Superconductivity in a new quaternary phase with HgSm 0.5 In 0.5 Pb 2 composition R. Corsini a , A.D. Bortolozo a , M.S. da Luz a , R. Ricardo da Silva b , C.A.M. dos Santos a , A.J.S. Machado a, ⁎ a Departamento de Engenharia de Materiais, Faenquil, C. P. 116, 12600-970, Lorena-SP, Brazil b Instituto de Física “Gleb Wataghin”, Unicamp, 13083-970, Campinas-SP, Brazil Received 21 February 2006; accepted 7 April 2006 Available online 24 May 2006 Abstract This work reports a systematic study on the structural and superconducting properties in the Hg–Sm–In–Pb system. It is investigated how In and Sm dopings influence the superconducting properties of the HgPb 2 phase which was recently reported. Results for the HgSm 1 - x Pb 2 system show that Sm atoms substitute Hg vacancies in the tetragonal phase which do not affect the onset critical temperature of the HgPb 2 compound. Simultaneous Sm and In doping effects on the properties of the HgPb 2 phase are also studied. Experimental and simulations of X-ray powder diffractograms suggest that the HgSm 1 - x In x Pb 2 system (0.2 ≤ x ≤ 0.7) has a pseudo-cubic structure with lattice parameters near 4.88 Å. Superconducting properties measurements show that this system has an optimum doping at x ∼ 0.5 with the onset critical temperature close to 6.9 K. This is the highest superconducting critical temperature for the AuCu prototype so far reported. © 2006 Elsevier B.V. All rights reserved. Keywords: Quaternary phase; Superconductors; Crystal structure; Intermetallic 1. Introduction Superconductivity in the HgPb 2 compound was recently reported in the literature with critical temperature ranging within the interval 5.7 ≤ T c ≤ 6.0 K [1]. The critical temperature of this compound depends on lead composition within the solubility limit which is between 65 and 73 at.% Pb. It was also observed that superconducting properties of this compound are strongly dependent on the synthesis conditions. Optimal results are obtained for heat treatments at 80 °C. The HgPb 2 phase crystallizes in a tetragonal cell, space group P4/mmm, with lattice parameters a = 4.985 Å and c = 4.55 Å which is isomorphic to the AuCu prototype [1]. In order to crystallize HgPb 2 in a AuCu prototype structure, vacancies must be created on the Hg site to fulfill the 1:2 Hg/Pb stoichiometric ratio [2]. With regard to possible substitutions on the mercury site of the HgPb 2 compound, it is necessary to consider atoms with size similar to the mercury atoms (atomic radium = 1.49 Å) and chemical affinity comparable to lead and/or mercury. Atoms of approximately 1.5 Å in radius would be able to occupy mercury sites in the HgPb 2 compound. Careful analyses of the binary- phase diagrams of several intermetallic alloys such as Hg–Sm, In–Pb, and Sm–Pb [3] suggest that Sm and In would be good candidates to occupy mercury site in the HgPb 2 compound, despite of their chemical affinity. It is also important to mention that both Sm and In elements have similar atomic radii compared to a Hg atom (1.66 and 1.49 Å, respectively). Thus, in order to investigate the influence of these atoms on the superconducting properties of the HgPb 2 compound, in this work a systematic study of the In and Sm dopings in HgPb 2 phase is reported. The results suggest the existence of a new superconducting quaternary phase. Materials Letters 61 (2007) 263 – 266 www.elsevier.com/locate/matlet ⁎ Corresponding author. Faenquil-Demar, Demar, Polo Urbo Industrial Gleba AI 6, C.P. 116, 12600970, Lorena-SP, Brazil. Tel.: +55 1231599911; fax: +55 1231533006. E-mail address: jefferson@demar.faenquil.br (A.J.S. Machado). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.094