Structure and Thermal Decomposition of Ammonium Metatungstate

Ammonium metatungstate, also known as AMT, is a tungsten chemical in the form of highly soluble hydrated crystals. Global Tungsten & Powders AMT is produced as minus 10-mesh powder to meet stringent demands in the catalyst, medical, semiconductor and other high-technology industries. GTP AMT is also used in materials research and in analytical chemistry. At room temperature, aqueous solutions can be saturated up to 70% by weight of contained WO3. This equates to 1.6 kilograms per liter or 13.3 pounds per gallon.

The structure and morphology of ammonium metatungstate (AMT), (NH4)6[H2W12O40]⋅4H2O, and its thermal decomposition in air and nitrogen atmospheres were investigated by SEM, FTIR, XRD, and TG/DTA-MS. The cell parameters of the AMT sample were determined and refined with a full profile fit. The thermal decomposition of AMT involved several steps in inert atmosphere: (i) release of crystal water between 25 and 200 °C resulting in dehydrated AMT, (ii) formation of an amorphous phase between 200 and 380 °C, (iii) from which hexagonal WO3 formed between 380 and 500 °C, and (iv) which then transformed into the more stable m-WO3 between 500 and 600 °C. As a difference in air, the as-formed NH3 ignited with an exothermic heat effect, and nitrous oxides formed as combustion products. The thermal behavior of AMT was similar to ammonium paratungstate (APT), (NH4)10[H2W12O42]⋅4H2O, the only main difference being the lack of dry NH3 evolution between 170 and 240 °C in the case of AMT.