Abstract: Compositions are described including free-flowing particles of controlled size of purified metal halides such as sodium iodide and mixtures of sodium iodide with scandium iodide. These ultra-pure metal halide particles are particularly useful as a vaporizable fill disposed within an arc tube of a high pressure electric discharge device. The preparation and identification of the coordination entity Na.sup.+ScI.sub.4 .sup.- (III) is also described. The apparatus for producing purified metal halides of accurately controlled particle size includes means for evaporating volatiles from the solid impure metal halide, melting the impure halide, passing hydrogen, hydrogen halide, hydrogen and halogen, or mixtures thereof through the halide, and then passing the molten halide through a vibrating discharge conduit or nozzle into an inert quenching atmosphere to form the particles of purified halide.