Preparation and thermochemical properties of alkali-metal diuranates (VI) and dineptunates (VI).

Alkali-metal diuranates (VI) and dineptunates (VI) have been prepared by controlled thermal decomposition of well-characterised alkali-metal uranyl (VI) and neptunyl (VI) triacetates having the desired stoicheiometricalkali-metal to actinide ratio. The compounds in the series MI2MVI207 (MI = Na - Cs incl., MVI = U or Np) have been obtained in this way. This preparative method failed to provide further evidence for the existence of the controversial ternary oxide, Li2U207. Isolation of the analogous neptunium complex, Li2Np207, was not achieved. Enthalpies of formation have been derived from the enthalpies of solution in 1 mol dm-3HC1 and auxiliary thermodynamic data. The values for Na2U207 and Cs2U207 are in good agreement with those assessed by Cordfunke and O'Hare and thus confirm the reliability of the method of preparation of such mixed oxides, as compared to the 'classical' methods which involve heating an actinide oxide with an alkali-metal binary oxide, nitrate or carbonate. The values for K2U207 and Rb2U207 are previously unreported. Confirmation of an earlier assessment of the enthalpy of formation of Na2Np207 and preliminary values for the enthalpies of formation of K2Np207, Rb2Np207 and Cs2Np207 have been obtained.