For variation of the electronic structure with pressure. I apply pressure (Isotropic pressure); I optimize the structure and do my band structure calculations and have my expected results.
Only when I want to completely change the structure into a new polymorph, then I really don’t know how to proceed?
In general, different polymorphs will have different symmetries and different numbers of atoms per unit cell, so you cannot model a phase transition between two polymorphs directly using a primitive cell of either polymorph.
Assuming that you're interested in the change in electronic structure under pressure between two polymorphs, you can simply study the two structures independently. If you calculate the enthalpy of each phase, the transition pressure is when the enthalpies are equal. Equivalently, you can plot the energy-volume curves of the two phases on the same graph, and the common tangent is the transition pressure. See, for example, my colleague Matt Probert's lecture notes here:
https://www-users.york.ac.uk/~mijp1/teaching/grad_FPMM/lecture_notes/lec5_structural_calculations.pdf
Note that this is effectively saying when the phase transition is favourable (would lower the free energy), it does not tell you anything about the actual mechanism or energy barrier to it happening. This means you don't know how quickly the transition would happen.
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