[Pw_forum] phonon instability

[Mike Mehl]

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).


On 08/23/2011 02:40 PM, bhabya sahoo wrote:
> sir
> i am calculating the phase transition of some carbides
> the transition from B1 to B2 transition occures at 130 gpa by enthalpy
> compparison
> but i am able to find phonon instability at 130 gpa for B1 structure
> but at the same time i am not able to find the dyanamic stability of B2
> phase
> but it occures high pressure (more than130 gpa )
> is there any explanation for this ?
>
> is there any relation of thermodyanamical unstability
> and dyanamical unstability simultaneously?
>
>
> answers are highly appriceted with some reson
>
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> Bd sahoo
> reserch scholar
> barc mumbai
>
>
>
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-- 
Michael J. Mehl
Head, Center for Computational Materials Science
Naval Research Laboratory Code 6390
Washington DC