Electron exchange between tin impurity U^– centers in PbS_zSe_1–z alloys

Using emission ^119mmSn(^119mSn) and ¹¹⁹Sb(^119mSn) Mössbauer spectroscopy, it is shown that impurity tin atoms in PbS_zSe_1–z alloys substitute lead atoms and are two-electron donors with negative correlation energy (U^– centers). It is found that the energy levels related to impurity tin atoms are in the lower half of the band gap at z ≥ 0.5 against the background of allowed valence-band states at z ≤ 0.4. The electron exchange between neutral and doubly ionized tin U^– centers in partially compensated Pb_0.99Sn_0.005Na_0.005S_zSe_1–z alloys is studied. The activation energy of this process decreases from 0.111(5) eV for a composition with z = 1 to 0.049(5) eV for compositions with c ≤ 0. For all z, the exchange is implemented via the simultaneous transfer of two electrons using delocalized valence-band states.