Communication pubs.acs.org/IC
The New Superconductor tP‑SrPd2Bi2: Structural Polymorphism and Superconductivity in Intermetallics Weiwei Xie,* Elizabeth M. Seibel, and Robert J. Cava* Department of Chemistry, Princeton University, Princeton, New Jersey, 08540 United States The origin of the polymorphism in SrPd2Bi2 is a symmetrylowering twist of the constituent tetrahedra, which we argue impacts the existence of superconductivity in a manner similar to the temperature-induced structural phase transition in the superconducting ThCr2Si2-type iron arsenides. In the CaBe2Ge2-type structure, M and X are typically heavy elements with a small electronegativity difference, so the distinction between transition metals and main group elements is not so clear. In the current case of SrPd2Bi2, there is no clear electronegativity differentiation between Pd and Bi. According to Pauling’s scale, the metal Pd (2.20) is more electronegative than Bi (2.02), while for Allen’s scale Bi (2.01) is more electronegative than Pd (1.59).12,13 As a consequence, SrPd2Bi2 should be considered an intermetallic compound, with a more subtle form of internal charge redistribution than is found in ThCr2Si2-type compounds. Polycrystalline pellets of SrPd2Bi2 were synthesized via solid-state reaction. Stoichiometric amounts of Pd (powder, 99.99%, Alfa Aesar) and Bi (powder, 99.999%, Alfa Aesar) were mixed with Sr (50% molar excess, as is commonly employed;14 99.9%, Alfa Aesar) and compacted into 300 mg pellets in a hydraulic press. The pellets were placed inside alumina crucibles and heated in evacuated quartz ampules at 1050 °C for 20 h, then reground, repelletized, and reheated for 20−40 h, until material with visible crystallites was observed. The tetragonal phase is metastable and slowly decomposes to the monoclinic phase at ambient temperature and pressure. The SrPd2Bi2 samples were examined by powder X-ray diffraction for identification and phase purity on a Bruker D8 ECO powder diffractometer employing Cu Kα radiation and a LynxEye-XE detector. The polycrystalline sample employed for superconductivity measurements matches the previously reported tetragonal primitive form of SrPd2Bi2.10 A Le Bail fit was carried out using the FullProf Suite with Thompson−Cox−Hastings pseudo-Voigt peak shapes (Figure 1).15 An impurity phase of SrPdBi was detected at a level of
