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

The hexahydrate of praseodymium nitrate hexahydrate Pr(NO₃)₃·6H₂O 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(NO₃)₃·6H₂O into a cyclic cluster 6[Pr(NO₃)₃·6H₂O]. This hexamer gradually loses water and nitric acid, and a series of intermediate amorphous oxynitrates is formed. The removal of 68% HNO₃–32% H₂O 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 Pr₂O₃. The latter absorbs approximately 1 mol of atomic oxygen from N₂O₅ disproportionation, giving rise to the non-stoichiometric higher oxide Pr₂O_3.33. All mass losses are satisfactorily accounted for under the proposed scheme of thermal decomposition.