Synthesis, structure and characterisation of the thorium zirconolite CaZr1‐xThxTi2O7 system

A series of zirconolite ceramics with composition CaZr1‐xThxTi2O7 (Δx = 0.10) were reactively sintered at 1350 ˚C for 20 h, in air (0 ≤ x ≤ 0.60) and 5% H2/N2 (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 h. Th4+ immobilisation was most readily achieved under oxidising conditions, with Th4+ 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 CaZr0.40Th0.60Ti2O7 when targeting x = 0.60. Th4+ 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 ThO2. Under reducing conditions, powder XRD data were consistent with zirconolite adopting the 3T polytype structure. The sample produced by HIP presented a non‐equilibrium phase assemblage, yielding a major phase of zirconolite‐2M alongside accessory Th4+ bearing phases ThTi2O6, ThO2 and perovskite. These data highlight the efficacy of Th4+ as a Pu4+ surrogate, with implications for the formation of Zr‐stabilised Th‐pyrochlore phases as matrices for waste with elevated Th4+ content.