PREPARATION OF BLOCKS Of RARE EARTH HALIDE

Production of polycrystalline blocks comprising at least 10 g of a rare earth metal halide (I) comprises heating a mixture or complex of a rare earth metal halide and an ammonium halide to form a melt and then cooling the melt. Having reached 300 degrees C, the temperature is never allowed to fall below 200 degrees C before melt formation. Production of polycrystalline blocks comprising at least 10 g of a rare earth metal halide of formula (I) comprises heating a mixture or complex of a rare earth metal halide and an ammonium halide to form a melt and then cooling the melt. Having reached 300 degrees C, the temperature is never allowed to fall below 200 degrees C before melt formation : AeLnfX(3f+e) (I) Ln = rare earth metal(s); X = Cl, Br or I; A = alkali metal(s); e = 0-3f; f = 1 or more. Independent claims are also included for : (1) polycrystalline block of at least 1 g (I) containing less than 0.1 wt.% water and less than 0.2 wt.% rare earth metal oxyhalide; (2) monocrystal produced by melting a block as above; (3) monocrystal of (I) containing less than 0.1 wt.% rare earth metal oxyhalide.