Synthesis, structure, and characterization of the thorium zirconolite CaZr _1‐x Th _x Ti ₂ O ₇ system

Abstract

Abstract A series of zirconolite ceramics with composition CaZr 1‐x Th x Ti 2 O 7 (Δx = 0.10) were reactively sintered at 1350°C for 20 h, in air (0 ≤ x ≤ 0.60) and 5% H 2 /N 2 (0 ≤ x ≤ 0.40). A sample with composition corresponding to x = 0.20 was also produced by hot isostatic pressing (HIP) at 1300°C and 100 MPa for 4 hours. Th 4+ immobilization was most readily achieved under oxidizing conditions, with Th 4+ preferentially incorporated within a pyrochlore‐structured phase in the range 0.10 ≤ x ≤ 0.50, yet formation of the zirconolite‐4M polytype was not observed. We report the novel synthesis of single‐phase pyrochlore with nominal composition CaZr 0.40 Th 0.60 Ti 2 O 7 when targeting x = 0.60. Th 4+ incorporation under reducing conditions produced a secondary Th‐bearing perovskite, comprising 24.2 ± 0.6 wt% of the phase assemblage when targeting x = 0.40, alongside 8.8 ± 0.3 wt% undigested ThO 2 . Under reducing conditions, powder XRD data were consistent with zirconolite adopting the 3T polytype structure. The sample produced by HIP presented a nonequilibrium phase assemblage, yielding a major phase of zirconolite‐2M alongside accessory Th 4+ ‐bearing phases ThTi 2 O 6 , ThO 2 , and perovskite. These data highlight the efficacy of Th 4+ as a Pu 4+ surrogate, with implications for the formation of Zr‐stabilized Th‐pyrochlore phases as matrices for waste with elevated Th 4+ content.