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.
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