The invention relates to a method of preparing a polycrystalline block of a halide of formula A.sub.eLn.sub.fX.sub.(3f+e) in which Ln represents one or more rare earths, X represents one or more halogen atoms selected from the group consisting of Cl, Br and I, and A represents one or more alkali metals selected from the group consisting of K, Li, Na, Rb and Cs, e, which may be zero, being less than or equal to 3f, and f being greater than or equal to 1, having a low water and oxyhalide content, in which the method comprises heating a mixture of, on the one hand, at least one compound having at least one Ln--X bond and, on the other hand, a sufficient amount of NH.sub.4X in order to obtain the oxyhalide content, resulting in a molten mass comprising the rare-earth halide, the heating being followed by cooling, and the heating, after having reached 300.degree. C., never going below 200.degree. C. before the molten mass has been obtained. The blocks thus produced allow very pure single crystals having remarkable scintillation properties to be grown.