[Wien] alakali anisotropy

[Peter Blaha]

Nice to discuss some physics and not just program technical details!

I would expect that C' is connected with the "Bain-path", i.e. with the
tetragonal distortion from bcc to fcc structure. All these alkali elements
transform easily between fcc and bcc and thus C' should be small, thus your
anisotropy is large. In a "strong bcc" d-metal like W I'd expect C' rather
large. For d metals the bcc-fcc issue is related to the generic form of the
DOS and the respective filling (number of electrons).
Zr should also have a larger anisotropy, since bcc and fcc (hcp) are close ?


> recently, I came across the elastic anisotropy
> ratio in cubic metals defined as a ratio of the
> trigonal shear modulus C_44 and the tetragonal shear
> modulus C'=(1/2)(C_11-C_12), i.e. A = C_44/C'.
> There is one striking fact I would like to understand:
>
> For tungsten, a 5d metal, A = 1, i.e. tungsten is
> elastically isotropic! On the other hand, the alkali
> metals, the most free-electron metals, exhibit the
> highest elastic anisotropy factor among the elements:
>
> Element  A
> ----------
> Li   9.391
> Na   8.286
> Rb   7.435
> K    7.250
> Cs   7.220
> ----------
>
> One would expect that alkali metals, as nearly free-electron
> metals, would be also nearly elastically isotropic. On the
> other hand, a high elastic anisotropy could be expected in
> d-metals, such as tungsten, exhibiting directional bonds. As I show
> above, just the opposite is true. Does anybody of you know what
> is the origin of such a high elastic anisotropy in alkali
> metals? And why is tungsten elastically isotropic? I will be
> grateful for any hint or reference!
>
> With Best Regards,
>
> Sincerely,
>
> Dominik Legut
>
> P.S. I know that these questions are not related to the Wien code, but
> still this is a physical community...
>
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>


                                      P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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