New ways for purifying lead iodide appropriate as spectrometric grade material

In order to overcome some unsolved problems of lead iodide purification such as lack of stoichiometry, decomposition and polytype admixture, several purification methods were compared. Lead iodide Alfa Aesar and synthesized from lead nitrate and potassium iodide was purified by zone refining, zone refining followed by sublimation, repeated sublimation and repeated evaporation, at different conditions. Zone refining was performed at 420°C, 3 cm/hr, 100 passes, repeated sublimation at 390°C and vacuum (10^-5 mmHg) or Ar atmosphere (500 to 580 mmHg) and repeated evaporation at 10^-5 mmHg or in Ar atmosphere (150 to 600 mmHg) and temperature from 450°C to 600°C. Purification methods were evaluated by studying parameters of the purified material like decomposition, stoichiometry, purity and polytype composition and also taking into account purification yield and rate. Stoichiometry was determined by wet procedures, purity by Inductively Coupled Plasma (ICP) and polytypes by powder X-ray diffraction. Evaporation of lead iodide at the highest temperature and moderate Ar pressure (600°C and 500-600 mmHg) proved to be the best way to avoid material decomposition. Sublimation and evaporation give the best stoichiometry (PbI_1.90) especially when compared with zone refining (PbI_1.40). Whatever the purification method, material has an appreciable polytype content. Purity showed similar results for 100 zone refining passes than for 3 evaporations. Furthermore, evaporation exhibits maximum yield and rate (16%/day). Therefore, the work performed points out evaporation in Ar atmosphere as the quickest and more efficient purification method for producing spectrometric grade lead iodide, avoiding material decomposition and achieving high purity, but maintaining the best stoichiometry