Differential scanning calorimetry as a tool for evaluating preparation techniques, and detecting impurities, metastable states, etc. in silver mercury iodide (Ag2HgI4)

Differential scanning calorimetry has been used for evaluating three different methods for the synthesis of Ag2HgI4. The information obtained from a DSC curve of the order-disorder transition at 52 °C (temperature, enthalphy, shape of the DSC peak) is very sensitive to the purity of a sample. Pure polycrystalline Ag2HgI4 can be obtained by a solid state reaction between stoichiometric amounts of powders of AgI and HgI2. Precipitation from aqueous solutions of AgNO3 and K2HgI4 is the most common way to prepare Ag2HgI4, but pure Ag2HgI4 is obtained only if an excess of K2HgI4 is present, while small amounts of AgI are coprecipitated as an impurity if stoichiometric solutions are used. Two procedures for growing single crystals of Ag2HgI4 are described in detail, and it is demonstrated that minor deviations from the exact proportions of the constituents can result in a very impure product. It is also shown that some preparation methods can result in metastable products, which can be converted into stable ones by thermal cycling.