Are delocalized holes and trapped holes (polarons) mutually exclusive? Most of the density functional theory literature that I read and related to holes in metal oxides treats polarons and delocalized holes as mutually exclusive entities. The one with the lower formation energy is always assumed to be the one that is only present in the material (may be until certain temperature upon which the domination flips). Experimentalists also tend to explain conductivity measurements in this sense of mutual exclusiveness. What I'm wondering, why isn't it possible to have two populations at the same time but at lower temperature the delocalized population dominates the conductivity and at higher temperatures the polarons do the the job? Any thoughts or suggested references are appreciated?