Structure of the M_o7O⁶⁻₂₄ Ion in a Crystal of Ammonium Heptamolybdate Tetrahydrate

Abstract

THERMAL decomposition investigations^1,2 have shown that ammonium heptamolybdate tetrahydrate gives three endothermic peaks on differential thermal analysis, four different crystalline states on X-ray analysis with a Guinier focusing camera, and four steps corresponding to them on thermo-gravimetric analysis. The first endothermic peak corresponds to dehydration, and the second and third to loss of ammonium. In general, hydrates of poly acids are classified as containing structural water only³. In the case of ammonium heptamolybdate tetrahydrate, however, the dehydration takes place without a change in the structure at first, and is eventually followed by structural collapse at the end of the dehydration. Some of the water molecules, therefore, seem to have a coordinating function. It seems likely that the stepwise deammoniation is due to two types of ammonium ions in the structure. To interpret the results of these thermal decomposition investigations from the standpoint of crystal chemistry, a complete structural analysis of ammonium heptamolybdate tetrahydrate is required.