Controlling the Luminescence of Rare-Earth Chalcogenide Iodides RE₃(Ge_1–xSi_x)₂S₈I (RE = La, Ce, and Pr) and Ce₃Si₂(S_1–ySe_y)₈I

The rare-earth chalcogenide iodides RE 3 (Ge 1– x Si x ) 2 S 8 I ( RE = La, Ce, and Pr) and Ce 3 Si 2 (S 1– y Se y ) 8 I were prepared by reactions of the elements at 900 °C. Phase-pure samples and complete solid solutions were obtained for all series, as determined by powder X-ray diffraction patterns, with the cell parameters evolving smoothly. They adopt the monoclinic La 3 Si 2 O 8 Cl-type structure (space group C 2/ c ), containing isolated tetrahedra centered by the tetrel atoms and separated by the RE and I atoms. Single-crystal X-ray diffraction studies indicate that substitution of Si for Ge atoms in RE 3 (Ge 1– x Si x ) 2 S 8 I leads to complete disorder of the tetrel atoms, whereas partial substitution of Se for S atoms in Ce 3 Si 2 (S 1– y Se y ) 8 I occurs with preferential occupation of Se atoms. The optical band gaps increase slightly upon substitution with Si in RE 3 (Ge 1– x Si x ) 2 S 8 I (or negligibly in the case of the Ce-containing series, remaining at 2.8 eV), and they decrease dramatically upon substitution with Se in Ce 3 Si 2 (S 1– y Se y ) 8 I, from 2.8 to 1.9 eV. These band gaps are direct according to electronic structure calculations. The strong blue photoluminescence emission in Ce 3 Si 2 S 8 I shifts slightly to blue-green in Ce 3 (Ge 0.25 Si 0.75 ) 2 S 8 I and significantly to green in Ce 3 Si 2 (S 0.81 Se 0.19 ) 8 I, as quantified by their chromaticity coordinates. Increasing the concentration of Ge or Se tends to decrease the photoluminescence intensity and lifetime. For Ce 3 Si 2 S 8 I, the activation energy for thermal quenching is 0.68 eV.