X‐ray powder diffraction data have been obtained for U3O5P2O7. This compound has not yet been mentioned in the literature, although similar diffraction data have been reported for a compound described as (UO)3(PO4)2.

The thermal decomposition of UO2NH4PO~ 9 3H20 and UO2HPO4 9 4H20 was studied in the temperature range 25-1600~ Both compounds gave U2OaP~O~ around 900~ after a two step dehydration and an orthophosphate-pyrophosphate transformation.UO2NH~PO4 " 3H20 did not form any pure intermediates, but (UO2)zP207 could be prepared from UO2HPO ~ 9 4H20. In air, U20~P207 lost phosphorus above 1250~ In argon, (UO)2P207 was first formed between 1000 and 1290~ and this product only lost phosphorus at still higher temperatures.(UO)2P207 was also obtained by reduction of (UOz)zP207 or U2OjP207 at 700~ in H2 or with carbon black in argon above 1000~ It oxidised in air above 250~ with the formation of UzOaPzOT. Interest recently revived in uranium phosphates when phosphate containing ores had to be processed [1-4].This led to the discovery that the uranyl ion, UO~ § could be removed completely from acid solutions as uranyl ammonium phosphate, UO2NH4PO4 " 3H20 or UAP [5, 6]. This product was consequently used for chemical analysis as it was believed to decompose to (UO2)zP207 between 700 ~ and 1000 ~ [6-8].More recently however, it was found that U2OjP20 7 will be formed if UAP is heated above 800 ~ [9, 10].A possible contaminant during the precipitation of UAP is the dibasic uranyl phosphate, UO2HPO4 9 4HzO, which is normally called acid uranyl phosphate or AUP. The thermal stability of this product in air has been discussed in a few papers [6, 11, 12]. However, a detailed study of the decomposition reactions and thermal stability of the different decomposition products under various conditions is not available.The monobasic uranyl phosphate, UO2(H2PO~)2 9 XH20, has also been reported in the literature [4, 13, 14]. Its thermal decomposition was studied thoroughly by Kamo et al. [14], who found that no definite intermediate products were formed. The endproduct was UP207, which seems to be the only stable uranium phosphate, * Some of the results in this paper were taken from a thesis, submitted in partial fulfilment of the requirements for an M. Sc. degree in Inorganic Chemistry.

feature of this technique is that it is not necessary to identify the reference peak or any peak other than that of chrysotile.

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