Thermogravimetric study of uranium phosphates

The thermal decomposition of UO₂NH₄PO₄ · 3H₂O and UO₂HPO₄ · 4H₂O was studied in the temperature range 25–1600?C. Both compounds gave U₂O₃P₂O₇ around 900?C after a two step dehydration and an orthophosphate-pyrophosphate transformation. UO₂NH₄PO₄ · 3H₂O did not form any pure intermediates, but (UO₂)₂P₂O₇ could be prepared from UO₂HPO₄ · 4H₂O. In air, U₂O₃P₂O₇ lost phosphorus above 1250?C. In argon, (UO)₂P₂O₇ was first formed between 1000 and 1290?C and this product only lost phosphorus at still higher temperatures. (UO)₂P₂O₇ was also obtained by reduction of (UO₂)₂P₂O₇ or U₂O₃P₂O₇ at 700?C in H₂ or with carbon black in argon above 1000?C. It oxidised in air above 250?C with the formation of U₂O₃P₂O₇.