Low charge compensator (Mg2+) causing a new REE-end 3O structure (REE=Rare Earth Element) and a different phase transformation in Nd3+ Co-doped zirconolite: Investigation by X-ray structural analysis

Co‐immobilization of a PuO2 surrogate and contaminated stainless steel within a zirconolite matrix

“…Typically, actinides and lanthanides substitute on the Ca 2+ or Zr 4+ sites, whereas lower valence charge compensation species (such as Al 3+ , Fe 3+ , Cr 3+ , and Mg 2+ ) replace Ti 4+ sites to maintain charge neutrality. [20][21][22][23][24][25] Depending on the substitution scheme, zirconolite-2M may transform into other polytypes, such as 3T (Ca 1−x Pu x ZrTi 2−2x Fe 2x O 7 , x = 0.30-0.40), 22 4M (CaU x Zr 1−x Ti 2 O 7 , x = 0.20-0.70) 24 and zirconolite-3O (Ca 0.99−2x Nd 2x ZrTi 2−x Mg x O 7 , x = 0.35-0.50). 25 In our recent work, zirconolite ceramics with the nominal composition of Ca 1−x Ce x ZrTi 2−2x Fe x Cr x O 7 (0 ≤ x ≤ 0.30) were produced by solid-state reaction sintering at 1400 • C, by starting from the oxide precursors of CaTiO 3 , ZrO 2 , TiO 2 , CeO 2 , Fe 2 O 3, and Cr 2 O 3 .…”

“…[20][21][22][23][24][25] Depending on the substitution scheme, zirconolite-2M may transform into other polytypes, such as 3T (Ca 1−x Pu x ZrTi 2−2x Fe 2x O 7 , x = 0.30-0.40), 22 4M (CaU x Zr 1−x Ti 2 O 7 , x = 0.20-0.70) 24 and zirconolite-3O (Ca 0.99−2x Nd 2x ZrTi 2−x Mg x O 7 , x = 0.35-0.50). 25 In our recent work, zirconolite ceramics with the nominal composition of Ca 1−x Ce x ZrTi 2−2x Fe x Cr x O 7 (0 ≤ x ≤ 0.30) were produced by solid-state reaction sintering at 1400 • C, by starting from the oxide precursors of CaTiO 3 , ZrO 2 , TiO 2 , CeO 2 , Fe 2 O 3, and Cr 2 O 3 . 26 It was found that zirconolite-2M phase was formed across the solid solution, where the solubility limit was reached at x = 0.30, and trivalent Fe 3+ and Cr 3+ were maintained.…”

“…The crystal structure of zirconolite‐2M has three available cation sites and hence has the requisite chemical flexibility to accommodate various actinides, lanthanides, and charge compensators, based on available oxidation states and ionic radii. Typically, actinides and lanthanides substitute on the Ca 2+ or Zr 4+ sites, whereas lower valence charge compensation species (such as Al 3+ , Fe 3+ , Cr 3+ , and Mg 2+ ) replace Ti 4+ sites to maintain charge neutrality 20–25 . Depending on the substitution scheme, zirconolite‐2M may transform into other polytypes, such as 3T (Ca 1− x Pu x ZrTi 2−2 x Fe 2 x O 7 , x = 0.30–0.40), 22 4M (CaU x Zr 1− x Ti 2 O 7 , x = 0.20–0.70) 24 and zirconolite‐3O (Ca 0.99−2 x Nd 2 x ZrTi 2− x Mg x O 7 , x = 0.35–0.50) 25 .…”