Thermal decomposition of metazeunerite—a high-resolution thermogravimetric and hot-stage Raman spectroscopic study

A combination of high-resolution thermogravimetric analysis coupled to a gas evolution mass spectrometer has been used to study the thermal decomposition of metazeunerite {Cu(H2O)4}(H2O)4[(UO2)(AsO4)]2. Five stages of weight loss are observed at 48, 88, 125, 882 and 913 °C. In the first three stages 2, 4 and 2 mol of water are lost. In stages 4 and 5, some As2O5 units are lost and it is probable that reduction of the anhydrous phase Cu[(UO2)(AsO4)]2 occurs. The stages of dehydration were confirmed by the use of evolved water vapour mass spectroscopy. Changes in the structure of metazeunerite were followed by the use of Raman microscopy in conjunction with a thermal stage. Two Raman bands are observed at 818 and 811 cm−1 and are assigned to the ν1 symmetric stretching modes of the UO2 units. The UO2 Raman antisymmetric stretching mode was observed at 890 cm−1. No AsO4 stretching vibrations were observed until after two stages of dehydration of the metazeunerite. The AsO4 ν4 bending modes show complexity with bands observed at 463, 446, 396 and 380 cm−1. Thermal treatment results in the removal of this degeneracy. The ν2 band of the AsO4 units is observed at 320 cm−1. Raman bands at 275 and 235 cm−1 are attributed to the ν2 bending modes of the (UO2)2+ units. The use of the hot-stage Raman microscope enables low-temperature phase changes brought about through dehydration to be studied.