Possible Electronic States of Gadolinium Impurity Centers in Pb1–xGdxS Narrow-Gap Semiconductor: Electron Paramagnetic Resonance Data

In the electron paramagnetic resonance spectra of crystals of narrow-gap semiconductor Pb1–xGdxS ( $$3.2 \times {{10}^{{ - 4}}} \leqslant x \leqslant 9.5 \times {{10}^{{ - 3}}}$$ ), unusual changes in the shape of the lines of Gd3+ $$(4{{f}^{7}},\;\,S = 7{\text{/}}2)$$ centers were detected, which are caused by a change in the concentration of impurity gadolinium. The spectra of the studied crystals with increased gadolinium concentrations also show weaker additional lines of two types of new gadolinium centers with cubic symmetry of magnetic properties and with effective electron spins of $$S = 7{\text{/}}2$$ and $$S = 5{\text{/}}2.$$ Two possible explanations for the observed properties of the discovered paramagnetic centers were proposed.