Phase composition and stability of Gd2−x Th x Zr2O7 under extreme conditions

Volodymyr Svitlyk, Stephan Weiss, Gaston Garbarino, René Hübner, Andreas Worbs, Nina Huittinen, Christoph Hennig
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Abstract

Introduction of Th into synthetic disordered fluorite-type Gd2Zr2O7 induces a transition to an ordered pyrochlore-type phase at a Th concentration of 10 % at the Gd site (Gd1.8Th0.2Zr2O7 composition). The degree of order of the fluorite-type phase reaches 50 % for a Th concentration of 25 % (Gd1.5Th0.5Zr2O7 composition). Upon application of high pressure, the Gd2Zr2O7 phase retains the fluorite-type structure until 33 GPa (K 0 = 167(1) GPa), where it undergoes reversible amorphization. The Gd1.7Th0.3Zr2O7 phase was found to be stable up to at least a pressure of 25 GPa (K 0 = 169(3) GPa). Upon heating to T max of 1135 K, the Gd2Zr2O7 phase retains its disordered fluorite-type structural arrangement (α = 3.03 × 10−5 K−1). The excellent stability of the Gd2−x Th x Zr2O7 phases under extreme conditions of temperature and pressure makes Gd2Zr2O7 a promising candidate as a host matrix for radioactive elements for safe long-term underground storage of nuclear waste.
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极端条件下 Gd2-x Th x Zr2O7 的相组成和稳定性
在合成的无序萤石型 Gd2Zr2O7 中引入钍,当钆位点上的钍浓度为 10%(Gd1.8Th0.2Zr2O7 成分)时,萤石型相的有序度就会过渡到有序的热长石型相;当钍浓度为 25%(Gd1.5Th0.5Zr2O7 成分)时,有序度就会达到 50%。当 Th 浓度为 25%(Gd1.5Th0.5Zr2O7 成分)时,萤石型相的有序度达到 50%。在施加高压时,Gd2Zr2O7 相在 33 GPa(K 0 = 167(1) GPa)之前一直保持萤石型结构,并在此发生可逆的非晶化。研究发现,Gd1.7Th0.3Zr2O7 相至少在 25 GPa 压力下(K 0 = 169(3) GPa)是稳定的。加热到最大温度 1135 K 时,Gd2Zr2O7 相仍保持无序的萤石型结构排列(α = 3.03 × 10-5 K-1)。在极端温度和压力条件下,Gd2-x Th x Zr2O7 相具有极佳的稳定性,这使得 Gd2Zr2O7 有希望成为放射性元素的宿主基质,用于核废料的长期安全地下贮存。
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