[Pw_forum] gap in the phonon DOS

[Stefano Baroni]

On Oct 25, 2009, at 3:43 PM, Reza Ebraahimi wrote:

>
> Hi
> Thanks for your reply
> What i meant to say was, does not the ionicity (Born charges and ..)  
> of the crystal, which splits the TO-LO phonons, show itself as a gap  
> in high-energy region of the phonon DOS?

it may, or may not, according to the relative magnitude of the LO-TO  
splitting vs. band-width

> If not, what is the sign of this splitting in the phonon dos diagram?

LO phonons have a higher frequency

Notice, however, that in low-simmetry crystals the distinction between  
transferse and longitudinal modes is not as meaningful as, say, in  
cubic crystals, because long-wavelengths phonons are neither purely  
transverse, nor longitudinal

Stefano B

> Thanks again
>
> R. Ebraahimi
> Grad. Student- Tehran Uni.
> > 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
>
>
>
>
>
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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