PROSPECT
It is interesting to note that there seems to be a close correlation between the
anharmonicity of the E2g in-plane modes and the B-B bond length
(dBB).
For MgB2, dBB is 1.764 A, significantly stretched
from its optimal value of 1.65 A in elemental planar boron, probably due to repulsive
interactions between the Mg and B ions.
This explains the unusual anharmonicity and observed high Tc in MgB2.
On the other hand, for BeB2 the optimized dBB is found to be
1.67 A with no significant anharmonicity. It is interesting that this compound does not
superconduct even though its band structure is quite similar to that of MgB2.
For hypothetical CaB2, the calculated dBB is quite large
(1.84 A) and we calculate very large anharmonicity, which may explain the instability
of CaB2.
It then appears likely that MgB2 is fortuitously just at the phase boundary.
It will be interesting to see if it is possible to expand the dBB in
MgB2 by Ca substitution on the Mg site to increase Tc further.