Electronic polaron model band narrowing

In many oxides, and particularly those with large percentage ionic bonding, periodic fluctuations of the electric potential associated with each ion become too large (and energy bands too narrow) so that the band model provides an inadequate description or theory. In this case the electrons or holes may be considered to be localised at the lattice atoms. Such localised electronic defects are termed polarons and may from a chemical point of view be considered to constitute valence defects. 

Another experimental evidence against the polaron lattice model for the metallic state of heavily doped trans-(CH)j comes from Electron-Energy-Loss Spectroscopy (EELS) data [21]. These data show levels spread well across the gap, which is more in agreement with the disordered incommensurate state than with the picture of narrow polaron bands in the gap. Band structure calculations using the Valence Effective Hamiltonian (VEH) technique [22] support this conclusion since it is shown that a large energy gap exists between the polaron bands in the band structure of the polaron lattice. On the other hand, experimental and theoretical results have been presented that support the polaronic metal state for doped polyaniline (emeraldine salt) [23]. 

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