* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Beryllium Silicide Clusters, BenSin, Be2nSin (n = 1 – 4) and possible MgB2-like Superconductivity in some of them 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 The symmetry of the Periodic Table makes it possible to predict certain properties of similar elements and compounds using one of them as a template. Magnesium diboride, MgB2, presents a useful template in the search for similar materials. Starting from electronegativity, valence electron and atomic number equivalency, we identify many potential similar materials. One of them is the beryllium silicide, Be2nSin cluster system. We establish that though not yet produced in bulk, Be2Si exists. We show from symmetry principles that beryllium silicide and some of its clusters will be MgB2-like superconductors with Tcs close to or higher than 39K. Introduction The discovery of superconductivity in MgB2 in 2001  came as a surprise to many researchers in superconductivity. A bigger surprise also came when application of electronic structure principles [2 - 5] and crystal symmetry concepts [6 – 13] failed to produce MgB2-like superconductors with Tcs close to the 39K of MgB2. In searching for MgB2-like superconductors and other superconductors, we decided to look for materials specific correlations, often ignored by standard theories of superconductivity. Some of the chemical correlations with superconductivity are electronegativity, valence electron count, atomic number and formula weight. Our studies [14 - 18] indicate that they play a very significant role in the search for superconductivity. In this paper, we first review studies [19 – 24] on the existence, properties and preparation of beryllium silicide clusters. We then use the material specific characterization system (MSCD), symmetry rules , and the Tc equation to compare the material specific characteristics of Be2nSin clusters which have same valence electron count and atomic number as MgB2 and to estimate the Tc of Be2Si, based on this symmetry. Beryllium Silicide Studies Group IIA-IV alkaline-earth silicides have been studied by the ab initio pseudopotential method  and the full-potential linearized augmented plane wave method , within the local density approximation (LDA). This family is found to have an anti-fluorite structure and semiconducting. Beryllium silicide, Be2Si, a member of this family was however computed to be a metallic , antifluorite, with Si atoms in face-centered cubic structure and the Be atoms arranged around them in tetrahedral structure. In addition, the Be-Si bonding has been studied [21, 22] by density functional theory (DFT) Monte Carlo simulated annealing (DFTMCSA) methods and cluster geometries discovered. In particular, the structures and energetics of BenSin and Be2nSin (n = 1 – 4) clusters [21, 22] strongly suggest that they could be explored for superconductivity like the carbon-60 fullerene . Hite et al  have explored the possible formation of Beryllium silicide. Using scanning tunneling microscopy (STM) and photoelectron spectroscopy, they studied the nucleation, growth and structure of beryllium on Si(111)-(7x7)surface at temperatures ranging from 120K to 1175K. They produced amorphous ring nanocluster structure with Be-Si bond length less than 2.5 A of Beryllium silicide. Further studies by Saranin et al.  confirmed the cluster structure and discovered four types of nanostructure arrays formed by Be interaction with Si(111)-(7x7) surface in a not fully understood “self assembly” process. MSCD of Be2Si, Symmetry Rules and Tc Estimation In [15, 16] we showed that a material may be characterized in terms of averages of electronegativity, , valence electron count, Ne, atomic number, Z, and formula weight, Fw, in a method described as material specific characterization dataset (MSCD). MSCD makes it possible to quickly compare two or more materials. Table 1 gives the MSCD of MgB2, LiBSi and Be2nSin clusters. We also showed in  that the maximum transition temperature, Tc, of a superconductor can be estimated in material specific properties of electronegativity, , valence electrons, Ne, atomic number, Z, and a parameter, Ko by the equation: Tc = Ko (1) where Ko is a parameter that determines the value of Tc. Ko = n(Fw/Z) and n is dependent on the family of superconductors. Fw represents formula weight of the superconductor. For MgB2, with Tc of 39K and Fw/Z of 6.26, Ko = 22.85, making n = 3.65. Note that the MSCDs of LiBSi and Be2Si are identical. One of the symmetry rules  states that two materials with exactly identical MSCDs will have the same Tc. Following this symmetry rule and the MSCD displayed in Table 1, we estimate that the Tc of Be2Si will be the same as that of LiBSi , which is 36K. Discussion Even though bulk Be2Si has yet to be produced and fully characterized, recent computations [19, 21, 22] and experiments [24, 25] strongly suggest that beryllium silicide exists. Be2Si is an image of LiBSi, formed by replacing LiB with 2 atoms of Be. Thus Be2Si and LiBSi have the same electronegativity, valence electrons and atomic number. Be2Si also has the same valence electron count and atomic number as MgB2 making it an MgB2-like material. From the symmetry rules for searching for superconductors , materials with the same valence electron count and same atomic number will have Tcs proportional to their electronegativities. The challenge of producing bulk Be2Si is akin to that of producing bulk C60 fullerene [23, 26]. We note too that well-ordered binary cluster Cs3C60 attained a Tc of 38K . It will be interesting if well-ordered Be2Si could achieve a higher Tc than the 38K of Cs3C60. The verification (or otherwise) of superconductivity in Be2Si and some of its clusters will be a strong test of the symmetry rules [15, 18] for searching for superconductors. Conclusion We have studied the hypothetical cluster compound beryllium silicide. Using symmetry rules for searching for equivalent superconductors and MgB2 as a template, we find that Be2Si, Be4Si2, Be8Si4 are copies of MgB2 but with reduced electronegativity. Be2Si too is very similar to LiBSi, which we had earlier shown  should be superconducting. We conclude that Be2Si will be found to be superconducting with a Tc of about 36.2K. The clusters: Be4Si2, Be6Si3, Be8Si4 may have Tcs higher than MgB2 since they have higher Fw/Z ratio . Acknowledgements The author acknowledges financial support for this research from M.J. Schaffer, then at General Atomics, enlightening discussions with M.B. Maple at UC San Diego and literature support from J.R. O’Brien at Quantum Design, San Diego. References 1. J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani and J. Akimitsu, “Superconductivity at 39K in Magnesium Diboride,” Nature 410, 63 (2001) 2. N.I. Medvedeva, A.L. Ivannovskii, J.E. Medvedeva and A.J. Freeman, “Electronic structure of superconducting MgB2 and related binary and ternary borides”, Phys. Rev. B 64, 020502 (R) 2001 and references therein 3. H. Rosner, A. Kitaigorodsky and W.E. Pickett, “Predictions of High Tc Superconductivity in Hole-doped LiBC”, Phys Rev. Lett. 88, 127001 (2002). 4. 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). 5. Hyoung Jeon Choi, Steven G. Louie and Marvin L. Cohen, “Prediction of superconducting properties of CaB2 using anisotropic Eliashberg Theory”, Phys. Rev. B 80, 064503 (2009) and References 1 - 21 in that paper. 6. A. Bharathi, S. Jemima Balaselvi, M. Premila, T.N. Sairam, G.L.N. Reddy, C.S. Sundar, Y. Hariharan “Synthesis and search for superconductivity in LiBC” Arxiv:cond-mat/0207448V1 and references therein., Solid State Comm, (2002), 124, 423 7. Renker, H. Schober, P. Adelmann, P. Schweiss, K.-P. Bohnen, R. Heid ,“LiBC - A prevented superconductor”,Cond-mat/0302036 8. A.M. Fogg, J.B. Calridge, G.R. Darling and M.J. Rossiensky “Synthesis and characterization of LixBC---hole doping does not induce superconductivity”. Condmat/0304662v1 9. A. Lazicki, C.S. Yoo, H. Cynn, W.J. Evans, W.E. Pickett, J. Olamit, Kai Liu and Y. Ohishi “Search for superconductivity in LiBC at high pressure: Diamond anvil cell experiments and first-principles calculations” Phys. Rev. B 75, 054507 (2007). 10. 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 11. 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) 12. 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). 13. R.L. Meng, B. Lorenz, J. Cmaidalka, Y.S. Wang, Y.Y. Sun, J. Lenzi, J.K. Meen, Y.Y. Xue and C.W. Chu, “Study of intermetallic compounds isostructural to MgB2,” IEEE Trans. Applied Superconductivity, Vol. 13, 3042- 3046 (2002). 14. O. Paul Isikaku-Ironkwe, “Search for Magnesium Diboride-like Binary Superconductors” http://meetings.aps.org/link/BAPS.2008.MAR.K1.7 15. O. P. Isikaku-Ironkwe, “Transition Temperatures of Superconductors estimated from Periodic Table Properties”, Arxiv: 1204.0233 (2012) 16. O. P. Isikaku-Ironkwe, “Possible High-Tc Superconductivity in LiMgN: A MgB2-like Material”, Arxiv: 1204.5389 (2012) and references therein. 17. O. P. Isikaku-Ironkwe, “Prospects for Superconductivity in LiBSi”, Arxiv: 1205.2407 (2012) 18. O. P. Isikaku-Ironkwe, “Is Lithium Sulfide a MgB2-like Superconductor?”, Arxiv: 1205.4051 (2012) 19. J. L. Corkill and M.L. Cohen, “Structural, bonding and electronic properties of IIAIV antifluorite compounds, Phys. Rev B 48, 17138 -17144 (1993) 20. F. Kalarasse, B. Bennecer, “Electronic and optical properties of the antifluorite semiconductors Be2C and Mg2X (X = C, Si, Ge) under hydrostatic pressure”, J. Phys. Chem. of Solids 69 (2008) 1775 - 1781 21. R.C. Binning, Jr. and D.E. Bacelo, “Structures and energetics of BenSin and Be2nSin (n = 1 – 4) clusters, J. Phys. Chem. A 2005, 109, 754 – 758 22. S. Fioressi, D. E. Bacelo, R.C. Binning Jr., “ A DFT study of dodecahedral beryllium silicide cage clusters”, Chem. Phys. Lett. 537 (2012) 75 – 79 23. H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley. “C60: Buckminsterfullerene” Nature 318 (1985) 162 - 163 24. D. A. Hite, S.-J. Tang, P.T. Sprunger, “Reactive epitaxy of beryllium on Si(1 1 1)-(7 x 7)”, Chem. Phys. Lett. 367 (2003) 129 -135 25. A. A. Saranin, A.V. Zotov, V.G. Kotlyar, T.V. Kasyanova, O. A. Utas, H. Okado, M. Katayama, K. Oura, “Self-assembly formation of the ordered nanostructure arrays induced by Be interaction with Si(1 1 1) surface”, Surface Science 574 (2005) 99 109 26. W. Kratschmer L. D. Lamb, K. Fostiropoulos, D.R. Huffman, “Solid C60: a new form of carbon”. Nature 347, 354 -358, 1990 27. Alexey Y. Ganin, Yasuhiro Takabayashi, Yaroslav Z. Khimyak, Serena Margadonna, Anna Tamai, Matthew J. Rosseinsky & Kosmas Prassides “Bulk superconductivity at 38 K in a molecular system”, Nature Materials 7, 367 - 371 (2008). TABLE Table 1: MSCDs of MgB2, LiBSi and Beryllium Silicide Clusters Be2nSin (n = 1 – 4) computed from equations in ref.  and equation (1) in this paper. Note that LiBSi and Be2Si have exactly the same electronegativity, , valence electron count, Ne, atomic number Z and almost the same formula weight Fw. Symmetry rule given in  predicts that their Tcs will be about the same. Material Ne Z Ne/ Fw Fw/Z Tc(K) Ko 1 MgB2 1.7333 2.6667 7.3333 0.9847 45.93 6.263 39 22.85 2 LiBSi 1.6 2.6667 7.3333 0.9847 45.84 6.251 35.95 22.85 3 Be2Si 1.6 2.6667 7.3333 0.9847 46.11 6.288 36.2 22.85 4 Be4S2 1.6 2.6667 7.3333 0.9847 92.22 12.58 >39? 22.85? 5 Be6S3 1.6 2.6667 7.3333 0.9847 138.33 18.86 >39? 22.85? 6 Be8S4 1.6 2.6667 7.3333 0.9847 184.44 25.15 >39? 22.85?