This paper deals with nanoscale uranyl peroxide clusters containing UO₂²⁺ groups bonded through peroxide bridges to form polynuclear molecular species (polyoxometalates) exist both in solution and in the solid state. There is an extensive family of clusters containing 28 uranium atoms (U₂₈ clusters), with an encapsulated anion in the center, for example, [UO₂(O₂)_3?x(OH)_x^4?], [Nb(O₂)₄^3?], or [Ta(O₂)₄^3?]. The negative charge of these clusters is balanced by alkali ions, both encapsulated, and located exterior to the cluster. The present study reports measurement of enthalpy of formation for two such U₂₈ compounds, one of which is uranyl centered and the other is peroxotantalate centered.The Energy Landscape of Uranyl–Peroxide Species
Eitan Tiferet, Adrià Gil, Carles Bo, Tatiana Y. Shvareva,May Nyman and Alexandra Navrotsky

Chem.Eur.J. 2014, 20, 3646-3651