Synthesis of Hg-added Bi2212 superconductors using various routes

Supercond. Sci. Technol. 10 (1997) 58–64. Printed in the UK

PII: S0953-2048(97)77157-4

Synthesis of Hg-added Bi2212 superconductors using various routes Vilas Shelke and R K Singh† Superconductivity Research Laboratory, Department of Physics, Barkatullah University, Bhopal 462026, India Received 14 August 1996 Abstract. We have studied the effect of Hg addition in the BiSrCaCuO system. Polycrystalline samples with nominal compositions Bi2 Sr2−x Hgx Ca1 Cu2 Oy and Bi2 Sr2 Hgx Ca1 Cu2 Oy (x = 0.3) were synthesized in an open atmosphere by three different synthesis routes, i.e. solid-state reaction, matrix method and double-matrix method. The samples were characterized through XRD, R –T measurement and SEM. The phase ultimately obtained was mainly Bi2212. Almost identical Tc depressions were observed in all the samples as a consequence of overdoping of charge carriers due to the high-oxygen ambient provided by HgO decomposition. Additional air annealing was found to be useful in attaining proper tuning of charge carriers, while quenching to room temperature had an adverse effect. Improvement in grain boundary linkage was observed for additionally annealed samples.

1. Introduction A great deal of effort has been devoted to superconductivity research since the discovery of high-Tc superconductors in copper-oxide-based compounds by Bednorz and Muller [1]. Subsequently, several families of superconductors having similar layered structures have been discovered. It is now believed that a homologous series of superconducting layered cuprates exists for each of the M–(Ba/Sr)–Ca– Cu–O systems, where M represents an element with atomic number between 80 and 83, i.e. Hg (80), Tl (81), Pb (82) and Bi (83). These systems belong to similar structural families. Among them, Hg- and Tl-based systems involve toxic oxides and solid-to-vapour types of reaction. Therefore, such materials are prepared in sealed containers [2–5], which makes the synthesis quite delicate. Similarly, Pb-based systems are synthesized through high-pressure techniques [6, 7]. Since Bi-based superconductors are comparatively easy to prepare and have more stability, it is no wonder that they are extensively studied. Even though there is no complete agreement on the exact composition of different phases, at least three phases exist, i.e. Bi2 (Sr, Ca)2 Cu1 Oy , Bi2 (Sr, Ca)3 Cu2 Oy and Bi2 (Sr, Ca)4 Cu3 Oy , in this family. More commonly these phases are known as Bi2201, Bi2212 and Bi2223 having Tc values of 20 K, 80 K and 110 K, respectively. Again, easy sintering and a Tc value above liquid nitrogen temperature have made the 2212 phase the most popular. Many attempts have † Present address: Vice Chancellor and Professor of Physics, Guru Ghasidas University, Bilaspur 495009, India. c 1997 IOP Publishing Ltd 0953-2048/97/010058+07$19.50

been made to increase the Tc value of this phase. For this purpose, different types of substitution [8–12], sintering process [13, 14] and annealing atmosphere [15, 16] have been explored. Since the recent discovery of HgBaCaCuO superconductors with Tc values normally up to 135 K [4, 5, 17] and under very high pressure up to 160 K [18, 19], mercury has received recognition as a potential element for Tc enhancement. Subsequently, many results have been published on this new system. Comparatively scant attention has been paid to the substitution of Hg in other systems [20–22]. Recently, one more tendency for Hg atoms to act as a catalyst for enhancing Tc was reported by Lahirey et al [23]. Similarly, Shan and Risbud [24] have reported the effect of Hg addition on the BiPbSrCaCuO system. In our recent study [25], we reported that the effect of Hg addition in the BiSrCaCuO system is composition dependent: starting with the 2223 composition, the 2212 phase is obtained with a Tc value as high as 92 K, while starting with the 2212 composition Tc has a lower value (72 K). The probable reasons for the Tc depression might be poor weak links due to the lower sintering temperature and overdoping of the charge carriers due to the high-oxygen ambient provided by HgO decomposition. To carry out a further investigation, we have tried to synthesize the Hg-added Bi2212 system by three different routes, i.e. solid-state reaction, single-matrix and double-matrix routes. All the samples were characterized through x-ray diffraction, resistance versus temperature measurement and scanning electron microscopy. The results of these investigations are reported in this paper.

Synthesis of Hg-added Bi2212

Figure 1. X-ray diffraction patterns of samples prepared by the solid-state reaction method: (a ) sample BH1; (b ) sample BH1A.

Figure 2. X-ray diffraction patterns of samples prepared by the solid-state reaction method and quenched to room temperature. , prominent Bi2201 peaks.

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Figure 3. X-ray diffraction patterns of samples prepared by matrix and double-matrix method: (a ) sample B1 with starting composition Bi2 Sr2 Hg0.3 Ca1 Cu2 Oy ; (b ) sample B2 having starting composition Bi2 Sr1.7 Hg0.3 Ca1 Cu2 Oy ; (c ) sample C1 having starting composition Bi2 Sr2 Hg0.3 Ca1 Cu2 Oy ; (d ) sample C2 having starting composition Bi2 Sr1.7 Hg0.3 Ca1 Cu2 Oy .

2. Experimental aspects 2.1. Synthesis routes The following three routes of synthesis have been used to prepare the samples. 2.1.1. Solid-state reaction method. Appropriate amounts of high-purity Bi2 O3 (99.9%), SrCO3 (99.995%), CuO (99.99%), CaCO3 (99.5%) and HgO (99.9%) were 60

first well mixed and ground in an agate mortar. The finely powdered mixture was calcined in air at 815 ◦ C for 16 h. The temperature during calcination was raised in steps to prevent the melting of oxides of low melting temperature. The calcined mass was ground for a few hours and recalcined at 815 ◦ C for 56 h with intermediate grindings. The reacted materials were pulverized and cold pressed into circular pellets (12 mm diameter) by applying a hydraulic pressure of 8 tf. The pellets were then sintered at 820 ◦ C for 65 h followed by furnace cooling. A few

Synthesis of Hg-added Bi2212

(b )

(a )

Figure 4. Resistance (normalized value) versus temperature variation of the Hg-added BiSrCaCuO compounds: (a ) samples BH1, BH1A and BH1Q prepared by the solid-state reaction method; (b ) samples B1, B2, C1 and C2 prepared by single- and double-matrix methods.

pellets were further annealed in air at 820 ◦ C for 24 h and then furnace cooled. A few pellets, after annealing for 24 h, were quenched to room temperature.

24 h and then cold pressed into pellets. These pellets were further sintered for 24 h and quenched to room temperature. Finally, the pellets were annealed at 820 ◦ C for 4 h.

2.1.2. Single-matrix route. Because of the addition of HgO, the melting point of the composition decreases to 820 ◦ C. Therefore, to avoid melting, the sintering temperature was restricted to 820 ◦ C, which is somewhat less than the usual sintering temperature for Bi2212. To avoid the problem of Bi2 O3 volatility, a matrix method was proposed by Sastry et al [26]. We have adopted a similar route in the present work. In this method, firstly a high-melting-point Sr-based compound was prepared by mixing stoichiometric amounts of SrCO3 , CaCO3 and CuO and firing at 972 ◦ C for about 72 h with intermediate grindings. Stoichiometric amounts of Bi2 O3 and HgO were mixed with the thus prepared Sr– Ca–Cu–O matrix. This mixture was calcined at 820 ◦ C for

2.1.3. Double-matrix method. In this method, the matrix formation was done twice. Firstly, the Sr–Ca–Cu– O matrix was prepared by the previous method. With this matrix an appropriate amount of Bi2 O3 was added and the mixture was fired at 845 ◦ C for 48 h with intermediate grindings. This second matrix was then mixed with HgO and the mass so obtained was ground thoroughly and cold pressed into pellets. These pellets were sintered at 820 ◦ C for 24 h followed by room-temperature quenching. Finally, the pellets were annealed at 820 ◦ C for 4 h. For comparison, two types of specimen were prepared by all three routes: one with Sr-deficient composition Bi2 Sr1.7 Hg0.3 Ca1 Cu2 Oy and the other composition Bi2 Sr2 Hg0.3 Ca1 Cu2 Oy without any Sr deficiency. 61

V Shelke and R K Singh

2.2. Characterization All the samples were characterized by x-ray powder diffraction to identify the phase formation. For this purpose, a Rigaku Geigerfelex diffractometer using Cu Kα radiation was employed in the range 5 ◦ ≤ 2θ ≤ 70 ◦ . The superconducting behaviour of all the samples was studied by measuring electrical resistance as a function of temperature using the standard four-probe method. The low-temperature measurements were carried out with a closed-cycle APD cryostat. A d.c. current of 10 mA was passed through the samples by a Keithley constantcurrent source and voltage was measured by a Keithley nanovoltmeter. Some of the samples were studied by scanning electron microscopy (Cambridge Stereoscan, model ST50 MK3). (a )

3. Results and discussion The x-ray diffractograms of all the samples are shown in figures 1, 2 and 3, which mainly reveal the Bi2212 phase. No peak corresponding to any mercury compound or newly discovered Hg-based superconductors was found. Since HgO decomposes at 476 ◦ C and Hg vapour escapes, the absence of Hg-related phases was obvious. The BH1Q samples have shown Bi2201 as a major phase. The strong (115) reflection of the 2201 phase was present at 2θ = 29.79◦ with highest intensity and the (001) and (200) reflections of this phase are prominent. In all cases, a greater or lesser shifting of the peaks towards the highBragg-angle side with change in intensity was observed. This might be a consequence of the lattice distortion and structural modifications introduced by additional oxygen absorption and possible vacancy creation at Sr sites. At the reaction temperature HgO decomposes and provides a highoxygen ambient which stimulates the kinetics of oxygen in-diffusion. As explained by Runde et al [27], the oxygen diffusion is highly anisotropic: in the BiO plane (a–b-plane) it takes place by the motion of interstitials, whereas in the c-direction it occurs by a vacancy mechanism. Also, since Hg is not incorporated in the system, there may be vacancy creation at Sr sites. These modifications are reflected by the changes in the intensity and broadening of the peaks. The variations in electrical resistance (normalized values) with temperature are depicted in figure 4. The composition, the process parameters and the Tc values are listed in table 1. All the samples, except BH1Q, have shown metallic behaviour followed by a superconducting transition. The sample BH1Q has, however, shown a semiconductor-to-superconductor transition with Tc zero around 20 K. In earlier studies [24, 25], Tc enhancement has been reported with Hg addition, but in these cases the starting compositions were 2223 types and the phase obtained was 2212. In our earlier study [26], we reported Tc enhancement with 2223 starting composition and Tc depression with 2212 starting composition. In the present case, all the samples prepared by different routes have shown Tc depression. Also, the Tc values are more or less the same, irrespective of the processing route. Thus, the processing route has least effect on the superconducting 62

(b )

(c ) Figure 5. Scanning electron micrographs of the Hg-added BiSrCaCuO samples: (a ) sample BH1 sintered for 65 h; (b ) sample BH1A annealed for an additional 24 h; (c ) sample BH1Q quenched to room temperature.

properties of Hg-added samples. The only effective parameter may be the final sintering temperature, which was the same in all cases and lower than the usual value

Synthesis of Hg-added Bi2212 Table 1. Starting compositions, process parameters an Tc values of Hg-added Bi2212 compounds.

Sample

Composition

Synthesis route

Sintering temperature (◦ C)

Sintering time (h)

Tc zero (K)

BH1 BH1A BH1Q B1 B2 C1 C2

Bi2 Sr1.7 Hg0.3 Ca1 Cu2 Oy Bi2 Sr1.7 Hg0.3 Ca1 Cu2 Oy Bi2 Sr1.7 Hg0.3 Ca1 Cu2 Oy Bi2 Sr2 Hg0.3 Ca1 Cu2 Oy Bi2 Sr1.7 Hg0.3 Ca1 Cu2 Oy Bi2 Sr2 Hg0.3 Ca1 Cu2 Oy Bi2 Sr1.7 Hg0.3 Ca1 Cu2 Oy

SSR SSR(A) SSR(Q) MM MM DMM DMM

820 820 820 820 820 820 820

65 89 69 52 52 52 52

64 74 20 50 61 62 64

SSR, solid-state reaction; SSR(A), solid-state reaction (annealed); SSR(Q), solid-state reaction (quenched to room temperature); MM, matrix method; DMM, double-matrix method.

(860 ◦ C). Attempts to increase the sintering temperature have resulted in melting of samples. However, the samples which have been annealed for the longest time have shown the highest Tc . The depression of Tc and the subsequent increase in Tc value caused by annealing may be attributed to the overdoping of charge carriers. The charge carrier density in the CuO2 plane is a crucial parameter influencing the superconducting properties of a system. The factors governing charge carrier density include oxygen nonstoichiometry, cation site disorders such as interstitials and vacancies, Cu–O bond length, Cu oxidation state, external pressure and structural modulations. These factors are closely correlated to each other. According to the basic quantum mechanical treatment of an electron in a potential well, its energy is inversely proportional to the dimension of the region in which it is confined. Thus, a region of crystal structure under compression causes the electron energy to increase and is more likely to experience a net outflow of charge. In the present case, the high-oxygen ambient provided by HgO decomposition causes electrons to move from the CuO2 plane to interfacial planes, resulting in the creation of more holes in the CuO2 plane. Such a hole doping mechanism is relevant to the appearance of superconductivity. However, there is a narrow range of dopant concentration; either over- or underdoping causes Tc degradation. The holes induced in this way may cause overdoping of charge carriers, thereby decreasing the Tc value. The lowest value of Tc in sample BH1Q is due to the presence of Bi2201 phase as seen from XRD. Since the resistive behaviours of all the samples (except BH1Q) are similar and Tc is unchanged with HgO, added at single- or double-matrix stage, the doping seems to be significant at all stages of calcination. Also, the role of vacancies at Sr sites is found to be less effective. Although ‘Sr’ deficiency was proposed to be favourable for the optimum oxidation state of copper (+2.33) [28], that effect was dominated by the doping mechanism. Scanning electron micrographs of some of the samples are shown in figure 5. It is clearly seen that sample BH1Q has randomly oriented small grains, which is obviously not in favour of good interlinkage. In the case of BH1 and BH1A samples, the grain diffusion is rather improved. A well diffused grain structure is observed for the BH1A

samples, which have shown the highest Tc of all the samples. 4. Conclusions We have prepared Hg-added Bi2212 samples by making use of the solid-state reaction, single-matrix and double-matrix routes. The starting composition was the 2212 type and the phase obtained was Bi2212. No trace of Hg incorporation in the system or formation of any Hg compound was observed. In all cases, the sintering temperature was restricted to a lower value (820 ◦ C) in order to avoid melting of the mass. Since HgO decomposition provides a highoxygen ambient at reaction temperature, the net effect was similar to that of high-pressure oxygen sintering. As such, all the samples have shown Tc depression. However, the samples when annealed for a longer duration have shown a little improvement in the Tc value. This type of behaviour of the samples can be attributed to the deviation of the charge carrier density from its optimum value. Because of the high-oxygen ambient, the samples became overdoped and this caused the Tc depression. The tuning of the charge carrier density by quenching was not possible. However, vacuum–inert or a prolonged air annealing may be helpful in achieving a Tc as high as that obtained with the 2223 starting composition. Acknowledgments The authors are grateful to the All India Council of Technical Education (AICTE), New Delhi (India), for the financial support under the Thrust Area Programme in Technical Education (TAPTEC) on Superconductivity and Electronic Ceramics. The authors are also grateful to the Inter University Consortium for the Department of Atomic Energy facilities and to Dr V S Tiwari, Centre for Advanced Technology, Indore (India), for providing characterization facilities. References [1] Bednorz J G and Muller K A 1986 Z. Phys. B 64 189 [2] Adachi S, Mizuno K, Setsune K and Wasa K 1990 Physica C 171 543 63

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