Structure and thermal decomposition of ammonium metatungstate

Hunyadi, Dávid and Sajó, István and Szilágyi, Imre Miklós (2014) Structure and thermal decomposition of ammonium metatungstate. JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 116 (1). pp. 329-337. ISSN 1388-6150

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Abstract

The structure and morphology of ammonium metatungstate (AMT), (NH 4)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[H2W12O 42]·4H2O, the only main difference being the lack of dry NH3 evolution between 170 and 240 °C in the case of AMT. © 2013 Akadémiai Kiadó, Budapest, Hungary.

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