Uranium substituted zirconolite samples based on CaZrTi₂O₇ have been synthesized by a conventional solid-state method in air and structurally characterized by powder X-ray diffraction. The uranium substituted compounds crystallize in a monoclinic C2/m structure like the parent phase. The oxidation state of uranium has been confirmed to be mostly U^VI by X-ray absorption spectroscopy (XAS). The enthalpies of formation from oxides (ΔH°_f,ox) determined by high temperature oxide melt solution calorimetry of uranium substituted compounds are more exothermic than that of the parent CaZrTi₂O₇. The enthalpies of decomposition of uranium substituted zirconolites into perovskite (CaTiO₃), UO₃, and other binary oxides are endothermic by about 20 kJ/mol, implying that decomposition is probably thermodynamically unfavorable. The results show that zirconolite is a thermodynamically stable host for uranium in high level nuclear waste.

中文翻译：

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铀取代锆钛矿的合成，表征和形成焓

基于CaZrTi ₂ O _7的铀取代锆沸石样品已通过常规固态方法在空气中合成，并通过粉末X射线衍射对其结构进行了表征。铀取代的化合物像母相一样以单斜C 2 / m结构结晶。通过X射线吸收光谱法（XAS）已确认铀的氧化态大部分为U ^VI。用高温氧化物熔融溶液量热法测定的铀取代化合物的氧化物生成焓（ΔH ° _f，ox）比母体CaZrTi ₂ O ₇放热大。。铀取代的锆钛矿分解成钙钛矿（CaTiO ₃），UO ₃和其他二元氧化物的分解焓是大约20 kJ / mol的吸热峰，这意味着分解可能在热力学上是不利的。结果表明，锆石是高含量核废料中铀的热力学稳定主体。