Evolution of Superconducting Transition Temperature (Tc) upon

Seong-Ju Hwang, Dae Hoon Park, and Jin-Ho Choy ... Jin-Ho Choy, Young-Il Kim, Seong-Ju Hwang, Yuji Muraoka, Naoyuki Ohnishi, Kenji Hiraga, and Pham V...
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J. Phys. Chem. 1996, 100, 3783-3787

3783

Evolution of Superconducting Transition Temperature (Tc) upon Intercalation of HgBr2 into the Bi2Sr1.5-xLaxCa1.5Cu2Oy Jin-Ho Choy,* Nam-Gyu Park, and Seong-Ju Hwang Department of Chemistry, Center for Molecular Catalysis, College of Natural Sciences, Seoul National UniVersity, Seoul 151-742, Korea

Zheong G. Khim Department of Physics, College of Natural Sciences, Seoul National UniVersity, Seoul 151-742, Korea ReceiVed: September 19, 1995; In Final Form: NoVember 13, 1995X

Intercalation of HgBr2 into Bi2Sr1.5-xLaxCa1.5Cu2Oy (0.0 e x e 0.4) superconductor has been carried out in order to elucidate the origin of Tc evolution upon intercalation. The Tc’s obtained from the dc magnetic susceptibility measurements were plotted against x. The plot of Tc vs x for the pristines showed the parabolic feature with overdoped (0.0 e x < 0.1), optimally doped (x ) 0.1), and underdoped (0.2 e x e 0.4) regions. The Tc’s of the HgBr2 intercalates in the overdoped region were reduced less than ∼6 K but increased by 4-6 K in the underdoped one compared with nonintercalated samples. Such changes in Tc upon intercalation indicate hole doping from intercalant to host lattice. An attempt of semiempirical calculation was made to determine the hole concentration doped by intercalation. Upon HgBr2 intercalation, the amount of hole doping was estimated to be ∼0.2 hole per formula unit of the sample with x ) 0.0, whereas the doping of ∼0.3 hole was estimated for the iodine intercalated sample. Considering the Tc depression (∆Tc) and lattice expansion (∆d) between the iodine intercalate (∆Tc ≈ 10 K and ∆d ≈ 3.6 Å) and the HgBr2 one (∆Tc ≈ 6 K and ∆d ≈ 6.3 Å), it can be concluded that the change in Tc upon intercalation clearly depends on the hole concentration due to the charge transfer between host and guest, rather than the interblock electronic coupling due to the lattice expansion.

Introduction Recently, we have developed the new superconducting intercalation compounds with the stoichiometry of (HgX2)0.5Bi2Sr2CaCu2Oy (X ) Br and I)1 and AgIBi2Sr2CaCu2Oy2-4 in which metal salt sheets are stabilized between the (Bi-O) double layers. From X-ray diffraction analysis, the newly prepared intercalation compounds have been determined to be single phases with a remarkable expansion along the c axis of 6.30, 7.15, and 7.35 Å upon HgBr2, HgI2, and AgI intercalation, respectively, which are almost twice as large as that for the previous iodine intercalated system with a 3.6 Å expansion.5,6 The most striking result obtained from the magnetic susceptibility measurements for the mercury salt intercalated compounds is that the intercalation of mercury salt does not alter the Tc to any significant degree before (Tc ) 76 K) and after (Tc ) 71 K) intercalation even with the large lattice expansion. Comparing the Tc depression (∆Tc) and lattice expansion (∆d) between IBi2Sr2CaCu2Oy and (HgX2)0.5Bi2Sr2CaCu2Oy, the small ∆Tc of about 5 K even with a large ∆d of ca. 7 Å for the latter compounds contrary to the large ∆Tc of about 10 K with a small ∆d of ca. 3.6 Å for the former one indicates that the Tc of the layered Bi2Sr2CaCu2Oy can be hardly tuned by an expansion along the crystal c axis upon intercalation. According to the X-ray absorption near edge structure (XANES) and X-ray photoelectron (XPE) spectroscopic studies of the (HgX2)0.5Bi2Sr2CaCu2Oy, the intercalated HgI2 or HgBr2 is found to be slightly reduced in the form of molecular ion (HgX2)δ-, but not significantly.7 This implies that a small hole doping from the intercalated HgX2 to the copper-oxygen sheet * To whom all correspondence should be addressed. E-mail: [email protected] X Abstract published in AdVance ACS Abstracts, January 15, 1996.

0022-3654/96/20100-3783$12.00/0

of the pristine results in the small Tc depression, in other words, the superconducting property of Bi2Sr2CaCu2Oy is associated with the copper valence as in the case of the superconductivitycopper valence relation in La2CuO4.8,9 On the basis of the recent results on the HgX2-intercalated system, it is obvious that the Tc variation is closely related to the change in hole concentration of the Bi2Sr2CaCu2Oy lattice and at least the effect of interblock electronic coupling may be negligible in this system. To get decisive evidence for the major effect on Tc depression, charge transfer, or interblock coupling upon intercalation, we intended to investigate how Tc is changed about when mercury salt is intercalated into the Bi2Sr2CaCu2Oy phases with a different hole concentration. As reported previously,10-18 it is well-known that the substitution of trivalent metal cations A3+ (A ) La , Y, Eu, Dy, or Tm) for Sr2+ or Ca2+ in Bi-Sr-Ca-Cu-O superconductors has led to a parabolic feature in the Tc vs hole concentration diagram, in which the cation substitution rate can realize the optimum hole concentration (nop) at Tc ) Tcmax, hole-overdoped and -underdoped region with n > nop and n < nop, respectively. Therefore, it is interesting to compare the Tc change for the holeunderdoped samples with that for the hole-overdoped samples upon the HgBr2 intercalation. This is because if the Tc change upon the HgBr2 intercalation is due to the weakening of interblock coupling owing to the expansion along the c axis, Tc is expected to decrease equally regardless of hole concentration, while if the Tc change is due to the hole doping by charge transfer, it is expected that the maximum Tc shift to holeunderdoped region for attaining the optimum hole concentration. In this paper, we investigate the Tc evolution upon the HgBr2 intercalation into the La-substituted Bi2Sr1.5-xLaxCa1.5Cu2Oy with the cation-substitution range 0.0 e x e 0.4. © 1996 American Chemical Society

3784 J. Phys. Chem., Vol. 100, No. 9, 1996

Choy et al.

Figure 2. Variation of lattice parameter c with respect to x in the pristines (filled circles with error bar) and their HgBr2 intercalates (empty circles with error bar).

Figure 1. Powder X-ray diffraction patterns for (a) the pristines and (b) their HgBr2 intercalates, (HgBr2)0.5Bi2Sr1.5-xLaxCa1.5Cu2Oy.

Experimental Section Polycrystalline samples of Bi2Sr1.5-xLaxCa1.5Cu2Oy (0.0 e x e 0.4) were synthesized by the conventional solid-state reaction and used as the pristine materials for the intercalation of HgBr2. The powder reagents of Bi2O3, SrCO3, La2O3, CaCO3, and CuO were mixed with atomic ratio of Bi:Sr:La:Ca:Cu ) 2:1.5 - x:x: 1.5:2, and the mixed powder was calcined at 800 °C for 12 h in air. After calcination, the prefired materials were pressed into 13 mm disk-shape pellets and sintered at 860-900 °C for 40 h depending on the La substitution rate. All the samples were characterized by powder X-ray diffraction (XRD) and were found to be single phase. The intercalation of HgBr2 was performed by heating the vacuum sealed tube containing the pristine samples of Bi2Sr1.5-xLaxCa1.5Cu2Oy and five equivalent of HgBr2 per formula unit of host sample. The intercalated compounds were obtained by heating at 235 °C for 4 h and confirmed to be single phase by XRD analysis. The amount of guest species, HgBr2, introduced into the pristine was determined quantitatively by thermogravimetric analysis (TGA) with a DuPont 2000 thermal analysis station. To gain information about the intracrystalline structure of intercalated HgBr2, X-ray absorption spectroscopy (XAS) experiment was carried out on the beam lines 7C and 10B at the Photon Factory, National Laboratory for High Energy Physics in Tsukuba, with a storage ring providing a 2.5 GeV electron beam at a current of about 300-360 mA. The Br K-edge and Hg LIII-edge spectra were obtained with a silicon (311) channel-cut monochromator. The extended X-ray absorption fine structure (EXAFS) data converted to k space were fitted by a curved wave ab initio EXAFS code FEFF 5.19 The dc magnetic susceptibilities of the pristine compounds and their intercalates were measured by a superconducting quantum interference device (SQUID) in the applied field of 20 Oe. Results and Discussion Figure 1 shows the powder XRD patterns for the pristines of Bi2Sr1.5-xLaxCa1.5Cu2Oy and their HgBr2 intercalates. Due to the intercalation of HgBr2 into the interlayer space of the (Bi-

Figure 3. One-dimensional Fourier map for (a) the pristine and (b) its HgBr2 intercalate together with the corresponding schematic structures.

O) double layers of the pristines, the (00l) reflection patterns for the intercalates shift to lower angles, indicating a lattice expansion along the c axis with the basal increment of ∼6.3 Å (Figure 2), and it was also confirmed from one-dimensional Fourier map (Figure 3). According to the XRD analysis, all the HgBr2 derivatives could be indexed as tetragonal symmetry. As shown in Figure 2, the c-axis parameters for the pristines are found to decrease continuously as the x value increases, which can be understood by the fact that the Sr2+ ion is replaced partly by the relatively smaller La3+ ion (Sr2+(8) ) 1.25 Å, La3+(8) ) 1.18 Å, where the number in parentheses represents the coordination number20). Upon HgBr2 intercalation, the c-axis parameter is drastically expanded with a basal increment of ca. 6.3 Å for all the HgBr2 intercalates regardless of x values which is almost twice as large as the iodine intercalate. Figure 4 shows the TGA curve for the intercalate with x ) 0.0 in the temperature range 20-800 °C. The total weight loss is determined to be 17% which is in good agreement with the calculated value of (HgBr2)0.5Bi2Sr1.5Ca1.5Cu2O8 (17.3%). For the above stoichiometric intercalate, an effort was made to understand the intracrystalline structure of the intercalated HgBr2 by EXAFS analysis, since the EXAFS can provide a useful structural information which can hardly be determined by powder XRD. The Fourier-filtered Hg LIII-edge EXAFS spectrum for the (HgBr2)0.5Bi2Sr1.5Ca1.5Cu2Oy is shown in Figure 5, where the EXAFS oscillation is resulted from the inverse Fourier transformation of the peak corresponding to the first shell in the Fourier transformed EXAFS in the course of the

Intercalation of HgBr2 into the Bi2Sr1.5-xLaxCa1.5Cu2Oy

J. Phys. Chem., Vol. 100, No. 9, 1996 3785

Figure 4. Weight change for the HgBr2 intercalate, Bi2Sr1.5-xLaxCa1.5Cu2Oy with x ) 0.0. (heating at a rate of 10 °C/min in nitrogen atmosphere).

Figure 6. Temperature dependence of the dc susceptibility for (a) the pristines and (b) their HgBr2-intercalates.

Figure 5. Fourier-filtered k3χ(k) data for the first shell of the Fourier transformed Hg LIII-edge EXAFS spectrum for (HgBr2)0.5Bi2Sr1.5-xLaxCa1.5Cu2Oy with x ) 0.0: the experimental data (open circle) are superimposed on the fitted one (solid line).

curve-fitting analysis to determine quantitatively the structural parameters such as bond length, coordination number, and Debye-Waller factor. From the result of EXAFS curve-fitting, it was found that the mercury in the HgBr2 intercalate is coordinated with two bromine atoms with the bond distance (Hg-Br) of 2.46 Å which is almost the same with the HgBr2 in vapor state with the coordination number of 2 and the bond distance of 2.44 Å, but different from the solid-state structure of HgBr2 with a distorted octahedron having two short bond lengths of 2.48 Å and four long ones of 3.23 Å. Therefore, the intercalated mercuric bromide is found to be stabilized in the form of a linear molecule inbetween the interlayer space of (BiO) double layers. Figure 6 shows the temperature-dependent dc susceptibilities of the pristines and their HgBr2 intercalates with x ) 0.0, 0.1, 0.2, 0.3, and 0.4, respectively. All the samples retain their superconducting properties before and after the intercalation regardless of the x values. The superconducting transition temperature (Tc) is determined from the mean value between 1% and 10% of the normalized χ(T)/χ(5 K) value instead of onset temperature because the heavily substituted compounds exhibit a rather broad transition behavior. Comparing the Tc’s before and after intercalation, the intercalation of HgBr2 into the heavily substituted samples with x g 0.3 enhances the Tc

Figure 7. Diamagnetic transitions, before ([) and after (]) the intercalation of HgBr2 into the Bi2Sr1.5-xLaxCa1.5Cu2Oy with x ) 0.4. Arrow indicates the increase in Tc upon intercalation.

compared to the pristine as can be clearly seen for that with x ) 0.4 (Figure 7). To investigate the evolution of Tc upon intercalation, the Tc dependence on the La substitution rate (x) is plotted before and after intercalation as shown in Figure 8a. For the pristines, the Tc dependence on x has a parabolic feature with a maximum Tc at x ) 0.1, as expected, and is similar to the feature in the Tc versus x curve of yttrium-substituted Bi2Sr2Ca1-xYxCu2Oy.12,13 According to the previous report on the relation between Tc and hole concentration in the cation-substituted Bi2Sr2CaCu2Oy system, though the hole concentration decreases monotonically with the substitution, the Tc increases to a maximum at an optimum hole concentration, and then decreases again.10-14 Such results imply that there are two regions (hole overdoping and underdoping regions) with an optimum hole concentration, where the maximum Tc appears. Therefore, our Tc versus x plot for

3786 J. Phys. Chem., Vol. 100, No. 9, 1996

Choy et al. the data in Figure 8b can be fitted by the equation;

Tc/Tcmax ) 1.0 - 7.47(∆n)2

(1)

If the Tc depression is caused by the increase of hole concentration due to the charge transfer, it is expected that the Tc/Tcmax vs ∆n curve moves to left by a without changing the maximum Tc/Tcmax, which can be described as

Tc/Tcmax ) 1.0 - 7.47(∆n + a)2

(2)

and if the decrease in Tc is due to only the weakening of interblock coupling upon intercalation, all the Tc values will shift downward by b and take the parabolic shape with a maximum value at ∆n ) 0.00, which can be represented as follows:

Tc/Tcmax ) (1.0 - b) - 7.47(∆n)2

(3)

On the basis of this hypothesis, an attempt has been made to fit the data for the HgBr2 intercalates in Figure 8b, and the data can be fitted to the following equation:

Tc/Tcmax ) (1.0 - 0.02) - 7.97(∆n + 0.05)2

Figure 8. (a) Variation of Tc with respect to x for the pristines (filled circles with error bar) and their HgBr2 intercalates (open circles with error bar) in which upper and lower limit in error bar denote 1% and 10% of the normalized χ(T)/χ(5 K) value, respectively. (b) Dependence of Tc/Tcmax on the formal copper valence for the pristines (b) and their HgBr2-intercalates (O). ∆n represents the difference of formal copper valence with respect to the valence at the Tcmax.

the pristine indicates that the region at x < 0.1 is an overdoped state and the region above x ) 0.1 is underdoped one. Upon HgBr2 intercalation, the Tc’s decrease less than ∼6 K in the overdoped region 0.0 e x e 0.1 but increase by about 4-6 K in the underdoped region 0.3 e x e 0.4. It is worthy to note here that the HgBr2 intercalation does not change the parabolic shape in the Tc-x plot; however, the optimum substitution concentration of x ) 0.1 at Tc ) Tcmax for the pristine is shifted to x = 0.2 upon intercalation. As mentioned in the introduction section, a shift of optimum concentration at Tc ) Tcmax is closely related with a variation of hole concentration or copper valence. To investigate a charge transfer effect of intercalation on Tc, the relative value of Tc with respect to the Tcmax of the pristine, Tc/Tcmax, has been plotted against the formal charge difference of copper, ∆n, where ∆n is obtained by subtracting n in Cun+ at Tc * Tcmax from n in Cun+ at Tc ) Tcmax, and the formal valence n is calculated by assuming that all bismuth ions are trivalent and the oxygen content is constant. In Figure 8b, we plotted the dependence of Tc/Tcmax on ∆n for the pristines and their HgBr2 intercalates. For the pristines, the hole overdoped and underdoped regions are clearly seen on the right and left sides at ∆n ) 0.00, respectively, in which ∆n becomes positive in the overdoped region but negative in the underdoped one. At an optimum value of ∆n ) 0.00, Tc/Tcmax has unity, however, this Tc/Tcmax value is lowered to 0.98 upon the HgBr2 intercalation and located at ∆n ) -0.05. Therefore, the new parabola formed by the intercalation seems to be resulted from a shift toward negative direction along ∆n accompanied by a slight depression of the maximum Tc/Tcmax value. For the pristine case,

(4)

where 0.02 and 0.05 correspond to the parameters b and a, respectively. In the previous result on the iodine intercalated system, it has been argued that a 10 K depression upon iodine intercalation was due to a hole doping and/or weakening of interblock coupling.5,6,21-26 If the Tc depression is caused only by the weakening of interblock coupling, one can easily expect that the b value will be at least 0.13 because Tc/Tcmax should have 0.87 ()69.6 K/79.6 K, the present system has a maximum Tc of 79.6 K). However, the present value of 0.02 found in the HgBr2 intercalated system is far below 0.13, from which we can deduce that the interblock coupling in the Bi2Sr2CaCu2Oy superconductor has little effect on its superconductivity. This experimental evidence demonstrates that the superconductivity mainly depends on the carrier hole concentration on the copperoxygen sheets, and therefore a shift by 0.05 indicates that the hole is doped to host lattice via charge transfer through intercalation. To investigate the relation between the variation of hole concentration and the change in Tc, we plot the ∆n dependence with respect to the relative Tc change, [Tc(P) - Tc(I)]/Tc(P), where Tc(P) and Tc(I) represent the Tc’s of the pristines and their intercalates, respectively (Figure 9). The value of [Tc(P) - Tc(I)]/Tc(P) is found to be directly proportional to the copper valence indicating that the Tc changes upon intercalation are clearly dependent on the doped hole concentration. Consequently, the amount of doped hole concentration could be determined from the Tc change before and after intercalation. Now we propose a semiempirical equation based on the eq 1, which can be represented by

∆n* )

x1 - (Tc/Tcmax) x7.47

(5)

where ∆n* is redefined as the amount of doped hole concentration or the increase of formal Cu valence and Tcmax is taken to be 79.6 K. At first, the iodine intercalated system is applied to this semiempirical postulation. Our iodine intercalate (x ) 0.0) shows the Tc of 67.9 K depressed by 10 K compared to the pristine and ∆n* is calculated to be 0.14 corresponding to an increase of formal Cu valence from 2.00 to 2.14, which also corresponds to a doping of ∼0.30 hole/formula unit. This result

Intercalation of HgBr2 into the Bi2Sr1.5-xLaxCa1.5Cu2Oy

J. Phys. Chem., Vol. 100, No. 9, 1996 3787 system has been found to be a good example to provide an answer why Tc is modified upon intercalation of the layered cuprate superconductor. From the dc magnetic susceptibility measurements for the pristines and their HgBr2 intercalates, we could prove that the electronic coupling between the adjacent blocks of the CuO2 sheets has little effect on the superconducting property of the layered Bi2Sr2CaCu2Oy superconductor, while the superconducting transition temperature tuned by intercalation is clearly related to the change of hole concentration induced by charge transfer between host and guest. Acknowledgment. This work was supported in part by the Ministry of Science and Technology (MOST) for high-Tc superconductivity research and the Ministry of Education (BSRI95-3413). References and Notes

Figure 9. Dependence of the relative Tc depression on the formal copper valence.

Figure 10. Relation between Tc and hole concentration for the pristine, and its HgBr2 and iodine intercalates.

is in good agreement with the previous result of a doping of 0.33 hole by the I3- formation within the stoichiometry of IBi2Sr2CaCu2O8.27,28 We have also attempted to determine the amount of hole transferred to the copper-oxygen sheet for the HgBr2 intercalate using the eq 5. On the basis of the data of Tc ) 72 K for the HgBr2-intercalated sample with x ) 0.0, the ∆n* value is calculated to be 0.11, indicative of an increase of Cu valence from 2.00 to 2.11 or a doping of ∼0.20 hole/formula unit. In Figure 10, it is clearly demonstrated that the smaller Tc depression for the HgBr2 intercalate compared to the iodine intercalate is due to the fact that the amount of hole doped by the HgBr2 intercalation is less than that by the iodine intercalation. Conclusion Intercalation of HgBr2 into the La-substituted Bi2Sr2CaCu2Oy has been systematically performed. The HgBr2-intercalated

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