research papers J. Appl. Cryst. (2015). 48, 37–43 doi:10.1107/S1600576714025175 37 Journal of Applied Crystallography ISSN 1600-5767 Received 27 August 2014 Accepted 17 November 2014 # 2015 International Union of Crystallography Crystalline characteristics and superconducting properties of Bi2212 thin films prepared by the Pechini sol–gel method: the effect of different substrates on the film growth Xiaoming Lu, Xuejiao Miao, Dalu Sun, M. Babar Shahzad and Yang Qi* Institute Of Material Physics and Chemistry, School of Sciences, Northeastern University, No. 3–11 Wenhua Road, Heping District, Shenyang, Liaoning 110819, People’s Republic of China. Correspondence e-mail: qiyang@imp.neu.edu.cn Bi 2 Sr 2 CaCu 2 O 8+ (Bi2212) superconducting thin films were prepared by the Pechini sol–gel method using nitrates as reactants. The precursor sol was deposited on LaAlO 3 (100) and MgO(100) single-crystal substrates via the spin- coating method. The results revealed that the phase formation temperature region of the pure Bi2212 phase and perfectly epitaxial Bi2212 films deposited on LaAlO 3 substrates was significantly expanded to 8 K, indicating an improvement in crystallization of Bi2212 films. In addition, the phase formation temperature region of the pure phase and c-axis-grown Bi2212 films deposited on MgO substrates was reduced to 5 K because of the deterioration of epitaxy of Bi2212 films sintered at higher temperature. According to the investigation, the T c,onset values of Bi2212 films grown on MgO substrates were about 92 K. Meanwhile, the optimal T c,onset value of Bi2212 superconducting films grown on LaAlO 3 substrates rose to 96 K. 1. Introduction Bismuth strontium calcium copper oxide (BSCCO) high- temperature superconductors, because of their high transition temperature (T c ) and high anisotropy ( c   ab ), are one of the most popular high-temperature superconducting mate- rials. It is well known that there are several phases in the BSCCO system. The compositions of each phase are expressed by the general formula Bi 2 Sr 2 Ca n1 Cu n O 2n+4+ (n = 1, 2 and 3), corresponding to phases with T c of 2, 85 and 110 K, respectively (Miche et al. , 1987; Khasanova & Antipov, 1995). Hereafter, we use the abbreviations Bi2201, Bi2212 and Bi2223 phases for n = 1, 2 and 3, respectively. It is difficult to prepare single phases of BSCCO because of the very narrow temperature region of the superconducting phase. Hence, the preparation of BSCCO thin films is much harder than that of yttrium barium copper oxide thin films, which have a T c value of 90 K. According to reports in the literature (Karaca et al., 2003; Kameli et al., 2006), the phase purity of the Bi2223 phase has been partially improved through the substitution of Pb for Bi, and the intergrain weak links, the stability of the Bi2223 phase, the degree of contamination by residual secondary phases and the weak flux pinning capability of films were improved, but reports on the fabrication of pure BSCCO thin films without additional components are relatively rare. So, the preparation of the Bi2212 phase is emerging as a new trend in research on BSCCO materials owing to its relatively high T c and good phase stability. In recent years, the sol–gel method has become one of the most popular and important techniques to synthesize various types of tailor-made nanomaterials, including high-temperature ceramic insulation coatings with high homogeneity and purity (Brinker & Scherer, 1990; Pierre, 1998), avoiding the use of sophisticated equipment requiring high power, vacuum conditions, industrial laser sources and expensive raw materials (i.e. molecular beam epitaxy and pulsed laser deposition methods). In our previous work, Bi2212 superconducting thin films were prepared by the Pechini sol–gel method (Pechini, 1967) using nitrates as reactants (Lu et al., 2013). The results showed that the purity and physical properties of Bi2212 films grown on SrTiO 3 (100) substrates were remarkably enhanced as a result of the preparation of a uniform and stable precursor gel (Zhang et al., 2010), providing a potential process to improve the purity and crystallinity of Bi2212 thin films. The optimal sintering temperature region (1096–1098 K) and sintering time (20– 45 min) of Bi2212 films were obtained, and then the sintering time was significantly reduced. The phase purity of the Bi2212 thin films was obviously improved to 97% owing to the stable gel mechanism. The T c,onset value of the c-axis Bi2212 thin films rose to about 95 K and T c,zero was about 86 K. The phase purity and superconductivity of the Bi2212 films were far better than those of Bi2212 films prepared by the conventional sol–gel method (Yu et al., 2011; Yang et al., 2012). Bi2212 films deposited on different single-crystal substrates have also been researched, with a view to applications in other fields. The lattice match between film and substrate is an important factor for the fabrication of epitaxial films.