Reactions of oxyfluoride precursors for the preparation of barium yttrium cuprate films M. Yoshizumi * , I. Seleznev, M.J. Cima Department of Materials Science and Engineering, Massachusetts Institute of Technology, Room 12-096, 77 Massachusetts Ave., Cambridge, MA 02139, USA Received 8 July 2003; received in revised form 26 November 2003; accepted 10 December 2003 Abstract Precursor films prepared by metal organic deposition are shown to be much richer in fluorine than previously suspected. The ratio of fluorine to barium is 2.7 when the film is prepared at 400 °C and decreases with increasing temperature. That is, the films contain more fluorine than can be explained by only barium fluoride being present. The remaining F/Ba ratio trajectory of MOD decreases from 2.7 to about 1.5 without the formation of detectable Ba 2 YCu 3 O 6:5 (YBCO). The trajectory is also shown to include compositions known to contain low melting liquids. These observations show that YBCO formation from MOD-derived precursor occurs in multiple steps: (1) decom- position of YF 3 , (2) decomposition of BaF 2 without YBCO nucleation, (3) formation of YBCO via liquid phase. This means small amount of oxyfluoride melt is present in the film during the conversion to YBCO. Thicker films have a larger F/Ba ratio at any given temperature which possibly leads to more melt. The melt may contribute to the densi- fication and the epitaxial growth of YBCO. It may also, however, cause reaction with substrate and/or buffer layer. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.72.B Keywords: Coated conductor; BaF 2 process 1. Introduction Metal organic deposition (MOD) or e-beam deposition of a precursor film consisting of mixed oxides of copper and yttrium and barium fluoride are known as ex situ process. The MOD process uses trifluoroacetate (TFA) solution for prepa- ration of precursor films [1,2]. TFA salts are decomposed into yttrium, barium and copper oxyfluorides during low temperature heat treat- ment [1–3]. Alternatively, the e-beam process in- volves co-evaporation of yttrium, copper, and barium fluoride [4,5]. The film composition is controlled by independently controlled power to three separate e-beam sources and by carefully positioning the substrate (or moving the substrate) to achieve the required flux balance. The precursor film is thought to be purely nanocrystalline whe- ther it is deposited by decomposition of organic precursors or by e-beam. X-ray diffraction yields only broad reflections roughly in positions of * Corresponding author. Tel.: +1-617-253-9450; fax: +1-617- 258-6936. E-mail address: yosizumi@mit.edu (M. Yoshizumi). 0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.12.004 Physica C 403 (2004) 191–199 www.elsevier.com/locate/physc