2-ELECTRON TIN CENTERS WITH NEGATIVE CORRELATION-ENERGY IN LEAD CHALCOGENIDES - DETERMINATION OF THE HUBBARD ENERGY

It is shown using Mossbauer spectroscopy on the isotope Sn-119 that an isovalent tin impurity in the solid solutions PbS1-zSez is a two-elec tron donor with negative correlation energy. The energy levels associa ted with the tin impurity are found in the lower half of the band gap for z<0.7 and are found among the allowed states of the valence band f or z>0.7. We have demonstrated electron transfer between neutral and i onized tin centers. The activation energy E(0) of this process decreas es monotonically as z is increased, which reflects the fact that the t in energy levels move toward the top of the valence band, and for z>0. 7 the activation energy is E(0)=U/2, where U is the Hubbard energy. (C ) 1997 American Institute of Physics.