- Consider a simple ionic compound or metal. What would be symmetry of its crystal structure under a given temperature and pressure?
- How and why free energy of crystal lattice with different symmetry vary differently with temperature and pressure? Why temperature and pressure selectively prefer some symmetry over other while P and T have themselves no spatial symmetry? (note, higher pressure do not universally prefer highest packing density phase, does it?)
- how would an atom's/ion's coordination number dependent on shape of its electronic orbital?
Ulrich Deiters
There is no simple answer to this question. If one assumes that the ions forming a lattice are rigid spheres, interacting with Coulomb potentials only, it is possible to state at which cation/anion size ratio the bcc lattice will become more stable than the fcc lattice.
But ions are not rigid. Furthermore, they also interact with dispersion (van der Waals) forces, and sometimes there are even stronger chemical interactions (for instance because of partially occupied d or f electron shells).
Increasing the pressure will always favour the denser phase (Le Châtelier's principle).
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