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 > > > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum --- Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype) La morale est une logique de l'action comme la logique est une morale de la pens?e - Jean Piaget Please, if possible, don't send me MS Word or PowerPoint attachments Why? See: http://www.gnu.org/philosophy/no-word-attachments.html -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20091025/03601ca9/attachment.htm
