[Pw_forum] phonon instability

[Mike Mehl]

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