Abstract: Anhydrous metal iodides of Sc, Dy, Er and Tm were prepared by the reaction of metallic rare earths with mercuric iodide at elevated temperature. Phase diagrams of the binary systems NaI-REI₃ (RE= Sc, Dy, Er and Tin) were obtained by DTA technique. All they are simple binary eutectic system. For RE=Dy, Er and Tm, the phase transition of the corresponding systems occured below eutectic temperature, indicating the possible existence of some unstable compounds. Metal halide lamps filled with the mixture of sodium iodide and rare earth iodides have been made and compared with the mercury arc lamp. It was shown that addition of sodium iodide and rare earth iodides improves the color temperature, color rendering indices and efficacy of the lamps.

Key words: Inorganic synthesis, Rare earth compounds, Iodides, Luminescent materials, Lamp