48 per supercage (or β-cage) and is abbreviated as K12 -LSX, here In the present paper, we load guest nK-atoms into K12 -LSX The total chemical formula is given as K12+n Al12 Si12 O48 (abbreviated as Kn /K12 -LSX, here) When potassium metal is heavily loaded into Na4 K8 LSX, N´eel’s N-type ferrimagnetism has been observed and is explained by assuming two non-equivalent magnetic sublattices of clusters in β-cages and supercages Alkali metals loaded into the regular nanospace of zeolites exhibit exotic electronic properties that depend on the structure of zeolites, the loading density, and the alkali metals The aluminosilicate frameworks of zeolite crystals provide different types of regular arrays of nanocages, such as the double-diamond structure of β cages and supercages, the CsCl structure of α and β cages, and the body centered cubic structure of β cages in zeolites LSX (low-silica X), A and sodalite, respectively The aluminosilicate framework has negative charges by the number of Al atoms Exchangeable cations (positive ions), such as K+ , are distributed in the space of the framework for the charge neutrality The s-electrons of guest alkali metals are shared with the zeolite cations to form cationic clusters and are confined in the space of cages of the framework These s-electrons exhibit exotic magnetisms, although bulk alkali metals are nonmagnetic [1,2] Ferromagnetism, ferrimagnetism and antiferromagnetism have been observed in zeolites A [2–4], ∗ E-mail: nozue@phys.sci.osaka-u.ac.jp -512- Insulator-to-metal Transition and Magnetism of Potassium Metals · · · – Takehito Nakano et al -513- at n ≈ A sudden decrease in the electrical resistivity is observed again at n ≈ 6, and a metallic phase is observed at n > Ferrimagnetic properties are observed at n ≈ In addition, a remarkable increase in the electrical resistivity is observed at very low temperatures in the metallic phase at n ≈ 9, and a Kondo insulator model is discussed II EXPERIMENTAL PROCEDURES Fig (Color online) (a) Schematic illustration of the aluminosilicate framework of zeolite LSX having the FAU framework structure β-cages are arrayed in a diamond structure by the sharing of double 6-membered rings Among them, supercages are formed (b) Illustration of alkali-metal clusters stabilized in β-cages and supercages of LSX zeolite [8–10] When nNa atoms are loaded into Na12 -LSX (Nan /Na12 -LSX), the optical spectrum shows an insulating phase up to n ≈ 10 and suddenly changes to a metallic spectrum at n ≈ 12 [11] The electrical resistivity dramatically decreases by several orders of magnitude with increasing n from 11 to 12 [12] Many paramagnetic moments are thermally excited at n ≈ 12 [12] The insulating and non-magnetic phase at n < 11 is explained by the polaron effect as follows: An s-electron has a finite interaction with the displacement of cations, which is called the electron-phonon interaction A small polaron, which is a self-trapped state of an electron, can be stabilized when the electron-phonon interaction is large enough to trap the electron at the local lattice deformation induced by the electron itself [13] If the electronphonon interaction is weak, a large polaron is stabilized and moves freely The small polaron is immobile because of a large lattice distortion Two electrons can be self-trapped by the strong electron-phonon interaction, and the small bipolaron in the spin-singlet state is stabilized If the electron-phonon interaction is large enough to combine bipolarons, small multiple-bipolarons can be stabilized They are the case at n < 11 Large polarons, however, are stabilized at n > 11 in the metallic state, if multiple-bipolarons become unstable due to the increase in the Coulomb repulsion among electrons The thermal excitation of the paramagnetic susceptibility has been observed in the metallic state and is assigned to paramagnetic moments of thermally excited small polarons The anomalous paramagnetic behavior has been observed in NMR study of 23 Na [14] This insulatorto-metal transition and the thermal excitation of paramagnetic moments are explained by both the electron correlation and the electron-phonon interaction in the deformable structure of cations [13] In the present research, we have studied the magnetic property and the electrical resistivity in Kn /K12 -LSX A remarkable increase in the paramagnetic moments and a sudden decrease in the electrical resistivity are observed We used synthetic zeolite powder of Na12 -LSX which were checked in terms of the chemical analysis for Si/Al ratio and the X-ray analysis for structural quality and purity We exchanged Na cations with K ones in KCl aqueous solution many times in order to prepare K12 LSX The complete dehydration of the zeolite powder was made by heating at 500 ◦ C for one day under high vacuum Distilled K metal and dehydrated zeolite powder were sealed in a glass tube, and K metal was adsorbed into zeolite powder at 150 ◦ C through the vapor phase as well as the direct contact with the zeolite powder In order to improve the homogeneity of loading density of K metal, we performed the heat treatment of zeolite powder for two weeks Finally, we obtained a homogeneous K-loading The average loading density of nK atoms per supercage (or β cage) was controlled by adjusting the weight ratio of K metal and zeolite No residual K metal was seen in either the optical spectrum or the optical microscope image Samples for magnetic measurement were sealed in quartz glass tubes The DC magnetization was measured by using a SQUID magnetometer (MPMS-XL, Quantum Design) in the temperature range 1.8 - 300 K For the electrical resistivity measurements, the sample powder was put between two gold electrodes, and an adequate compression force ≈ MPa was applied during the measurements The electrical resistivity of the sample was obtained by multiplying the measured resistance by the dimensional factor (area/thickness) of compressed powder Due to the constriction resistance between powder particles, the observed electrical resistivity is about oneorder of magnitude larger than the true value The relative values, however, can be compared with each other, because of the constant compression force Because of the extreme air-sensitivity of the sample, the sample powder was kept in a handmade airproof cell These procedures were completed in a glovebox filled with pure He gas containing less than ppm O2 and H2 O Then, the cell was set in the sample chamber of Physical Property Measurement System (PPMS, Quantum Design) The sample temperature was controlled between 300 and K Impedance measurements on the cell were made by the 4-terminal measurement method by using Agilent 4824A LCR meter in the frequency range from 20 Hz to MHz and DC We analyzed the frequency dependence of the complex impedance by the Cole-Cole plot and checked -514- Journal of the Korean Physical Society, Vol 63, No 3, August 2013 Curie constant at n ≤ indicates that about 20% of supercages have magnetic moments of spin 1/2 Electrons in β cages are not observed in the optical spectra at low loading densities [11] In Fig 2(a), the Curie constant has a clear peak at n ≈ and quickly decreases at n ≈ The peak value at n ≈ amounts to ≈ 100% distribution of magnetic moments with spin 1/2 The Curie constant gradually increases for n > 4, and has the large value corresponding to ≈ 100% distribution of magnetic moments at n ≈ The Weiss temperature (TW ) estimated from the Curie-Weiss law is plotted in Fig 2(b) It shows small negative values up to n ≈ 8.5, and quickly decreases down to –10 K at n ≈ Spontaneous magnetization is clearly observed at n ≈ The extrapolated Curie temperature (TC ) is plotted in the same figure From the negative value of the Weiss temperature, the existence of an antiferromagnetic interaction is very clear Hence, the observed spontaneous magnetization is assigned to the ferrimagnetism, where two non-equivalent magnetic sublattices, possibly clusters in supercage- and β-cagenetworks, have an antiferromagnetic interaction through 6MRs, likely, N´eel’s N-type ferrimagnetism observed in Kn /Na4 K8 -LSX [8–10] Electrical resistivity Fig (Color online) (a) Loading density dependence of the Curie constant in Kn /K12 -LSX, and (b) that of the Curie (TC ) and the Weiss (TW ) temperatures the reliability of the resistivity at < 109 Ωcm A very small background resistivity, originating from the electric circuit inside the cell, was included at the order of 0.1 Ωcm This background was subtracted from the value III EXPERIMENTAL RESULTS AND DISCUSSION The electrical resistivity at 300 K is quite n-dependent as shown in Fig 3(a) The resistivity at n ≤ is very high, as expected from the optical spectrum [15], but suddenly decreases at n > in Fig 3(a) The resistivity gradually increases up to n = However, the resistivity suddenly decreases again at n ≈ and shows very small values at n > As shown in Fig 3(b), the resistivity at n = 6.2 is very low even at low temperatures This result implies that some amounts of carriers exist at low temperatures, indicating that a nearly metallic phase is realized at n > With the increase in n, the resistivity decreases at higher temperatures (T > 20 K), but quickly increases at very low temperatures (T < 20 K) At n = 9.0, the value at the lowest temperature is more than 100 times of those at higher temperatures This result clearly indicates that a very small gap, such as ≈ meV, exists at the Fermi energy Samples showing these strange temperature dependences exhibit ferrimagnetic properties as well Magnetic properties The Curie-Weiss behavior is observed in the temperature dependence of the magnetic susceptibility of Kn /K12 -LSX The loading density dependence of the Curie constant is estimated from the Curie-Weiss law and is plotted in Fig 2(a) If each supercage (or β-cage) has the magnetic moment of spin 1/2, the Curie constant is expected to be 3.21 × 104 Kemu/cm3 The observed Polaron effects In order to explain the high Curie constant and the low resistivity at n ≈ found in Figs 2(a) and 3(a), respectively, we propose the polaron effect for s-electrons in zeolite According to the theory of self-trapping of an electron in the deformable lattice [13], the self-trapped Insulator-to-metal Transition and Magnetism of Potassium Metals · · · – Takehito Nakano et al Fig (Color online) (a) Loading density dependence of the electrical resistivity at 300 K in Kn /K12 -LSX, and (b) the temperature dependence of the electrical resistivity at n = 6.2, 8.4, 9.0 Fig (Color online) Schematic illustration of adiabatic potentials for polarons expected at n < and n > in Kn /K12 -LSX See the text for the details -515- state (small polaron), can be stabilized in the case of a strong electron-phonon interaction In the small polaron, the depth of the deformation potential for electron must be deeper than the kinetic energy If the Coulomb repulsive interaction U between two electrons bound in the deformation potential well is smaller than the energy gain by the lattice distortion for two electrons at n < 2, the small bipolaron will be stabilized, as shown in Fig 4, where adiabatic potentials for different types of polarons are illustrated for n < and n > Small bipolarons have a heavy effective mass and are immobile They have a very small contribution to the electrical conductivity Small bipolarons have a closed electronic shell and are non-magnetic (spin-singlet) Hence, the hopping of an electron to neighboring small bipolaron states will be suppressed The small Curie constant and the high resistivity at n < in Figs 2(a) and 3(a), respectively, can be explained by small bipolarons However, at n ≈ 3, small tripolarons become more stable than small tetrapolarons, because the Coulomb repulsion energy among four electrons is significant in small tetrapolarons Tripolarons are paramagnetic and can contribute to the hopping conduction because of the open electronic shell Adiabatic potentials of these small multiple-polarons are illustrated schematically in Fig The increases in the Curie constant and the hopping conduction at n ≈ can be explained With increasing n, small multiple-polarons are generated successively These small multiple-polarons can become unstable suddenly above a certain critical value of n, and large polarons, which are mobile, may become stable, indicating that the stability of large polarons show the insulator-to-metal transition This type of insulatorto-metal transition has been observed in Nan /Na12 -LSX at n ≈ 12 [12] In Kn /K12 -LSX, a similar insulator-tometal transition may occur at n ≈ The smaller critical value of n in the K-system is due to the weaker electronphonon interaction compared with the Na-system The electrons (polarons) in supercages mainly contribute to the electrical conductivity, because of the large windows (12MRs) of supercages Electrons in β cages, however, may have no contribution to the conductivity, because of both the well-localized wave functions in β cages and the high potential barriers by D6MRs between them, as shown in Fig An electron in β cage can have magnetic moment and contribute to the remarkable increase in the Curie constant at higher loading densities in Fig 2(a) A sudden decrease in the resistivity at n ≈ 6, however, has no correlation to the Curie constant Hence, the insulator-to-metal transition is independent of β cage clusters, but occurs in the clusters in the supercage network The localized electronic state in β cage can have a finite hybridization with supercage electrons through 6MRs In order to explain the ferrimagnetism observed at n ≈ 9, an antiferromagnetic interaction through 6MRs is supposed between non-equivalent magnetic sublattices of clusters in β cages and supercages This interaction -516- Journal of the Korean Physical Society, Vol 63, No 3, August 2013 Kn /K12 -LSX The Curie constant has a clear peak at n ≈ 3, and the electrical resistivity suddenly decreases simultaneously A sudden decrease in the electrical resistivity is observed at n ≈ 6, and a metallic phase appears at n > These properties are explained by the polaron effect Ferrimagnetic properties are observed at n ≈ A remarkable increase in the resistivity is observed at very low temperatures at n ≈ This result is interpreted in terms of the hypothesis of the Kondo insulator ACKNOWLEDGMENTS Fig (Color online) Schematic illustration of density of states at the supercage network and the localized state at β-cage One-electron and two-electron states of β-cage cluster are located at below and above the Fermi energy of the supercage metallic network, where the Fermi energy is located at the center of the narrow band and the electron correlation in narrow β cage can lead to the model of the Kondo lattice, as discussed in the next section Possibility of a Kondo insulator As seen in Fig 3(b), the electrical resistivity at the metallic phase shows a remarkable increase at very low temperatures At least, a very narrow gap may exist at n ≈ 9, but no gap at n ≈ 6.2 Such a narrow gap at n ≈ is hardly expected from the ordinary electronic model Hence, we propose a model shown in Fig [5], where the Fermi energy is located at the center of the narrow band provided by the clusters in the supercage network, and the localized state at β cage is located below (above) the Fermi energy for one- (two-) electron state The Coulomb repulsion energy U is supposed for two electrons in the β cage Differently from the ordinary Kondo scheme, metallic electrons at the supercage network have the spin polarization, because both of the supercage and the β-cage networks have magnetic moments in the ferrimagnetic state If a small gap can be opened at the Fermi energy, likely a Kondo insulator, the electrical resistivity increases at very low temperatures This model is quite speculative, and further study is needed IV CONCLUSION Remarkable loading-density dependences are observed in the Curie constant and the electrical resistivity in This work was supported by Grant-in-Aid for Scientific Research (24244059 and 19051009) and by G-COE Program (Core Research and Engineering of Advanced Materials-Interdisciplinary Education Center for Materials Science) REFERENCES [1] T Nakano, N H Nam, T C Duan, D T Hanh, S Araki and Y Nozue, to be published in J Kor Phys Soc [2] Y Nozue, T Kodaira and T Goto, Phys Rev Lett 68, 3789 (1992) [3] Y Nozue, T Kodaira, S Ohwashi, T Goto and O Terasaki, Phys Rev B 48, 12253 (1993) [4] T Nakano and Y Nozue, J 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Mat 226-230, 229 (2001) ... sharing of double 6-membered rings Among them, supercages are formed (b) Illustration of alkali-metal clusters stabilized in β -cages and supercages of LSX zeolite [8–10] When nNa atoms are loaded into. .. observed at very low temperatures at n ≈ This result is interpreted in terms of the hypothesis of the Kondo insulator ACKNOWLEDGMENTS Fig (Color online) Schematic illustration of density of states at. .. value of n, and large polarons, which are mobile, may become stable, indicating that the stability of large polarons show the insulator-to-metal transition This type of insulatorto-metal transition