EPR studies of the valence states of thulium in thulium monochalcogenides

We have studied the Gd3+ EPR at 9.2 HGz in 0.1 at % GdS in TmS, 1 at % GdTe in TmTe, and 1 at % GdSe in TmSe. From the temperature dependence of the Gd3+ linewidth we have concluded that Tm in TmS is trivalent. The temperature variation of the Gd3+ linewidth in TmTe has been interpreted as the result of the ''delocalization'' of the extra electron ''localized'' around Gd3+. The g-value and linewidth data are consistent with the identification that Tm in TmTe is divalent. The linewidth and the g-value of the Gd3+ resonance in the TmSe have been found to be temperature independent. These striking results have been attributed to the ''motional narrowing'' effect arising from valence fluctuations of Tm in TmSe