Probing the Long- and Short-Range Structural Chemistry in the C-Type Bixbyite Oxides Th_0.40Nd_0.48Ce_0.12O_1.76,...

The long- and short-range structural chemistry of the C-type bixbyite compounds Th_0.40Nd_0.48Ce_0.12O_1.76, Th_0.47Nd_0.43Ce_0.10O_1.785, and Th_0.45Nd_0.37Ce_0.18O_1.815 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 X-ray absorption fine structure spectroscopy (EXAFS) measurements supported by electronic structure calculations. S-PXRD measurements revealed that the title compounds all form classical C-type bixbyite structures in space group Ia3̅ 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 Ce⁴⁺ into the structures but involves local distortion that resembles cluster behavior and loss of nearest-neighbors. In comparison, HERFD-XANES measurements on the Nd L₃-edge supported by electronic structure calculations reveal that Nd³⁺ adopts a local coordination environment similar to the long-range C-type structure while 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 the 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 help explain these observations, where the role of oxygen defects, via Nd³⁺ incorporation, induces local bond contraction and enhanced Th cation valence, leading to the observed increased lattice expansion with progressive Nd³⁺ incorporation. Overall, the investigation points to the significance of dissimilar cations exhibiting variable short-range chemical behavior and how it can affect the long-range structural chemistry of complex oxides.