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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.
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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
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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.
References
1. J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani and J. Akimitsu,
“Superconductivity at 39K in Magnesium Diboride,” Nature 410, 63 (2001)
2. A.S. Cooper, E. Corenzwit, L.D. longinotti, B.T. Matthias and W.H. Zachariassen, “
Superconductivity: the transition temperature peak below four electrons per
atom”, Proc. Nat. Acad. Sci , 67, 313-319 (1970)
3. Cristina Buzea, Tsutomu Yamashita, “Review of superconducting properties of
MgB2”, Superconductor Science & Technology, Vol. 14, No. 11 (2001) R115-R146
4. I. Felner “Absence of superconductivity in BeB2”, Physica C 353 (2001) 11 – 13.;
D.P. Young, P.W. Adams, J.Y. Chan and F.R. Franczek, “Structure and
superconducting properties of BeB2” Cond-mat/0104063
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5. B. Lorenz, J. Lenzi, J. Cmaidalka, R.L. Meng, Y.Y. Sun, Y.Y. Xue and C.W. Chu,
“Superconductivity in the C32 intermetallic compounds AAl2−xSix, with A=Ca and Sr;
and 0.6<x<1.2” Physica C, 383, 191 (2002)
6. R.L. Meng, B. Lorenz, Y.S. Wang, J. Cmaidalka, Y.Y. Xue, J.K. Meen. C.W. Chu“Study
of binary and pseudo-binary intermetallic compounds with AlB2 structure” Physica
C: 382, 113–116(2002).
7. H. Rosner, A. Kitaigorodsky and W.E. Pickett, “Predictions of High Tc
Superconductivity in Hole-doped LiBC”, Phys Rev. Lett. 88, 127001 (2002).
8. C. Bersier, A. Floris, A. Sanna, G. Profeta, A. Continenza, E.K.U. Gross and
“Electronic, dynamical and superconducting properties of CaBeSi”, ArXiv:
0803.1044 (2008).
9. O. Paul Isikaku-Ironkwe, “Search for Magnesium Diboride-like Binary
Superconductors” http://meetings.aps.org/link/BAPS.2008.MAR.K1.7
10. O. P. Isikaku-Ironkwe, “Transition Temperatures of Superconductors estimated
from Periodic Table Properties”, Arxiv: 1204.0233 (2012)
11. O. P. Isikaku-Ironkwe, “Possible High-Tc Superconductivity in LiMgN: A MgB2-like
Material”, Arxiv: 1204.5389 (2012) and references therein.
12. O. P. Isikaku-Ironkwe, “Prospects for Superconductivity in LiBSi”, Arxiv: 1205.2407
(2012)
13. W. Buehrer, F. Altorfer, J. Mesot, H. Bill, P. Carron and H.G. Smith, “ Lattice
dynamics and the diffuse phase transition of lithium sulfide investigated by
coherent neutron scattering”, J. Phys Condensed Matter, 3, 1033-1064, (1991)
14. P. Pandit, B. Rakshit and S. P. Sanyal, “Electronic and elastic properties of alkalimetal sulphides-Li2S and Na2S”, Indian J. Pure & Appl. Phys. 47, 804-807 (2009)
15. Materials Project, Li2S, (2012), www.materialsproject.org/tasks/1125
16. Materials Project, Li2S, (2012), www.materialsproject.org/tasks/1153
17. V.V. Struzhkin, R.J. Hemley, H-K Mao, Y.A. Timofeev, “Superconductivity at 10 17K in compressed sulphur, Nature 390, 382-384, (1997)
18. K. Shimizu, H. Ishikawa, D. Takao, T. Yagi and K. Amaya “Superconductivity in
compressed lithium at 20K”, Nature 419, 597 – 599, (2002)
19. B.T. Matthias, “Symmetries of superconducting sulfides”, Intl. J. of Quantum
Chem, S 10, 435 - 436 (1976).
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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.
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