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Various types of binary or ternary complex halides between alkaline and rare earth halides such as K2LaBr5:Ce, RbGd2Br7:Ce, Cs2LiYCl6:Ce and LiGdCl4:Ce have been developed as scintillators in the last two decades. Many of them, especially the elpasolite series, show excellent scintillation performance and good application prospect. Current crystal growth of complex rare earth halides usually uses high purity simple rare earth halides as starting materials. For example, in order to grow K2LaBr5:Ce crystal, high purity anhydrous LaBr3 and CeBr3 are needed as starting materials, which are very expensive due to their extremely hygroscopic nature and difficulty in preparation. As a result, K2LaBr5:Ce crystal is commercially unattractive because it costs similar but performs worse than LaBr3:Ce crystal, which is grown from merely the same rare earth starting materials. Therefore, new preparation technologies for low-cost complex rare earth halides must be developed.
Previous investigations of G. Meyer [1] on the ammonium-bromide route to anhydrous rare earth bromide revealed that when LaBr3 hydrates were dehydrated with NH4Br, (NH4)2LaBr5 formed as an intermediate product. This inspired us that if LaBr3 hydrates were dehydrated with KBr, K2LaBr5 might be easily synthesized as the product. We named the method mixed-salts-dehydration. Actually, G. Meyer had reported early that complex rare earth halides could be synthesized from the mixed solution of rare earths and other metal halides [2]. D.K. Ingole also used this method to prepare Ce3+-activated K2LaX5 (X = Cl, Br or I) phosphors before [3]. However, whether or not the purity of such products can meet the requirement of crystal growth has not been verified.
Our experiments revealed that the mixed-salts-dehydration method can yield K2LaBr5 with high phase purity. However, the oxygen content of the product is as high as 500 ppm, which is due to hydrolysis of the halide during the dehydration. Unfortunately, such purity can hardly meet the requirement of crystal growth, for which use, the oxygen content should be as low as possible and usually less than 100 ppm.
In order to restrain the hydrolysis and hence improve the purity of the product, the mixed-salts-dehydration method was slightly modified by adding some NH4Br into the solution at the first step. Correspondently, after the dehydration, the solids were further heated above 500℃ in order to remove NH4Br. High purity K2LaBr5 with oxygen content less than 50 ppm was successfully obtained.
The modified mixed-salts-dehydration method was further implied to prepare a lot of other complex halides including K2LaCl5, K2LaI5, Cs2LiYCl6, Cs2LiCeCl6, Cs2LiY0.995Ce0.005Cl6, RbGd2Br7 and LiGdCl4, and high purity samples were all successfully obtained with high phase purity and very low oxygen content (<100 ppm). These results indicate that the modified mixed-salts-dehydration method is a universal route to high purity anhydrous complex rare earth halides. Our experiments also revealed that it is much easier to obtain high purity complex rare earth halides by the modified mixed-salts-dehydration method than to obtain simple rare earth halides by the ammonium halide route, which may provide cost advantage to rare earth complex halides towards simple rare earth halides, and shed light on their commercial development and applications.
References:
[1] G Meyer, S Dotsch, T Staffel, J Less-Comm. Metals, 1987, 127, 155.
[2] G Meyer, M S Wickleder, Handbook on the Physics and Chemistry of Rare Earths, Vol.28, Chapter 177, Elsevier Science B V, 2000.
[3] D.K. Ingole, C. P. Joshi, S. V. Moharilb, P. L. Muthal, S. M. Dhopte, Lumin., 2012, 27, 24.
