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Dear all :
In order to studied the phonon specter for SrO in B2 phase, we calculated static transition pressure, using the condition of equal enthalpy, the transition pressure obtained is p (t) =30 GPa, (expt =36 GPa), then we calculated phonon specter at 30, 40, 50, 60, 70 and 80 GPa for B2 phase, our calculated have a soft phonon at p (t) to 66 GPa. (B2 phase unstable at p (t), 30, 40, 50, 60) but stable at and up 66 GPa.
My question is:
For B2 phase, is it necessary around to p(t) the phonon specter stable?
How to explain the instability for B2 phase up to p(t) to 66 GPa?
There is a correlation between phonon stability for B2 phase and transition pressure?

Best regards

Dear somourad,

somourad wrote:For B2 phase, is it necessary around to p(t) the phonon specter stable? Read for expample, R.~E.~Peirls, {\it Quantum Theory of Solids} (Claredon Press, Oxford, 1956).

Yes, it is. More precisely, above p(t).

somourad wrote:How to explain the instability for B2 phase up to p(t) to 66 GPa?

You could explain it in terms of your thermodynamical stability condition, which you got from the enthalpy comparison (see A.~Mujica {\it et al.}, Rev. Mod. Phys. 75, 863 (2003)). Or from the lattice dynamical point of view (see the book above).

somourad wrote:There is a correlation between phonon stability for B2 phase and transition pressure?

Yes, there is. For these kind of phase transitions, Cochrans' and Andersons' theory of soft modes applies. Please, read W.~Cochran, Adv. Phys. {\bf 9}, 387 (1960) and P.~W.~Anderson, {\it Fizika dielektrikov} (Acad. Nauk SSSR, Moscow, 1960).
Usually, enthalpy comparison and lattice dynamics agree on the transition pressure, but not necessarily. Why is it so, I don't know. But it would be nice to find out for specific cases.

As it happened to be, a paper on high pressure lattice dynamics of SrO is being published in PRB. Maybe it would be interesting to You to check when it comes out.

Cheers!

Igor Lukacevic

Dear Igor Lukacevic
Thank you very much for your reply.
Best regards.
somourad