[Pw_forum] phonon instability

[bhabya sahoo]

ok but i have seen many papers that this B2 unstability due to some
anaharmonic contributions
or it may be due to band closing phenomenon
can u explain this more correctly because i am doing calculation on
transition metal carbides
On Wed, Aug 24, 2011 at 9:01 PM, Mike Mehl <Michael.Mehl at nrl.navy.mil>wrote:

> ZnS, WC, and NiAs are the likeliest candidates, but not the only ones.
>
> The unstable B1 phonons lead to structures which relax to something
> lower in energy than B1.  Since all of your candidate structures have a
> higher enthalpy than B1 at 130 GPa, it can't be one of those structures.
>  Hence, it's a different structure.
>
> It should be frowned upon to hype one's own publications, but if you look
> at
>
> M. J. Mehl, G. L. W. Hart, and S. Curtarolo
> Journal of Alloys and Compounds, 509, pp. 560-567 (2011)
>
> (http://www.sciencedirect.com/science/article/pii/S0925838810021225 or
> http://arxiv.org/abs/1006.0236)
>
> There are examples of how you can find new structures from unstable
> phonon modes.  This case sounds simpler than IrV.
>
> It would, of course, help us if we knew which carbides you were working on.
>
> On 08/24/2011 10:35 AM, bhabya sahoo wrote:
> > no i have calculated correctly because the convergence is good with
> > higher k points
> > second is that i have tested zns,wc,NiAs phase none of them have less
> > enthalpy at 130 gpa
> > other thing is that it is stable at above 250 gpa
> >
> >
> > On Wed, Aug 24, 2011 at 7:38 PM, Mike Mehl <Michael.Mehl at nrl.navy.mil
> > <mailto:Michael.Mehl at nrl.navy.mil>> wrote:
> >
> >     If the B1 structure is unstable with respect to the B2 structure
> above
> >     130 GPa, but the B2 structure is dynamically unstable at and above
> 130
> >     GPa, there are two possible conclusions:
> >
> >     1) The phonons for the B2 structure were not calculated accurately,
> or
> >
> >     2) There is a competing structure that is lower in enthalpy than B1
> or
> >     B2 at 130 GPa
> >
> >     Without more information, no one can judge which conclusion is
> correct,
> >     suggest remedies for 1), or possible structure for 2).
> >
>
>
> --
> Michael J. Mehl
> Head, Center for Computational Materials Science
> Naval Research Laboratory Code 6390
> Washington DC
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