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J. Phys. Chem. B 2001, 105, 5174-5177
Origin of the Metallization of c-Axis Resistivity upon Iodine Intercalation into Bi2Sr2CaCu2O8+δ Jin-Ho Choy,* Seong-Ju Hwang, Sung-Ho Hwang, and Woo Lee National Nanohybrid Materials Laboratory, School of Chemistry and Molecular Engineering, Seoul National UniVersity, Seoul 151-747, Korea
Dongwoon Jung Department of Chemistry, Wonkwang UniVersity, Iksan, Joenbuk 570-749, Korea
Minhyea Lee and Hu-Jong Lee Department of Physics, Pohang UniVersity of Science and Technology, Pohang, Kyungbuk 790-784, Korea ReceiVed: February 7, 2001
The origin of the metallization of c-axis resistivity upon intercalation of iodine into Bi2Sr2CaCu2O8+δ has been studied by performing I LI-edge angle-resolved X-ray absorption spectroscopic (XAS) analysis and the tight binding band calculation with the extended Hu¨ckel method. According to the polarized I LI-edge XAS analysis, it has become clear that there is a significant anisotropy in the I 5p hole distribution of the intercalated iodine layer. Compared to the E⊥c spectrum, the E//c spectrum shows weaker intensity and higher energy for the white line feature corresponding to the 2s f 5pz transition, indicative of a strong interaction between the BiO layer and the intercalated iodine molecule along the c-axis. Such an interpretation is further supported by the band calculation results showing a significant hybridization between the Bi 6s orbital and the I 5pz one. On the basis of these findings, it is concluded that the orbital overlap between Bi 6s and I 5pz opens a conduction channel along the c-axis, which leads to the metallization of out-of-plane resistivity upon iodine intercalation.
Introduction One of the most striking features of high-Tc superconductors is their prominent anisotropy in physical properties as well as in crystal and electronic structures. Among various high-Tc superconductors, the Bi-based cuprates are found to be extremely anisotropic, since their normal state resistivity and magnetoresistivity for H//c show remarkable differences between the c-axis and the ab-plane.1,2 To explain such a large anisotropy in these compounds, several models assuming interlayer tunneling or weak coupling have been proposed for c-axis conduction.2-4 However, the validity of these theoretical models has remained unconfirmed. In this regard, the application of intercalation into Bi-based cuprates is expected to provide useful information on the mechanism for c-axis conduction, since the intercalation allows us to control the interlayer distance along the c-axis. Previously, X. D. Xiang et al. measured the anisotropic conductivity of iodine-intercalated Bi2Sr2CaCu2O8+δ (denoted as IBi2212), in comparison with that of pristine Bi2Sr2CaCu2O8+δ (denoted as Bi2212).5 They found that the iodine intercalation dramatically alters the semiconducting c-axis resistivity of the pristine compound to the metallic one, whereas it has little influence on the in-plane conduction. In an earlier report, the Anderson and Zou (AZ) model was applied to explain such a remarkable change in c-axis transport upon intercalation.4 However, this attempt proved to be unsuccessful, since the AZ model predicts that the 1/T dependence of c-axis resistivity * Author to whom correspondence should be addressed. Tel: +82-2880-6658. Fax: +82-2-872-9864. E-mail):
[email protected].
becomes more significant by expansion of the basal spacing, which surely conflicts with the observed metallization caused by iodine intercalation. The failure of this model suggests that, besides a change in interlayer coupling strength, there are other intercalation effects that affect transport properties such as modification of band structure. In this regard, the detailed electronic structure of IBi2212 should be probed by employing the experimental tools or theoretical calculations in order to understand the metallization of c-axis resistivity upon iodine intercalation. In this work, we have first measured the anisotropic resistivity of an iodine-intercalated Bi2212 single crystal by fabricating several nanosized stacks on the freshly cleaved surfaces of samples. Taking into account the fact that the severe elastic deformation during the intercalation reaction often causes considerable crystal defects, the present measurement technique with nanosized probes is very useful in verifying the intrinsic effect of intercalation on the c-axis transport property of Bibased cuprates.6 Second, the origin of the variation of c-axis resistivity upon intercalation has been studied by performing angle-resolved X-ray absorption spectroscopic (XAS) analysis complemented by tight-binding band calculations with the extended Hu¨ckel method. Experimental Section Single-crystalline Bi2212 was synthesized by the self-flux method as reported previously.7 Iodine intercalation into the pristine Bi2212 compound was achieved by reacting the host single crystal with excess iodine molecule in an evacuated
10.1021/jp010501m CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001
Iodine Intercalation into Bi2Sr2CaCu2O8+δ
J. Phys. Chem. B, Vol. 105, No. 22, 2001 5175
Figure 1. Variation of c-axis resistivity of Bi2212 upon iodine intercalation. The inset provides a schematic view of nanosized stacks fabricated on the crystal surface.
ampule at 190 °C for 6 days. The formation of single-phase Bi2212 and its iodine intercalate was confirmed by X-ray diffraction analysis using a Phillips PW3710 diffractometer equipped with Ni-filtered Cu KR radiation (λR1 ) 1.54056 Å). For the measurement of electrical resistivity, the platelets of a sample with a typical size of 0.7 × 0.2 × 0.03 mm3 were glued on MgO substrates using negative photoresist (OMR-83) and were cleaved with adhesive tape until optically smooth surfaces were obtained. Then, a 500 Å-thick layer of gold was thermally deposited on the crystal surface to protect it from contamination during the further fabrication process as well as to obtain a clean interface between normal electrodes and samples. Stacks with area 25 × 25 µm2 and height 15 nm were patterned using conventional photolithography with positive photoresist (Microposit 1400-23) and the Ar-ion-etching technique, as illustrated in the inset of Figure 1. The temperature dependence of the c-axis resistance of a stack was measured by the conventional lock-in technique in the temperature range 4-300 K. The present XAS data were recorded with synchrotron radiation by using the extended X-ray absorption fine structure facility (BL 7C) installed at Photon Factory in Tsukuba, operated at 2.5 GeV and 300-360 mA. The polarized spectra for singlecrystalline samples were measured using fluorescence mode while the unpolarized spectra for powder samples were obtained using transmission mode. To obtain freshly exposed sample surfaces, micaceous single crystals were cleaved and then carefully tiled on adhesive tape with sample area of 12 × 15 mm2 just before measurements were taken. All the XAS data were processed using the standard data reduction procedure, as reported previously.8 The tight binding band calculation based on the extended Hu¨ckel method was carried out by using CAESAR program written by Ren, J. and Whangbo, M. H. (1998, North Carolina State University). Results and Discussion The out-of-plane resistivity Fc of the pristine Bi2212 and its iodine intercalate is plotted as a function of temperature in Figure 1. There is a dramatic change in Fc before and after intercalation, as reported previously;5 the iodine intercalation gives rise to the metallization of semiconducting c-axis resistivity of Bi2212. Since the fabrication of a nanosized stack allows us to exclude the possible effect of crystal defects that might be formed during
Figure 2. The polarized I LI-edge XANES spectra of IBi2212: (a) E⊥c (θ ) 90°) and (b) E//c (θ ) 10°), together with the unpolarized spectra of polycrystalline (c) KI, (d) IBi2212, (e) I2, (f) KIO3, and (g) KIO4.
intercalation reaction, the metallic behavior displayed by Fc is surely attributable to the intrinsic transport property of IBi2212. According to the AZ model,4 the c-axis resistivity of bismuth cuprate is inversely proportional to the coupling strength between adjacent CuO2 layers as well as to the temperature. Since the intercalation would suppress the interaction between these layers through lattice expansion, the semiconducting behavior of Fc is expected to become enhanced, which is not consistent with the observed metallization of Fc upon iodine intercalation. Therefore, such a model based on the interlayer coupling theory cannot explain the variation of out-of-plane resistivity caused by iodine intercalation. To probe other effects of intercalation affecting the c-axis conduction, angle-resolved X-ray absorption near-edge structure (XANES) analyses have been carried out at the I LI-edge for the single-crystalline Bi2212 and its iodine intercalate (Figure 2). All the spectra except for KI exhibit a characteristic white line (WL) peak at around 5185-5195 eV, which is ascribed to the transition from the 2s core level to the unoccupied 5p state.8,9 The intensity of this WL feature is dependent on the density of the unoccupied 5p final state,8 and therefore the most intense peak is discernible for both KIO3 and KIO4 references, since their I 5p orbitals are completely empty. On the contrary, such a 2s f 5p transition is not possible for KI due to the full occupancy of the I 5p orbital. In the case of polycrystalline IBi2212, however, this peak shows a rather reduced intensity compared to the free iodine, indicating that the I 5p orbital of intercalated iodine is partially filled with electrons transferred from the host Bi2212 lattice with an electronic configuration of 5p5+x. As can be seen from Figure 2, the polarized spectra of IBi2212 single crystals show significant anisotropy in I 5p hole distribution with different WL peak positions at around 5186 and 5188 eV for the E⊥c and E//c polarization geometries,
5176 J. Phys. Chem. B, Vol. 105, No. 22, 2001
Choy et al.
TABLE 1: Lorentzian and Sigmoid Line Fitting Results of I LI-edge XANES Spectra for I2, IBi2212 with Polarization Geometries of E//c and E⊥c Lorentzian linea I2 IBi2212 (E//c) IBi2212 (E⊥c)
Sigmoid lineb
a0
a1 (eV)
E0 (eV)
b0
b1 (eV)
E1 (eV)
WL area (eV)c
hole (h)
total hole (h)
1.17 1.22 1.09
4.19 1.61 3.47
5186.54 5187.78 5186.41
0.98 0.94 1.00
0.91 0.30 1.74
5191.37 5190.46 5189.75
7.43 3.02 5.78
1.00 0.13d 0.52d
1 0.65
a The symbols a , a , and E represent the maximum amplitude, the full width at the half-maximum (fwhm), and the energy at a of the Lorentzian 0 1 0 0 line, respectively, determined by fitting the following Lorentzian equation to the normalized XANES data: f(E) ) a0[(w/2)2/((w/2)2+ (E - E0)2], where a0 is c/[π × (w/2)] with amplitude constant (c). b The symbols b0, b1, and E2 represent the step, the fwhm, and the inflection position of the sigmoid line in energy, respectively, determined by fitting the following sigmoid step function to the normalized XANES data: f(E) ) b0/[1 + exp{-(E - E1)}/b1]. c The WL area was calculated by integrating the Lorentzian line. d The amount of holes was estimated by imposing spectral weight (i.e., the number of contributing in-plane and out-of-plane 5p components) to the area of each WL peak.
TABLE 2: Atomic Parameters Used in the Calculations atom
orbital
Hii (eV)a
ζb
Bi
6s 6p 2s 2p 5s 5p
-21.2 -12.6 -32.3 -14.8 -18.0 -12.7
2.760 2.290 2.275 2.275 2.679 2.322
O I
b
a The symbol H (eV) represents valence orbital ionization potential. ii The symbol ζ represents exponent of the Slater-type orbital.12
Figure 3. The density of states curves weighted by overlap population for the I-Bi (solid lines) and I-O bonds (dashed lines). The Fermi energy (EF) is denoted by a vertical dotted line.
respectively. Judging from the geometry of the X-ray beam with respect to the crystal axes, these anisotropic WL features can be assigned as the 2s f 5px (or 5py) transition for E⊥c and the 2s f 5pz one for E//c, respectively. Compared to the E⊥c spectrum, the E//c spectrum shows a higher peak energy, even though in the case of free iodine molecule the energy of the antibonding 5px (or 5py) orbital is higher than that of the 5pz one due to the formation of a covalent I-I bond along the ab plane. In this context, the higher WL energy in the E//c spectrum can be regarded as evidence of a strong interaction between the BiO layer and the intercalated iodine molecule along the c-axis, leading to the marked destabilization of the antibonding 5pz state. On the other hand, the degree of electron transfer (i.e., the amount of hole density in iodine layer) could be quantitatively estimated by comparing each WL area for IBi2212 single crystal to that for free iodine. As summarized in Table 1, the out-of-plane (pz) and in-plane components (px, py) of the I 5p
Figure 4. The projected density of states (DOS) of the Bi 6s (solid lines) and I 5p (dashed lines) orbitals for (a) Bi2212 and (b) IBi2212. The Fermi energy (EF) is denoted by vertical dotted lines.
orbital contain different amounts of holes (h); 0.13 h for the former and 0.52 h for the latter. The smaller hole density in the I 5pz orbital provides further evidence of electron transfer from the Bi 6s orbital to the out-of-plane I 5pz orbital. To cross-confirm the above conclusion drawn from I LI-edge XANES analyses, we have also examined theoretically the electronic structure of IBi2212 by performing the tight binding band calculation based on the extended Hu¨ckel method. From the previous transmission electron microscopy investigations,10 it has been clarified that all the intercalated iodine atoms are sandwiched by oxygen or bismuth atoms to form O‚‚‚I‚‚‚O or Bi‚‚‚I‚‚‚Bi connections with bond distances of d(I-O) ) 3.1 Å and d(I-Bi) ) 3.0 Å. The intercalated iodine species are also found to form triiodide I3- chains aligned along the b-axis.10 On the basis of this crystal structure data,10 the crystal orbital
Iodine Intercalation into Bi2Sr2CaCu2O8+δ
J. Phys. Chem. B, Vol. 105, No. 22, 2001 5177 orbitals implies the marked overlap of both orbitals, which provides an electron conduction channel along the c-axis and hence induces a metallization of the c-axis resistivity. Such a significant overlap between both orbitals is further evidenced from the fragment molecular orbital (FMO) calculation for (Bi2O9)-12 and I3- performed on the basis of the local structure around the intercalant layer. As illustrated in Figure 5, there is a prominent hybridization between Bi 6s and I 5p orbitals, which gives rise to splitting orbitals in the molecular orbital diagram. In this regard, the higher peak energy in the E//c XANES spectrum can be interpreted as a result of hybridization between Bi 6s and I 5pz orbitals, which leads to the prominent destabilization of the final 5pz (or hybridized sp) state with respect to the nonhybridized 5px (or 5py) states. Summarizing the present experimental findings, it is certain that there is a significant interaction between I 5p and Bi 6s orbitals, which opens a conduction channel along the c-axis. Such an orbital overlap along the c-axis is responsible for the metallization of Fc upon iodine intercalation.
Figure 5. The fragment molecular orbital (FMO) calculated for (Bi2O9)-12 and I3- slabs.
overlap population (i.e., the density of states curves weighted by overlap population) was calculated for the I-Bi and I-O bonds in IBi2212.11 As shown in Figure 3, there is a strong overlap population below, above, and around EF for I-Bi, but the overlap population for the I-O bond is almost zero. Such findings clarify that the intercalated iodine interacts significantly with the host lattice through I 5pz and Bi 6s orbitals. In this respect, the degree and the direction of electron transfer between host and guest are interesting questions to solve. As plotted in Figure 4, the projected density of states (DOS) has been calculated for the I 5p and Bi 6s orbitals in IBi2212 as well as for Bi 6s orbital in Bi2212. In the pristine Bi2212 superconductor, the 6s and 6p bands of Bi are located well below and above EF, respectively,13 which is well consistent with the previous band calculation results showing that the oxidation state of Bi is fixed to +3.14 In the case of iodine intercalate, the 6s lonepair orbital of Bi interacts with I 5p and hence the resultant antibonding part of Bi 6s lies above EF. This implies that the Bi 6s band in IBi2212 is partially empty, indicating the increase of Bi oxidation state upon iodine intercalation. On the other hand, the amount of I 5p below EF is found to be somewhat larger than that expected for the neutral iodine atom. From the present DOS diagrams, it is evident that some electrons in the Bi 6s orbital flow to iodine to form an I3- linear chain. According to the valence shell electron pair repulsion theory, the mononegative I3- molecule is stabilized in the linear structure while the positively charged I3+ molecule shows a bent form. In this regard, the linear structure of molecular iodide intercalated in the Bi2212 lattice can be regarded as further evidence of electron transfer from host to iodine. Such an electron transfer becomes possible because the interaction between Bi 6s and I 5p is quite strong. From the viewpoint of electronic structure, this interaction between I 5pz and Bi 6s
Acknowledgment. This work was supported in part by the Ministry of Science and Technology through the 1999 National Research Laboratory (NRL) project and in part by the Ministry of Education through the Brain Korea 21 program. D. Jung thanks for the financial support from the Korea Research Foundation, made in the program year of 1998. And our thanks are extended to Prof. M. Nomura for helping us to get the XAS data in the Photon Factory. References and Notes (1) Martin, S.; Fiory, A. T.; Fleming, R. M.; Schneemeyer, L. F.; Waszczak, J. V. Phys. ReV. Lett. 1988, 60, 2194. (2) Briceno, G.; Crommie, M. F.; Zettl, A. Phys. ReV. Lett. 1991, 66, 2164. (3) Gray, K. E.; Kim, D. H. Phys. ReV. Lett. 1993, 70, 1693. (4) Anderson, P. W.; Zou, Z. Phys. ReV. Lett. 1988, 60, 132. (5) Xiang, X. D.; Wareka, W. A.; Zettl, A.; Corkill, J. L.; Cohen, M. L.; Kijima, N.; Gronsky, R. Phys. ReV. Lett. 1992, 68, 530. (6) Yurgens, A.; Winkler, D.; Claeson, T.; Hwang, S. J.; Choy, J. H. Int. J. Mod. Phys. B 1999, 13, 3758. (7) Fujii, T.; Nagano, Y.; Shirafuji, J. J. Crystal Growth 1991, 110, 994. (8) Hwang, S. J.; Park, N. G.; Kim, D. H.; Choy, J. H. J. Solid State Chem. 1998, 138, 66. (9) Liang; G.; Sahiner, A.; Croft, M.; Xu, W.; Xiang, X. D.; Badresingh, D.; Li, W.; Chen, J.; Peng, J.; Zettl, A.; Lu, F. Phys. ReV. B 1993, 47, 1029. (10) (a) Kijima, N.; Gronsky, R.; Xiang, X. D.; Wareka, W. A.; Zettl, A.; Corkill, J. L.; Cohen, M. L. Physica C 1991, 181, 18. (b) Stoto, T.; Pooke, D.; Kishio, K. Phys. ReV. B 1995, 51, 16220. (11) The atomic parameters used in the calculation are summarized in Table 2. (12) (a) Clementi, E.; Toetti, C. Atomic Data Nuclear Data Tables 1974, 14, 177. (b) McLeen, A. D.; McLeen, R. S. Atomic Data Nuclear Data Tables 1981, 26, 197. (13) Since the whole Bi 6p block bands in Bi2212 and IBi2212 are well above the energy range drawn in Figure 4, so the corresponding peaks are not shown. (14) Ren; J.; Jung, D.; Whangbo, M. H.; Tarascon, J. M.; Le Page, Y.; McKinnon, W. R.; Torardi, C. C. Physica C 1989, 159, 151.
