Download Beryllium Silicide Clusters, BenSin, Be2nSin (n

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 [1] 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 [15], 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 [19] and the full-potential linearized augmented plane wave method [20], 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 [19], 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 [23]. Hite et al [24] 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. [25] 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 [15] 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 [15] 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 [17],
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 [15], 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
[27]. 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 [17] 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 [15].
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.
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TABLE
Table 1: MSCDs of MgB2, LiBSi and Beryllium Silicide Clusters Be2nSin (n = 1 – 4) computed from
equations in ref. [15] 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 [15] 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?