Thermal decomposition of synthetic ammonium jarosite

Ammonium jarosite (NH4Fe3(OH)6(SO4)2) was produced by forced hydrolysis of a NH4Fe(SO4)2 solution. Upon heating of ammonium jarosite at 400 °C, the IR band at 1426 cm−1 corresponding to ν(N-H) vibrations almost disappeared, thus indicating decomposition of the NH4+ ion. The IR bands at 1197, 1081 and 1003 cm−1 can be assigned to sulphate groups in ammonium jarosite. Upon heating of ammonium jarosite at 400 °C the IR bands showed positions at 1200, 1092 and 1011 cm−1, with a shoulder at 1026 cm−1. The FT-IR spectrum did not change by heating ammonium jarosite at 500 °C. Assignations of IR bands are given. The changes in the FT-IR spectra recorded for the samples produced up to 500 °C were not sufficient for the phase analysis of these samples. 57Fe Mössbauer spectroscopy was very sensitive to the phase changes in the thermal decomposition products of ammonium jarosite. Upon heating of ammonium jarosite at 400 °C, the corresponding Mössbauer spectrum was deconvoluted into two doublets which were assigned to Fe(OH)SO4 and Fe2O(SO4)2. The Mössbauer spectrum recorded for the sample produced by heating of ammonium jarosite at 500 °C, was deconvoluted into three doublets which were assigned to Fe2(SO4)3, Fe(OH)SO4 and Fe2O(SO4)2. Hematite (α-Fe2O3) was produced upon heating of ammonium jarosite at 600 °C, as found by Mössbauer and FT-IR spectroscopies. Quantitative data obtained by deconvolution of the Mössbauer spectra are summarized.