Structural characterization of Nd-doped Hf-zirconolite Ca1−xNdxHfTi2−xAlxO7 ceramics

Because of its high incorporation capacity and of the high thermal neutron capture cross-section of hafnium, Hf-zirconolite (CaHfTi2O7) ceramic can be envisaged as a potential waste form for minor actinides (Np, Am, Cm) and plutonium immobilization. In this work, Nd-doped Hf-zirconolite Ca1−xNdxHfTi2−xAlxO7 (x = 0; 0.01 and 0.2) ceramics have been prepared by solid state reaction. Neodymium has been used as trivalent actinide surrogate. The ceramic samples structure has been studied by X-ray diffraction and refined by the Rietveld method. This revealed that Nd3+ ions only enter the Ca site, whereas part of Hf4+ ions substitute titanium into Ti(1) sites and Al3+ ions mainly occupy the Ti(2) split sites and Ti(3) sites of the zirconolite structure. Using various spectroscopic techniques (electron spin resonance, optical absorption and fluorescence), the environment of Nd3+ cations in Hf-zirconolite has been studied and compared with that of Nd3+ cations in Zr-zirconolite (CaZrTi2O7). Different local environments of Nd3+ cations have been detected in Hf-zirconolite that can be attributed to the existence of an important disorder around Nd in the Ca site probably due to the statistical occupancy of the next nearest cationic site of neodymium (a split Ti site) by Ti4+, Al3+ cations and vacancies. No significant differences were observed concerning Nd3+ cations environment and distribution in Hf- and Zr-zirconolite ceramics.