[Pw_forum] measuring covalent vs. ionic nature of Li bonding in LiC6

[Natalie Holzwarth]

We studied LiC6 many years ago -- Phys. Rev. B 28, 1013 (1983) and there
have been quite a few others who have studied it since.  If I remember
correctly, to a very rough approximation you can interpret the band
structure results as Li donating its charge to the graphite pi bands. To
complicate the picture, Posternak and co-workers Phys. Rev. Lett. 50, 761
- 764 (1983) pointed out an existence of an interlayer band which we later
(Phys. Rev. B 30, 2219 (1984)) tried to reconcile with the Li donor band
picture. However, what is the physical distribution of the charge in the
vicinity of the Li site is another story.  Within the picture of the
top valence  bands of LiC6 being primarily composed of C pi states, there
will still be electron density near the Li site.   To determine whether
this electron density is due to a superposition of C pi states or whether
there is some hybridization with Li 2s states is very difficult to
determine quantitatively in the sense there is not an obvious way to
partition the charge.    As long as you are careful to report how you
you do your analysis you will at least have a reproducible parameter....

That's my 2 cents worth.
Natalie Holzwarth

On Fri, 2 May 2008, Yaser Rehem wrote:

> Dear Pwscf Users-
>
> Can anyone offer advice on how best to measure the nature of Li
> bonding in LiC6?  I've used projwfc.x to calculat the pDOS of the
> valence Li electron in a relaxed LiC6 structure, and I find the
> results surprising.  If I use a gga ps, I find that the valence Li
> electron in LiC6 has an approximately 55% overlap with it's free
> valence states, and an approximately 69% overlap if I use an lda ps
> with a semi-core.   Many studies show the charge transfer from Li to C
> in LiC6 to be almost complete--i.e., the Li is essentially fully
> ionized--hence my surprise at the pDOS results.
>
> Can anyone offer alternate method(s) by which I can assess the
> relative covalent vs. ionic nature of Li bonding in this structure?
>
> Thank you,
>
> -Yaser Rehem
> Rehem Research & Consulting
> (a.k.a. Yaser working in his basement :-)
>
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N. A. W. Holzwarth			email: natalie at wfu.edu
Department of Physics			www: http://www.wfu.edu/~natalie
Wake Forest University		        voice: 336-758-5510
Winston-Salem, NC 27109-7507            fax: 336-758-6142
U. S. A.