ИСТИНА

In spite of several experimental evidences of the existence of higher oxides of plutonium, americium, and curium, and significant attention paid to these oxides because of their hypothetical role in the volatilization of the two former elements from oxidative media, reliable information on these species is extremely scarce. We report the results of two-component relativistic DFT electronic structure calculations on simple molecules of higher oxides (actinide oxidation state VI through VIII) and peroxides of Pu, Am, and Cm. The calculations employed accurate “small-core” two-component pseudopotentials, hybrid approximations for the exchange-correlation functional and flexible contracted Gaussian bases. Ground-state equilibrium structures, harmonic vibrational frequencies and main parameters of gas-phase thermodynamics were determined; electronic density distributions were analyzed in terms of Bader atomic charges. All “true” An(VIII) oxides (An = Pu, Am, Cm), both monomers and dimers, were predicted to be thermodynami-cally unstable in realistic gas-phase conditions with respect to the decay into lower oxides (An2O6 or An2O7) and molecular oxygen as well as to the conversion into dioxoperoxides AnO2(O2). It is probable that the experimentally detected volatile compounds with presumable stoichiometry AnO4 are peroxides. The stability of higher oxidation states rapidly decreases from Pu to Am and further to Cm. The formation of heterooxide molecules PuAmO6 and especially PuAmO7 from the corresponding pure oxides (An2O6 and An2O7) should be accompanied by a decrease in energy; one can suppose that the experimentally observed enhancement of the volatilization of an americium compound from alkaline solutions under ozonation in the presence of plutonium is related to the particular stability of these heterooxides. A strong correlation between Bader net charges of actinide atoms in AnOn and An2On molecules and formal An oxidation state was detected. Charge distribution analysis allowed us to identify PuAmO7 as mixed oxide of Pu(VII) and Am(VII), and not of Pu(VIII) and Am(VI), as could be supposed on the basis of structure data.