Polaron Band Model and Its Application to Ce–S Semiconductors

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Abstract
The low‐lying eigenstates of the ``large polarons'' have been calculated by several authors for arbitrary strengths of the electron lattice interaction α. However, if α > 1 the large polaron picture becomes questionable, since for finite temperatures the polaron eigenstates may be strongly affected by the presence of thermal phonons; then a new approach to the polaron theory is applicable which takes into account the atomicity of the lattice and the presence of thermal phonons and which results in the ``small polaron'' picture. The eigenstates of small polarons depend on T. If the eigenstates form a band, the bandwidth is a function of T, and the eigenstates near the band extremum can be expressed in terms of a T‐dependent effective mass. From measurements of the high‐ and low‐frequency dielectric constants, of the temperature dependence of the Seebeck coefficient, and of the electronic mobility, it appears that the eigenstates of the electronic charge carriers in Ce–S semiconductors may be adequately described by the small polaron picture.

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