Magnetism, Metal-Insulator Transition, and Optical Properties in Sm- and Some Other Divalent Rare-Earth Monochalcogenides

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Abstract
In Sm2+X compounds (X = S, Se, Te) an unusualy large dependence of the low‐temperature Van Vleck susceptibility on the ligand has been observed. SmSe, SmTe, and also TmTe undergo a continuous insulator‐metal transition under pressure. In contrast, SmS shows a discontinuous transition to the metal at 6.5 kbar. In all cases the transition is accompanied by a large volume change of around 20%, which is due to the promotion of a 4f electron into the 5d‐6s conduction band. Optical spectroscopy yields three distinct absorption peaks. They may be interpreted as transitions from 4f6↔4f55dt2g and 4f6↔4f55deg and probably an excitonic peak. The activation energy of a 4f electron is found to be 0.63, 0.46, 0.20, and 0.22 eV for SmTe, SmSe, SmS, and TmTe, respectively, from both the pressure‐dependent electrical resistivity at room temperature and the observed absorption edge.