Thermal decomposition of praseodymium nitrate hexahydrate Pr.sub.3·6H.sub.2O

The hexahydrate of praseodymium nitrate hexahydrate Pr(NO.sub.3).sub.3·6H.sub.2O 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.sub.3).sub.3·6H.sub.2O into a cyclic cluster 6[Pr(NO.sub.3).sub.3·6H.sub.2O]. This hexamer gradually loses water and nitric acid, and a series of intermediate amorphous oxynitrates is formed. The removal of 68% HNO.sub.3-32% H.sub.2O 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.sub.2O.sub.3. The latter absorbs approximately 1 mol of atomic oxygen from N.sub.2O.sub.5 disproportionation, giving rise to the non-stoichiometric higher oxide Pr.sub.2O.sub.3.33. All mass losses are satisfactorily accounted for under the proposed scheme of thermal decomposition.