Thermal decomposition of praseodymium nitrate hexahydrate Pr(NO3)3·6H2O

The hexahydrate of praseodymium nitrate hexahydrate Pr(NO 3 ) 3 ·6H 2 O does not show phase transitions in the range of 233–328 K when the compound melts in its own water of crystallization. It is suggested that the thermal decomposition is a complex step-wise process, which involves the condensation of 6 mol of the initial monomer Pr(NO 3 ) 3 ·6H 2 O into a cyclic cluster 6[Pr(NO 3 ) 3 ·6H 2 O]. This hexamer gradually loses water and nitric acid, and a series of intermediate amorphous oxynitrates is formed. The removal of 68% HNO 3 –32% H 2 O azeotrope is essentially a continuous process occurring in the liquid phase. At higher temperatures, oxynitrates undergo thermal degradation and lose water, nitrogen dioxide and oxygen, leaving behind normal praseodymium oxide Pr 2 O 3 . The latter absorbs approximately 1 mol of atomic oxygen from N 2 O 5 disproportionation, giving rise to the non-stoichiometric higher oxide Pr 2 O 3.33 . All mass losses are satisfactorily accounted for under the proposed scheme of thermal decomposition.