Dear all, I have calculated phonon frequency for perovskite compound at cubic phase. As it is known a presence of soft modes at this structure give new structure more stable than a cubic phase. But in my calculations I found these new structures less stable than parent structure (cubic) with little difference about 2 meV. So in this case, my calculation is wrong or it is correct and just it has no phase transition to these structures. Any help please?
From where you can say it is stable or not?
Do you use XRD or XPS?
No, i didn’t use XRD because I'm doing a theoretical study of stability.
Theoretically, one of the methods to demonstrate a stability of structure is calculation of phonon at this structure. If we find soft mode (frequency with a negative value) we can say this structure is not stable. This is meaning we have phase transition from this structure to another structure more stable (it has less energy than parent structure). These new structures are determined by condensation of soft modes found in parent structure.
