Temperature dependence of the frequency of two-electron exchange between impurity negative-$U$ tin centers in lead sulfide

Fast two-electron exchange between neutral Sn$^{2+}$ and doubly ionized Sn$^{4+}$ impurity negative-$U$ tin centers in partially compensated Pb$_{0.98}$Sn$_{0.02}$Na$_{0.01}$Tl$_{0.01}$S solid solutions has been found by emission Mössbauer spectroscopy on $^{119mm}$Sn($^{119m}$Sn) isotope; the lifetime of the Sn$^{2+}$ and Sn$^{4+}$ states changes from $\sim$ 6 $\times$ 10$^{-4}$ to $\sim$ 8 $\times$ 10$^{-9}$ s with a change in temperature from 295 to 900 K.