Synthesis, structure, and characterization of the thorium zirconolite CaZr _1‐x Th _x Ti ₂ O ₇ 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 0.40). sample corresponding to 0.20 was also produced by hot isostatic pressing (HIP) 1300 100 MPa 4 h. Th4+ immobilisation most readily achieved under oxidising conditions, preferentially incorporated within a pyrochlore‐structured phase the range 0.10 0.50, yet formation zirconolite‐4M polytype not observed. We report novel synthesis single pyrochlore nominal CaZr0.40Th0.60Ti2O7 when targeting 0.60. incorporation reducing conditions secondary Th‐bearing perovskite, comprising 24.2 ± 0.6 wt. % assemblage 0.40, alongside 8.8 0.3 undigested ThO2. Under powder XRD data consistent adopting 3T structure. The HIP presented non‐equilibrium assemblage, yielding major zirconolite‐2M accessory bearing phases ThTi2O6, ThO2 perovskite. These highlight efficacy as Pu4+ surrogate, implications Zr‐stabilised Th‐pyrochlore matrices waste elevated content.