[Pw_forum] Pressure-induced phase transition

[pascal boulet]

Dear Amin,

Regarding your question:

And how can I use phonon calculations to help me find the phase transition?
I've heard about some "mode softening", but I am not sure how it can be used
in this context?


When pressure changes you may observe that one or several phonon modes
are softening, meaning, their frequencies are decreasing, eventually
becoming imaginary in values. To see this you have to calculate the
phonon dispersion band, that is, the phonon modes over the whole
Brillouin zone (BZ), not just at the gamma point. If for some pressure
you observe a negative (or more correctly imaginary) frequency at a
particular q-point of the BZ, it means that the system undergoes an
instability and is likely to transform into another structure.

This you can see in the following paper: Phys. Rev. B, 78, 100102(R),
2008 in Figure 3b where the phonon softening is observed at the X q-point.


Regards
Pascal




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