Magnetism, metal-insulator transition, and optical properties in Sm- and some other divalent rare-earth moonochalcogenides

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.