Two-electron tin centers with negative correlation energy in lead chalcogenides. Determination of the Hubbard energy

Abstract

It is shown using Mossbauer spectroscopy on the isotope 119Sn that an isovalent tin impurity in the solid solutions PbS1−z Se z is a two-electron donor with negative correlation energy. The energy levels associated with the tin impurity are found in the lower half of the band gap for z 0.7. We have demonstrated electron transfer between neutral and ionized tin centers. The activation energy E o of this process decreases monotonically as z is increased, which reflects the fact that the tin energy levels move toward the top of the valence band, and for z>0.7 the activation energy is E o =U/2, where U is the Hubbard energy.