J. MATER. CHEM., 1994, 4(8), 1267-1270 Sol-Gel Synthesis of Superconducting YBa,Cu,O, using Acetate and Tartrate Precursors Aivaras Kareiva,+ Maarit Karppinen and Lauri Niinisto Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02 150 Espoo, Finland A simple sol-gel route starting from an aqueous mixture of Y, Ba and Cu acetates and tartrates has been refined to prepare superconducting YBa,Cu,O, at an oxygen pressure of 1 atm. Thermal decomposition of the fabricated gels was studied by means of thermogravimetry. In order to interpret the decomposition mechanisms, both individual and double gels of Y, Ba and Cu were studied. The synthesis products were characterized by X-ray diffraction for phase purity, by thermal analysis for oxygen stability and by SQUID measurements for superconducting properties.High-T, superconducting YBa,CU,O, (124) is a member of the homologous series of compounds of general formula Y2Ba,Cu6 +n (n=0, 1, 2). The thermodynamically fav- oured 124 phase differs from YBa,Cu,O, (123) in having a double, instead of single, Cu-0 chain running parallel to the h axis. It exhibits a transition temperature, T,, at around 80 K, and unlike the 123 phase its oxygen content has excellent thermal stability. With decreasing oxygen content in 123, T, decreases gradually and the compound loses its supercon- ducting properties. On the other hand, all oxygen atoms in the 124 phase are stable even at high temperatures; the orthorhombic-to-tetragonal phase transition and the macro- scopic twins observed in the 123 structure are also absent.All these properties make the 124 phase very interesting from a theoretical point of view as a slightly different model system for testing the general trends of high-T, materials and also in view of possible applications. The YBa2Cu,08 phase was first isolated under high oxygen pressures,' but synthesis under ambient conditions is also p~ssible.~-~A major problem encountered in the preparation of the 124 phase under normal pressure is the difficulty in obtaining a single-phase material. At higher temperatures the 124 phase decomposes into 123 and CuO, while at lower temperatures the synthesis does not proceed to completion. The sol-gel meth~d~,~ of synthesizing multicomponent cer- amics from metal alkoxides or salts provides many advantages over conventional solid-state methods.Homogeneous prod- ucts are easily obtained by mixing the metal precursors in solution in a molecular scale, and synthesis temperatures can be lowered noticeably. Furthermore, the rheological properties of the precursor gel allow the preparation of film^.^,^ In the sol-gel process the most critical step is the choice of starting materials since the greatest difficulty lies in the preparation of a stable precursor sol. Two types of route have been described in the literature for the preparation of the superconducting powders, depending on whether the precur- sor is an aqueous solution of inorganic saltsg-'l or a non- aqueous solution of metal-organic corn pound^.^^^^^^^ Most aqueous sol-gel processes employ tartaric acid for metal complexation and gel formation, but recently a novel synthetic route has been developed where the yttrium and barium oxides are dissolved into an aqueous solution of copper acetate.' The purpose of the present work was to refine a simple and reliable sol-gel route for preparing superconducting YBa2Cu,08 under low oxygen pressure.An acetate solution t Permanent address: Department of General and Inorganic Chemistry, Vilnius University, Lithuania. of Y, Ba and Cu was selected for the precursor. Irr order to prevent partial crystallization of copper acetate hydriite during the gelation process the ability of tartaric acid to form soluble complexes with copper was investigated.The synthesis param- eters optimized included pH and temperature profile during the sol-gel process as well as the annealing conditions during the final heat treatments. Special attention was dso paid to the thermal decomposition mechanism of the precursor gel. Individual and double gels of Y, Ba and Cu were prepared and investigated to assist the interpretation of the reactions leading to the formation of the ternary 124 system. Synthesis products were characterized by X-ray diffraction (XKD) ther- mal analysis and SQUID measurements. Experimental The gels were prepared using stoichiometric amounts of analytical-grade Y203,Ba(CH,C0,)2 and Cu(CH3C( )2)2.H20 as starting materials.Yttrium oxide (3.125 mmol) was first dissolved in 100 ml of 0.2 mol 1-' CH,C02H. After stirring the mixture for 10h at 55-60 "C in a beaker covered with a watch-glass a clear solution was obtained. Next, 50ml of 0.5mol 1-' Cu(CH,CO,),.H,O was added and the solution was stirred for 2 h at the same temperature. Finally, 25 ml of 0.5mol 1-' Ba(CH,CO,), was added and the solufion was stirred for another 2 h at room temperature. The pli of the metal acetate solution was 6.1. In most experiments small amounts (1-2 g) of tartaric acid, dissolved in 20 ml of distilled water, was added to the above solution, decreasing the pH value to 5.6. After concentrating the solution for 8 h at 65°C in an open beaker under stirring the acetate-tartrate solution turned into a blue gel while the acetate gels were light green.The gels were dried in a furnace at 80 "C. The synthesis of the individual and double gels of Y, Ba and Cu were carried out under the same conditions. To check the metal contents in the dried gel powders photometric (Y, Cu)15 and gravimetric (Ba) methods were applied. The gel powders were ground in an agate mortar and preheated for 10 h at 780 "C in flowing oxygen. Since the gels are very combustible slow heating ( 1 "C min- ') especially between 150 and 350°C was found to be essential. After an intermediate grinding the powders were sintered at various temperatures from 750 to 820°C in air or flowing oxygen. The annealing times varied between 10 and 45 h.For the thermogravimetric ( TG) analyses a Perkin-Elmer System 7 thermobalance was used. The thermal decomposition of the individual, double and ternary acetate-tartrate gels of Y, Ba and Cu was studied up to 1200°C in a dynamic oxygen atmosphere using a heating rate of 2°C min-'. The sample J. MATER. CHEM., 1994, VOL. 4 weight was 30-60mg. Also, the oxygen stability of the final products under oxygen and argon atmospheres was confirmed by TG measurements (heating and cooling rates 5 "C min-'; sample weight 25-50mg). The phase purity and the crystal- linity of the products were studied by XRD analyses using a Philips MPD 1880 diffractometer equipped with a graphite secondary monochromator and a Cu tube. The reported critical temperatures of superconductivity are onset tempera- tures of the diamagnetic signal determined by a SQUID magnetometer (Quantum Design MPMS2).Results and Discussion Optimization of the Sol-Gel Process To ascertain the homogeneity of the acetate gel, the pH of the starting solution has to be adjusted carefully. If the solution is too acidic (pH<5.6), copper acetate hydrate is crystallized during the concentration process, while in solu- tions that are too basic (pH>6.1) copper hydroxide may floc~ulate.~In the acetate solution of Y, Ba and Cu a considerable quantity of CU(CH,CO~)~*H,O was precipitated when the evaporation process was started from pH 6.1. This could be seen, for example, in the resemblance of the TG curves recorded for the Y-Ba-Cu acetate gel and for the pure copper acetate hydrate (Fig.1). The decomposition of anhy- drous acetate occurs around 250 "C, which coincides with the decomposition temperature of the gel. However, when tartaric acid was added to the acetate solution a transparent gel was easily obtained even when the pH of the starting solution was 5.7. Evidently, tartrate ligands form stable16 and sufficiently soluble complexes with Cu2+ ions to prevent the crystalliz- ation of Cu(CH3C02),-H20. The observed critical limits for the C4H606:Cu ratio are 0.3 and 0.6, restricted by the solubilities of copper acetate and the metal tartrates, respect- ively. For the further experiments a C4H606:Cu ratio of 0.44" was selected. Thermal Decomposition of the Gels The mechanism of the thermal decomposition of the dried gels was investigated in an oxygen atmosphere by means of TG measurements in the temperature range 40-1200 "C.Fig. 2 shows the TG curve for the ternary yttrium, barium and copper acetate-tartrate gel. The decomposition occurs in several steps. The weight loss below 200°C (8.5 %) is due to the evolution of water and to the initial decomposition of the copper constituent. The final decomposition of the Y-Ba-Cu acetate-tartrate precursor above 200 "C proceeds uia horno-40 -I CUO -+20 -----. -, -. ----. -. -. -20 -I I I I I I II Fig. 1 TG curves for the ternary Y-Ba-Cu acetate gel (solid line) and Cu(CH,CO,),.H,O powder (broken line; theoretical weights of the decomposition products are indicated) samples recorded in a flowing oxygen atmosphere.The heating rate was 2 "C min-l. r-"or----7 60 \ 200 400 600 800 TI'C Fig. 2 Thermal decomposition of the ternary Y-Ba-Cu acetate-tartrate gel under a flowing oxygen atmosphere. The heating rate was 2 "C min-l. geneously distributed intermediate species, e.g. BaCO,. Since the structures of the gel complexes are not known, a more detailed interpretation of the decomposition mechanism is complicated. In order to clarify the possible reactions, individ- ual and double Y, Ba and Cu acetate-tartrate gels were studied. The TG curves for the individual nietal gels are shown in Fig 3-5. The decomposition of the acetate-tartrate gels differs considerably from that of the corresponding solid acetates.CUO -cue------- - ---------20 u 200 400 600 800 T1°C Fig. 3 TG curves for the copper acetate-tartrate gel (solid line) and Cu(C,H,O,) powder (broken line; theoretical weights of the decompo- sition products are indicated) samples recorded in a flowing oxygen atmosphere. The heating rate was 2 "C min-l. LuJ 200 400 600 800 1000 TI'C Fig. 4 TG curves for the barium acetatetartrate gel (solid line) and Ba(CH,CO,), powder (broken line; theoretical weights of the decomposition products are indicated) samples recorded in a flowing oxygen atmosphere. The heating rate was 2 "C min-'. J. MATER. CHEM., 1994, VOL. 4 40 II 200 400 600 800 T1"C Fig. 5 TG curves for the yttrium acetate-tartrate gel (solid line) and Y (CH,CO,),H,O powder (broken line; theoretical weights of the decomposition products are indicated) samples recorded in a flowing oxygen atmosphere.The heating rate was 2 "C min-'. A comparison between the TG curves recorded for Cu(CH,CO,),-H,O (Fig. l), Cu(C4H406) and the copper acetate-tartrate gel (Fig. 3) confirms that the main part of the gel is decomposed at the same temperature as the tartrate but at a much lower temperature than the acetate. In the temperature range 40-175 "C (weight loss 10.0%)the residual solvent and the coordinated water molecules are evolved, followed by a rapid decomposition of the gel above 175°C (46.9%) to a mixture of Cu20 and CuO. With increasing temperature the oxidation of Cu' takes place.Judging from the TG data and the analysed copper content the probable composition of the gel is either CU,(C4H2O,)(H20), or Cu2(OH)2(C4H40,)(H20)2.17The decomposition of Ba(CH,C02)2 to BaO proceeds via BaCO,, but is incomplete up to 1100°C owing to the thermal stability of the carbonate (Fig. 4). On the other hand, the decomposition of the barium gel occurs in three steps. By assuming that all the available C,H,O,,-ligands (C,H,O, :Ba =0.88) form barium tartrate, the TG curve obtained for the Ba gel (Fig. 4) could be explained by the following reactions: (i) between 160 and 280°C (weight loss 1.8%) Ba(OH),.H,O is dehydrated to BaO; (ii) between 280 and 345°C (28.7%) the tartrate is decomposed to BaCO,; and (iii) above 1000°C decomposed further to the oxide.However, in the presence of copper (Ba-Cu and Y-Ba-Cu gels) barium tartrate was not precipitated any longer, but a gelatinous product with a complicated decomposition scheme was formed. In the case of Y(CH,C02)3-H,0 the dehydration starts at 80cC, and the resulting anhydrous yttrium acetate is stable up to 300"C, at which temperature Y,02C03 is formed (Fig. 5). Further heating leads to the oxide as the final product above 600°C. The TG data of the yttrium acetate-tartrate gel (Fig. 5) reveal weight losses of 4.0% (40-160°C), 6.6% (160-215 "C), 14.4% (215-265 "C), 15.9% (265-330°C) and 28.1 O/O (330-365 "C), which could be associated, respect- ively, with the desorption of residual solvent, the escape of water molecules from the coordination sphere of yttrium and with a multi-step decomposition of the yttrium hydroxo and/or acetato tartrate complex.According to charge-balance considerations and the above assumptions, the gel might be composed of Y,(OH)(C,H,O,),(H,O), or Y,(CH,COO)(C4H,06),( H,O), (z=4-8). In the Y-Cu double and Y-Ba-Cu ternary gels the tartrate ligands tend to form complexes with copper, and the situation may be different. Optimization of the Annealing Conditions The 124 phase can be grown in a flowing oxygen atmosphere above 750 "C. The growth is promoted by increasing annealing temperature and time. XRD data of the powder heated for 10 h at 780 "C indicated the presence of various oxide phases such as CuO, BaCuO,, YBa2Cu307 and YBa,Cu,O,.Further heat treatment with intermediate grinding improved the phase purity of YBa,Cu40,. Essentially single-phase 124 was obtained at 780°C after annealing for 30 h. In the XRD spectrum shown in Fig. 6 the reflections of the 124 phase are indicated; the only unidentified peaks seen at the 2t' values of 29" and 31" are most probably due to small amounts of BaCuO, impurities. Above 800 "C the 124 phase already starts to decompose to 123 and CuO, and these phases wcre clearly present in the XRD spectrum recorded for the sample annealed at 820 "C for 15 h. Characterizationof the Products Because of the structural similarities, the d values of the strongest peaks in the XRD patterns of YBa2Cu3O7 and YBa,Cu,O, coincide, which makes it difficult to distinguish between these two phases.However, in order to detect small amounts of 123 impurities the differences in the oxygen stability or in the superconducting transition temperature may be utilized. The 123 phase starts to lose oxygen around 330-400 "C depending on the atmosphere, while the 124 phase should be stable up to 800-900°C11~12~'8~19in oxygen and up to 670-700"C1918 in an inert atmosphere. Fig. 7 shows the I I I I I I I I lu 20 30 40 50 60 70 28ldegrees Fig. 6 Powder X-ray diffraction pattern (Cu-Ka radiation) for the ternary Y-Ba-Cu acetate-tartrate system annealed in flowing oxygen at 780 "C for 10+30 h. The reflections of the 124 phase are mdicated. I I I I I I I IL 200 400 600 800 TI'C Fig.7 TG heating and cooling curves for the synthesized YBa,Cu,O, material showing sufficient thermal stabilitities under flowing argon (solid line) and oxygen (broken line) atmospheres. The heating and cooling rates were 5 "C min-l. J. MATER. CHEM., 1994, VOL. 4 The authors thank Prof. K. V. Rao and Ms. T. Turkki, Royal Institute of Technology, Stockholm for carrying out the SQUID measurements. Prof. S. Pejovnik, National Institute of Chemistry, Slovenia is thanked for stimulating discussions. Financial support by the Nordic Council of Ministers to A.K. in form of a scholarship administrated by the Finnish Center for Mobility (CIMO) is gratefully acknowledged. References 1 J. Karpinski, E. Kaldis, E.Jilek, S. Rusiecki and B. Bucher, Nature (London),1988,336,660.1R. J. Cava, J. J. Krajewski, W. F. Peck Jr., B. Batlogg, L. W. Rupp2 0 20 40 60 80 100 TIK Fig. 8 SQUID data for the synthesized YBa,Cu,O, material TG curves of subsequent heating and cooling cycles under oxygen and argon atmospheres for one of the synthesized 124 samples (annealed at 780 "C for 10+ 30 h). No indication of weight loss could be seen below 690°C even in the argon atmosphere, and in the oxygen atmosphere the oxygen content was stable up to 880°C. Finally, the SQUID measurement of the same sample showed the onset of superconductivity at 78 K (Fig. 8), but owing to the low density of the non-pelletized powder sample the transition was found to be rather broad.Nevertheless, the TG and SQUID results above con- firm the absence of the other superconducting impurity phases, and together with the XRD data verify sufficient phase purity of the synthesized 124 material. Conclusions A sol-gel method, starting from an aqueous acetate solution of the metals, was investigated for synthesizing super-conducting YBa,Cu,O, under low oxygen pressure. 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Belford and I. C. Paul, J. Coord. Chem., 1972, 2, 145. 18 J. Mullens, A. Vos, A. DeBacker. D. Franco, J. Yperman and L. C. Van Poucke, J. Thermal And., 1993,40, 303. 19 T. Wada, N. Suzuki, A. Ichinose, Y. Yaegashi, H. Yamauchi and S. Tanaka, Jpn. J. Appl. Phys., 1990,29, L915. 20 J. LindCn, M. Lippmaa, J. Miettinen, I. Tittonen, T. Katila, A. Kareiva, M. Karppinen, L. Niinisto, J. Val0 and M. Leskela, Phys. Rev. B, in press. 21 M. Karppinen, A. Kareiva, J. LindCn, M. Lippmaa and L. Niinisto, 2nd International Conference on $Elements, Helsinki 1994, to be presented. Paper 4/00765D; Received 8th February, 1994