Black Current: Structure, Characterization, and Optoelectronic Properties of Ce₃ Cl₃ [MoO₆ ]

The admixture of CeO₂ , Ce, CeCl₃ , and MoO₃ with an excess of LiCl as flux in evacuated silica ampules leads to large black single crystals as well as a black microcrystalline powder of Ce₃ Cl₃ [MoO₆ ] after tempering at 850 °C for three days. The title compound crystallizes in the hexagonal space group P6₃ /m (a=934.93(4), c=538.86(2) pm) with two formula units per unit cell. The crystal structure consists of rather unusual trigonal-prismatic [MoO₆ ]^6- units besides Ce³⁺ ions in a tetra-capped trigonal-prismatic coordination, formed by four Cl^- and six O^2- ions. The black color is related to an optical band gap of 1.35(2) eV, which was determined by diffuse reflectance spectroscopy and confirmed by theoretical calculations. The low band gap between the 4f¹ state of cerium (HOMO) and the 5d⁰ state of molybdenum (LUMO) gave rise to the idea of electronic excitation between these two states by IR irradiation, creating a drop in the resistivity of the material, which was detected by appropriate measurements.