Cryst. Res. Technol. 39, No. 8, 718 – 725 (2004) / DOI 10.1002/crat.200310244 © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim On the synthesis and characterization of La doped MgB 2 superconductor C. Shekhar, R. Giri, R. S. Tiwari, and O. N. Srivastava* Department of Physics, Banaras Hindu University, Varanasi 221005, India Received 25 November 2003, accepted 10 February 2004 Published online 15 July 2004 Key words superlattice structure, partial dislocation, moiré fringe, transport critical current density. PACS 81.10.-h We report the synthesis of La doped MgB 2 superconductors with nominal compositions Mg 1-x La x B 2 (with x=0.01, 0.03, 0.05, 0.07) by solid state reaction at ambient pressure. A special encapsulation technique has been used by us to prepare high quality superconducting MgB 2 samples. The bulk polycrystalline samples possess superconducting transition temperature T c(R=0) ranging between 36-39K. It has been found that transport critical current density J c of the samples change significantly with the doping level of La. A high transport (J c ) value ~1.9 x 10 3 A/cm 2 at 15K has been achieved for Mg 0.97 La 0.03 B 2 sample. The XRD and TEM investigations indicate that the samples prepared by encapsulation method are devoid of MgO, which is generally found when synthesis of MgB 2 is done through sintering of Mg and B powders. The detailed microstructural investigations of Mg 0.97 La 0.03 B 2 specimens by transmission electron microscopy (TEM) reveal the presence of partial dislocation network, moiré fringes and superlattice structure in the as synthesized samples. The higher transport critical current density observed in Mg 0.97 La 0.03 B 2 superconductor has been attributed to the partial dislocations which are capable of providing pinning centres. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction The discovery of superconductivity at 39K in binary intermetallic compound, magnesium diboride (MgB 2 ) [1] has sparked new research to further understand this interesting material. MgB 2 has the highest transition temperature till date among non-cuprate superconductors. It could be promising in applications in the temperature range 20-30K, where conventional superconductors will not be able to operate. Unlike high temperature cuprate superconductor, MgB 2 has simple chemical composition and hexagonal AlB 2 -type crystal structure (space group P6/mmm). Significant aspect of MgB 2 superconductor is that the current density in MgB 2 is not suppressed by grain boundaries [2-4]. However, the synthesis of high quality MgB 2 is rather difficult. To date high quality MgB 2 bulk samples have been fabricated by using high pressure (3.5GPa) synthesis technique [5,6]. The proton irradiation, can be used to improve the flux pinning [7]. Therefore, synthesis of dense bulk MgB 2 at ambient pressure is of considerable interest from application point of view. In this connection chemical doping may play an important role. As is now known, the chemical doping is found to be easily controlled, non-destructive and highly efficient in improving microstructure and flux pinning in high-T c superconductors [8]. It may be mentioned that earlier Hg bearing cuprate superconductors have been stabilized and appropriate flux pinning centres introduced by doping of Tl and Bi at Hg site [9]. The investigations on chemical doping in MgB 2 has been mostly focused on monitoring the effect of doping on the critical temperature T c . In some such studies doping of MgB 2 prepared through high pressure ____________________ * Corresponding author: e-mail: hepons@yahoo.com