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

Abstract

Zirconolite is considered to be a suitable wasteform material for the immobilization of Pu and other minor actinide species produced through advanced nuclear separations. Here, we present a comprehensive investigation of Dy 3+ incorporation within the self-charge balancing zirconolite Ca 1– x Zr 1– x Dy 2 x Ti 2 O 7 solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x ≤ 1.00 (Δ x = 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 °C in air for 48 h. Three distinct coexisting phase fields were identified, with single-phase zirconolite-2M identified only for x = 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ x ≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ x ≤ 0.50. Compositions for which x ≥ 0.60 were consistent with single-phase pyrochlore. The formation of zirconolite-4M and pyrochlore polytype phases, with increasing Dy content, was confirmed by high-resolution transmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L 3 -edge XANES region confirmed that Dy was present uniformly as Dy 3+ , remaining analogous to Am 3+ . Fitting of the EXAFS region was consistent with Dy 3+ cations distributed across both Ca 2+ and Zr 4+ sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy 3+ was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi 2 O 7 –REE 3+ Ti 2 O 7 binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE 3+ cations.

Structural and electronic behavior of yttrium doped zirconolite ceramic; a potential waste form for burning minor actinides

“…These extra peaks due to ZrO 2 and TiO 2 are indicative of incomplete reaction of oxides at 1200 °C [28,29]. The XRD pattern of sample synthesized at 1300 °C shows formation of single monoclinic phase of 2M-zirconolite along with presence of minor perovskite phase represented by minute peak at 33.1°, marked by # in figure 1(a), and is in agreement with the literature [13][14][15][16][17][18][19][20][21][22] and JCPDS-PDF-01-084-0163 data card. The existence of minor phase of perovskite in the structure may be attributed to the substitution of the Y at the Ca and Zr site [18][19][20][21].…”

“…The existence of minor phase of perovskite in the structure may be attributed to the substitution of the Y at the Ca and Zr site [18][19][20][21]. For further authentication, the multiple phase refinement has been performed on XRD patterns of both the samples using Rietveld refinement performed through TOPAS software by taking the standard data of 2M-zirconolite, perovskite, rutile and baddeleyite from crystallographic open database as initial models for structural details [17,29]. From the analysis of Rietveld refinement of XRD data of sample sintered at 1200 °C, the phase assemblages consist of zirconolite-2M (CaZrTi 2 O 7 )-88.37%, perovskite (CaTiO 3 )-1.45%, baddeleyite (ZrO 2 )-5.62%, and Rutile (TiO 2 )-4.57%.…”

“…XRD patterns were recorded in 2θ range of 5°-80°at a step size and scan speed of 0.02°and 3 sec/step, respectively. Quantitative phase analysis was achieved with Rietveld refinement of the XRD data using TOPAS V6.0 Bruker AXS software package [17,24]. The profile fitting and structural refinement were performed using Fundamental Parameter Function [25].…”

“…Due to layered structure, zirconolite has capacity to accommodate wastes mainly at Ca and Zr cation sites without affecting its structural integrality [3]. Various studies have been reported on phase and structural evolution of zirconolite by the incorporation of lanthanides and actinides, the surrogates to long-lived minor actinides and fission fragments [1,4,5,[14][15][16][17].…”