Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The long- and short- range structural chemistry of the C-type bixbyite compounds Th0.40Nd0.48Ce0.12O1.76, Th0.47Nd0.43Ce0.10O1.785 and Th0.45Nd0.37Ce0.18O1.815 is systematically examined using synchrotron X-ray powder diffraction (S-PXRD), high energy resolution fluorescence detection X-ray absorption near edge (HERFD-XANES) and extended absorption fine structure spectroscopy (EXAFS) measurements supported by electronic structure calculations. S-PXRD measurements revealed the title compounds all form classical C-type bixbyite structures in space group Ia3 ̅ that have disordered cationic crystallographic sites with further observation of characteristic superlattice reflections corresponding to oxygen vacancies. Despite the occurrence of oxygen vacancies, HERFD-XANES measurements on the Ce L3-edge revealed that Ce incorporates as Ce+4 into the structures but involves significant local distortion akin to cluster behavior and loss of nearest-neighbors. In comparison, HERFD-XANES measurements on the Nd+3 L3-edge supported by electronic structure calculations reveal that Nd+3 adopts a local coordination environment similar to the long-range C-type structure whilst providing charge balancing for the formation of oxygen defects. Th L3-edge EXAFS analysis reveals shorter average Th-O distances in the title compounds in comparison to pristine ThO2 in addition to shorter Th-O and Th-Ce distances compared to Th-Th or Ce-Ce in corresponding F-type binary oxides (ThO2 and CeO2). These distances are further found to decrease with the increased Nd content of the structures despite simultaneous observation of the overall lattice structure progressively expanding. Linear combination calculations of the M-O bond lengths are used to explain these observations, where the role of oxygen defects, via Nd+3 incorporation, induces local bond contraction and enhanced Th+4 cation valence leading to the observed increased lattice expansion with progressive Nd+3 incorporation. Overall, the investigation points to the significance of dissimilar cations exhibiting variable short-range chemical behavior and how it can affect long-range structural chemistry of complex oxides.