Preparation and Structure of Uranium-Incorporated Gd₂Zr₂O₇ Compounds and Their Thermodynamic Stabilities under Oxidizing and Reducing Conditions

Gd₂Zr₂O₇ is being contemplated as a futuristic matrix for the incorporation of high-level radioactive nuclear waste. This compound has the unique ability to incorporate several fission products and heavy metal ions like uranium and thorium into its lattice sites without undergoing structural changes. X-ray diffraction analyses of Gd_2–<i>x</i>U_<i>x</i>Zr₂O_7+δ samples indicate that the parent compound can incorporate a substantial amount of uranium, both under oxidizing and reducing conditions. The oxidation state of these samples was investigated by X-ray photoelectron spectroscopy. The thermodynamic stability of uranium-substituted Gd₂Zr₂O₇ is an important parameter that will govern the long-term storage of uranium and minor actinides in this matrix. High-temperature calorimetry has been used to investigate the stability of the Gd_2–<i>x</i>U_<i>x</i>Zr₂O_7+δ (0.00 ≤ <i>x</i> ≤ 0.15) compositions. The standard molar free energy of the formation of Gd_2–<i>x</i>U_<i>x</i>Zr₂O_7+δ (0.00 ≤ <i>x</i> ≤ 0.15) compositions has been estimated. From the free energy of formation data, the sample corresponding to <i>x</i> = 0.15 was found to be most stable in the Gd_2–<i>x</i>U_<i>x</i>Zr₂O_7+δ (0.00 ≤ <i>x</i> ≤ 0.15) series. The relative stabilities of U⁴⁺ and U⁶⁺ substituted gadolinium zirconate have been discussed on the basis of the charge on the uranium ion and the incorporation of corresponding extra oxygen atoms into the lattice for charge compensation