Combined experimental and first-principles studies of a hydrated uranyl carbonate: Insight into phonon spectra for a core environmental class of uranium materials

Abstract

Density functional theory calculations for an environmentally important crystalline phase of a hydrated uranyl carbonate, liebigite (Ca2UO2(CO3)3·11H2O), were performed at the generalized gradient approximation Perdew-Burke-Ernzherhof functional level to support experimental interpretation of vibrational data. This phase contains the uranyl tricarbonate center (UO2(CO3)34−), which represents the coordination core of many uranium materials found in nature. This hydrated crystal is surprisingly found to have a theoretical band gap of about 2.45 eV. The electronic structure shows strong covalent U–O and C–O bonds accounting for the marked relative intensity of the Raman bands. Vibrational and photoluminescence spectra are presented. Original assignments of the vibrational lines are proposed on the basis of the theoretical analysis of the normal modes. The role of water in the vibrational properties of the crystal is discussed by investigation of, in particular, a “dry” hypothetical framework structure without water molecules.