Electron exchange between tin impurity $U^-$ centers in PbS$_{z}$Se$_{1-z}$ alloys

Using emission $^{119mm}$Sn($^{119m}$Sn) and $^{119}$Sb($^{119m}$Sn) Mössbauer spectroscopy, it is shown that impurity tin atoms in PbS$_{z}$Se$_{1-z}$ 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 $z\ge$ 0.5 against the background of allowed valence-band states at $z\le$ 0.4. The electron exchange between neutral and doubly ionized tin $U^-$ centers in partially compensated Pb$_{0.99}$Sn$_{0.005}$Na$_{0.005}$SzSe$_{1-z}$ 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 $c\le$ 0. For all $z$, the exchange is implemented via the simultaneous transfer of two electrons using delocalized valence-band states.