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Is Lithium Sulfide a MgB2-like Superconductor? O. P. Isikaku-Ironkwe1, 2 1 The Center for Superconductivity Technologies (TCST) Department of Physics, Michael Okpara University of Agriculture, Umudike (MOUAU), Umuahia, Abia State, Nigeria and 2 RTS Technologies, San Diego, CA 92122 Abstract Lithium Sulfide, Li2S, is an anti-fluorite semiconductor with a band-gap of 3.865 eV. It also has exactly the same valence electron count, Ne, and atomic number, Z, as magnesium diboride, MgB2. Both have almost the same formula weight. This qualifies Li2S as a magnesium-diboride like material. Li2S passes the same computational material specific test for superconductivity as MgB2. Using our recently developed symmetry rules for searching for superconductors, we predict that Li2S, with electronegativity of 1.5, will be superconducting with a Tc of 33.8K, if double gapped or 16.9K if single gapped. Introduction The discovery of MgB2 as a superconductor [1], long overlooked by experts [2], led to an avalanche of renewed interest and publications [3] in binary and ternary superconductors in a relatively short time. Though the electronic structure and properties were quickly deciphered, the progress of reverse engineering of MgB2-like superconductors was not equally a successful venture [4, 5, 6]. In the unsuccessful search, structure and iso-valency were the primary theoretical model and experimental strategy [5 - 8]. Using a new model based on similarities of electronegativity, valence electrons, atomic number and formula weight [9, 10] we have been able to predict many MgB2-like superconductors awaiting experimental verification. Two of them are Lithium magnesium nitride LiMgN [11] and LiBSi [12]. In this paper, we review the structure and properties of another material, lithium sulfide Li2S. We then apply the material specific characterization analysis scheme [10] and compare Li2S, with MgB2. We apply the symmetry rules for similar superconductors [10, 11], and estimate the Tc of Li2S. 1 Structure and Properties of Li2S Lithium sulfide is a much studied material [13, 14], though never tested for superconductivity. Li2S can exist in two forms: orthorhombic and cubic. The orthorhombic form [15] belongs to space group Pmnb and has dimensions: a = 3.808Å; b = 6.311Å; c = 7.262Å. It has density of 1.75g/cm3. The cubic version [16] has density of 1.63g/ cm3, belongs to space group Fm-3m and has cubic dimensions 4.046Å. The electronic structure and density of states [16] indicate that cubic Li2S is an indirect band-gap semiconductor with a band gap of 3.865 eV. Lithium sulfide melts between 900 – 975 degrees centigrade. Symmetry Rules and Tc Estimation Guided by atomic number symmetry we find that Li2S has same average atomic number as MgB2. A MSCD [10] characterization of Li2S yields the data displayed in table 1 and figure 1, showing that it is a MgB2-like material. Earlier [10], we derived a formula for estimating maximum Tc, given by Tc = Ko (1) where , Ne, Z are averages respectively of electronegativity, valence electron count and atomic number and Ko is a parameter that determines the value to Tc. Ko = n(Fw/Z) and n is dependent on the family of superconductors. Fw represents formula weight of the superconductor. For MgB2, Ko = 22.85 and Fw/Z is 6.26, making n = 3.65. The computational material specific test for superconductivity is that 0.75 <1.0. Lithium sulfide meets this test range, implying that it is a superconductor. The applicable symmetry rule [10, 11 ] is: “If two or more materials have the same average valence electrons Ne, and atomic number Z, then their Tcs will be proportional to their electronegativities.” Applying this rule, we find that the Tc of Li2S will then be approximately 1.5/1.7333 of 39K or 33.8K. Discussion Sulphur has been found under pressure to show superconductivity up to 17K [17]. Lithium too is known to be a metallic superconductor [18] with Tc of 20K under pressure of 48 GP. One 2 would expect that a combination of lithium and sulphur should produce an even higher Tc. It is interesting to observe that the band-gap of Li2S of 3.865 eV is close to that of LiMgN [11] which we predicted as having same Tc as MgB2. Generally we have found that ternary or three-atom materials with whose atomic numbers add up to 22 tend to have their valence electrons adding up to 8, just like magnesium diboride. Matthias [19] who was one of the first to study superconducting sulfides, used the concept of symmetry of crystals in the Periodic Table to raise Tc in a number of sulfides. Lithium and its compounds find strong commercial applications in the battery industry. If confirmed, this will be the first lithium-sulphur compound superconductor. Conclusion Simple symmetry rules [10, 11], involving electronegativity, valence electrons and atomic number and formula weight, discovered to apply to many other superconductors, strongly suggest that Lithium sulfide, Li2S, though an ionic compound may prove to be a superconductor with Tc of 33.8K, comparable to that of magnesium diboride. Acknowledgements Discussions with A.O.E. Animalu on symmetry in mathematics, physics, chemistry and nature influenced my search strategy. My thanks go to J.B. O’Brien at Quantum Design who provided literature support and useful discussions while M.J. Schaffer, then at General Atomics, fully sponsored this research. 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J. of Quantum Chem, S 10, 435 - 436 (1976). 4 Material Fw/Z Tc(K) Ko Ne/ Fw Ne Z 1 MgB2 1.7333 2.667 7.3333 0.9847 45.93 6.263 39 22.85 2 Li2S 1.5 2.6667 7.3333 0.9847 45.95 6.266 33.8 22.85 Table 1: Material specific characterization datasets (MSCDs) for MgB2 and Li2S. Note that Ne and Z are the same for MgB2 and Li2S. Their formula weights (Fw) differ by just .02. The Tc of Li2S was computed with equation (1) and the symmetry rule. The Ne/ tells us that Li2S is a superconductor like MgB2. 8 7 6 5 MgB2 4 Li2S 3 0 2 1 0 Fw/z Z Ne X Figure 1: MgB2 and Li2S compared in terms of Fw/Z, Z, Ne and X. This similarity suggests that their Tcs will be close. 5