Synthesis, structure, and characterization of the thorium zirconolite CaZr_1‐xTh_xTi₂O₇ system

Abstract

The United Kingdom is in the process of consolidating all civil Pu inventories at the Sellafield site, in an effort to satisfy security and proliferation requirements, presenting a significant decommissioning challenge in terms of materials degradation. 1 The Nuclear Decommissioning Authority (NDA) is liable for the management and disposition of all nuclear wastes under UK safeguards, and has commissioned a credible options analysis to identify technically mature reuse and disposition options, one of which is a strategy of immobilization and disposal. 2 Zirconolite (nominally CaZrTi 2 O 7 ) is the proposed candidate ceramic wasteform for the immobilization of Pu oxides. [3][4][5][6][7] The zirconolite-2M parent structure is derived from the anion-deficient fluorite unit cell, and is closely related to the A 2 B 2 O 7 pyrochlore structure (Fd-3m) by a compression along the (111) direction. Furthermore, a range of polytypical zirconolite structures (e.g., 4M, 3O, 3T) have been reported, the formation of which is controlled by cation substitution and processing conditions. [8][9][10] The zirconolite-2M unit cell is considered the archetype, with observed stability over the compositional range CaZr x Ti 3-x Ti 2 O 7 (0.8 < x < 1.3), with a lamellar unit cell crystallizing in the space group C2/c. 11 The nomenclature "2M" is a

Zirconolite Polytypes and Murataite Polysomes in Matrices for the REE—Actinide Fraction of HLW

“…A description of all five zirconolite polytypes known so far is given in [ 103 ]. Their formation depends on the type of substitutions, charge and radius of cations, temperature, and oxidizing conditions [ 104 ]. Polytypes 2M and 4M are monoclinic (sp.…”

“…C2/c ), 3O is orthorhombic ( Acam ), and 3T and 6T are hexagonal ( P3 1 21 ). The most common polytypes in actinide matrices are 2M and 4M [ 104 ], less common are 3O [ 82 ] and 3T [ 70 , 105 ], there are no data for the 6T polytype. For phases of composition (Ca 1−x 239 Pu x )Zr (Ti 2−2x Fe 2x )O 7 (x = 0.1–0.7), the 2M polytype is stable up to “x” = 0.3, and 3T appears at “x” = 0.3 and 0.4.…”

“…Zirconolite 3T is often formed using thorium as a simulator. A reducing medium (5% H 2 /N 2 ) is favorable for the formation of 3T zirconolite CaZr 1−x Th x Ti 2 O 7 at x ≥ 0.20 [ 104 ], Ca 0.8 Ti 1.35 Zr 1.3 Th 0.15 Al 0.4 O 7 crystallizes in the same 3T type [ 106 ]. Fine intergrowth of 2M and 3T polytypes was established in glass ceramics obtained in the SiO 2 –Al 2 O 3 –CaO–ZrO 2 –TiO 2 –ThO 2 system [ 80 ].…”

Phase Evolution in the CaZrTi2O7–Dy2Ti2O7 System: A Potential Host Phase for Minor Actinide Immobilization

“… 16 Further isovalent substitution of cations within the Zr 4+ site promotes a structural transformation from zirconolite-4M to pyrochlore, although it should be noted that this does not occur for the corresponding CaZr 1– x Th x Ti 2 O 7 solid solution, for which the intermediate 4M phase does not form. 17 Cubic pyrochlore-structured materials (parent structure A 2 B 2 O 7 —space group Fd 3̅ m ; Z = 8) have attracted significant interest in many areas of solid-state chemistry, with titanate and zirconate pyrochlores (A 2 Ti 2 O 7 and A 2 Zr 2 O 7 , respectively) developed as potential wasteforms for actinides, due to high radiation stability. 18 − 24 The rare earth pyrochlore structure is derived from the fluorite (AO 2 ) superstructure, with one-eighth of the oxygen atoms replaced by vacancies, and the A 3+ and B 4+ cations in 8- and 6-fold coordination with oxygen, respectively.…”

Controllable sites and high-capacity immobilization of uranium in Nd2Zr2O7 pyrochlore

“…Among them, zirconate pyrochlore is the most important class of host materials due to its remarkable resistance to amorphization under ion beam irradiation Lu et al, 2019;Shu et al, 2017). Previous studies have demonstrated that actinides can be effectively incorporated into the zirconate pyrochlore matrix at both the A and B sites (Blackburn et al, 2021;Jafar et al, 2015;Lu et al, 2017;Shu et al, 2016;Wang et al, 2021), but many impurities, such as ZrO 2 , U 3 O 8 , ThO 2 and other complex uranium oxides, were introduced at high actinide concentrations, and the stability of the pyrochlore was reduced. Moreover, the immobilization capacity, local structure and valence state of the incorporated actinides (i.e.…”