Higher valency ion substitution causing different fluorite-derived structures in CaZr1-Nd Ti2-Nb O7 (0.05 ≤ x ≤ 1) solid solution
Higher valency ion substitution causing different fluorite-derived structures in CaZr1-Nd Ti2-Nb O7 (0.05 ≤ x ≤ 1) solid solution
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高价离子取代导致CaZr1-Nd Ti2-Nb O7 (0.05××1)固溶体中不同的萤石衍生结构
DOI：
10.1016/j.ceramint.2020.09.119
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发表时间：
2021
影响因子：
5.2
通讯作者：
Zhiguang Wang
中科院分区：
文献类型：
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作者：
Shiyin Ji;Chang-Zhong Liao;Shuangqiang Chen;Kuibao Zhang;Kaimin Shih;Chung-Kai Chang;Hwo-shuenn Sheu;Shan Yan;Yuhong Li;Zhiguang Wang
萤石衍生的A2B2O7和ABC2O7结构，即烧绿石和锆石，是一种很有前途的陶瓷基质，可用于高放废物的固定化，特别是对微量鳗系元素的固定化。虽然许多研究报道了低价阳离子的取代，但关于高价元素在这些萤石衍生基质上的掺杂的研究较少。本文通过对一系列CaZr1-xNdxTi2-xNbxO7(x=0.05~1.0)陶瓷的研究，揭示了天然锆石中的主要杂质Nb5+对其晶体结构演化、取代机理和相变的影响。用X射线光电子能谱证实了样品中Nb5+的存在。同步辐射X射线衍射和扫描电子显微镜-能谱分析表明，随着Nb~(5+)含量的增加，萤石衍生结构(锆石-2M、3T、4M和焦绿石)在基质中同时出现单相或多相组合。几乎只有单相的锆石-2M和焦绿石分别在0.05g≤x≤0.1g和0.5g≤x≤0.9g范围内观察到。对锆石-2M相的Rietveld结构分析表明，Nd3+离子优先占据了锆位，而Nb5+取代了钛位。这一结果不同于文献中报道的低价(≤+3)作为电荷补偿剂或稀土掺杂锆石中没有电荷补偿剂的机理。在烧绿石结构中，Ca~(2+)、Nd~(3+)和Zr~(4+)离子占据了A亚晶格，而Ti4+和Nb~(5+)离子填充了B亚晶格，A和B亚晶格之间存在电荷补偿。研究结果表明，当高价元素进入晶格时，萤石衍生结构具有不同的电荷补偿机制，这将有助于高放废物的固定化。
Fluorite-derived A2B2O7and ABC2O7structures, i.e. pyrochlore and zirconolite, are promising ceramic matrices for high-level radioactive waste immobilization, especially for minor actinides. Though substitutions of low valent cations were reported by many studies, fewer researches were carried out on the incorporation of high valency elements on these fluorite-derived matrices. In this study, a series of CaZr1-xNdxTi2-xNbxO7(x= 0.05–1.0) ceramics was investigated to unravel the effect of Nb5+- one of the main impurities in natural zirconolite, on the evolutions of crystal structure, substitution mechanism and phase transformation. X-ray photoelectron spectroscopy was used to confirm the existence of Nb5+in the samples. Fluorite-derived structures (zirconolite-2M, 3T, 4 M and pyrochlore) with increasing Nb5+contents simultaneously appeared as single-phase or multi-phase assemblages in the matrices, revealed by the combination of synchrotron X-ray diffraction and scanning electron microscopy-energy dispersive x-ray spectroscopy. Almost single-phase zirconolite-2M and pyrochlore were observed only in the ranges of 0.05 ≤x≤ 0.1 and 0.5 ≤x≤ 0.9, respectively. Rietveld structural analysis of the zirconolite-2M phase showed that Nd3+ions preferably occupied the Zr sites while Nb5+ions substituted the Ti sites. This result is different from the mechanisms reported in the literature when lower valency (≤+3) was as charge compensator or there was no charge compensator in the REE doped zirconolite (REE = rare earth element). In the pyrochlore structure, the Ca2+, Nd3+and Zr4+ions occupied the A sublattice while Ti4+and Nb5+ions filled in the B sublattice, and there is charge compensation between A and B sublattices. The results in this study showed a different charge compensation mechanism of the fluorite-derived structures when higher valency elements incorporate into the lattice and will be very helpful for high-level radioactive waste immobilization.