Purification of bismuth tri-iodide as material for radiation detector purposes

First attempts to purify bismuth tri-iodide for radiation detector applications are presented. Bismuth tri-iodide synthesized from bismuth subcarbonate and potassium iodide was used as starting material. Sample composition was confirmed by powder X-ray diffraction. A first set of samples (at 400 mmHg of Ar) was purified by zone refining in a vertical zone refining furnace at 460°C and at a speed of about 3 cm/hr down the length of the ingot, 100 passes. One of these samples was then purified by sublimation. A second set of samples was purified by repeated sublimation under Ar atmosphere (400 mmHg) with the source sample at 370°C and the cold extreme of the ampoule at room temperature. The starting and purified products´ stoichiometry was determined. Also the starting and purified materials purity was determined by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). Zone refining appears as an appropriate method for purification of several impurities, while sublimation, for others. On the other hand, zone refining clearly deteriorates the material stoichiometry (from BiI_2.82 to BiI_2.75), while a combination of the two methods, that is, a first zone refining process of about 100 passes, followed by, at least, one sublimation, results to be the most appropriate purification method, in order of obtaining a pure but also stoichiometric material (BiI_3.08). This process has a yield of 20%. The purification way proposed here turns over to be the same what was found for lead iodide purification, although easier, because of the lower tendency for bismuth tri-iodide to decompose.