[Pw_forum] gap in the phonon DOS

[Eyvaz Isaev]

Dear Reza,

--- On Sat, 10/24/09, Reza Ebraahimi <rezaebraahimi at yahoo.com> wrote:

> Does the existence of a gap in the high-energy region of the phonon DOS > means ionicity of the  crystal 

No, does not. It means just the mass difference, i.e. one of compound componenet is rather light, than other(s), as examlpe, NiAl, FeAl, FeH, PdH, etc. 

> (splitting of the TO and LO-like phonons)? 

This one is more relevant for the ionicity issue. This happens in polar semiconductors/insulatoras. Another excitnig example is gamma-boron phase where a cage of B-atoms (icosahedra, B12) is charged negatively, but dumbbell B-atoms B2 are charged positively giving rise for LO-TO splitting. 

> The higher the gap, means the more ionicity?

The gap depends on both  the Born effective charge and static dielectric constant (see any textbook, or Rev.Mod.Phys. 73, 515, 2002, see page 527). So, the gap is the result of the competition of these parameters.  
I suppose analyzing this relation for yourself will give you more insight.  
 
Hope this helps.

Bests,
Eyvaz.

-------------------------------------------------------------------
Prof. Eyvaz Isaev, 
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia, 
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden 
Condensed Matter Theory Group, Uppsala University, Sweden 
Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com