Authors: Gabriel L. Murphy | Elena Bazarkina | Volodymyr Svitlyk | André Roßberg | Shannon Kimberly Potts | Christoph Hennig | Maximilian Henkes | Kristina O. Kvashnina | Nina Huittinen
Source: ACS Omega (article, 2024-06-12)
Field: Materials Science — Materials Chemistry

Probing the Long- and Short-Range Structural Chemistry in the C-Type Bixbyite Oxides Th<sub>0.40</sub>Nd<sub>0.48</sub>Ce<sub>0.12</sub>O<sub>1.76</sub>, Th<sub>0.47</sub>Nd<sub>0.43</sub>Ce<sub>0.10</sub>O<sub>1.785</sub>, and Th<sub>0.45</sub>Nd<sub>0.37</sub>Ce<sub>0.18</sub>O<sub>1.815</sub> via Synchrotron X-ray Diffraction and Absorption Spectroscopy The long- and short-range structural chemistry of the C-type bixbyite compounds Th<sub>0.40</sub>Nd<sub>0.48</sub>Ce<sub>0.12</sub>O<sub>1.76</sub>, Th<sub>0.47</sub>Nd<sub>0.43</sub>Ce<sub>0.10</sub>O<sub>1.785</sub>, and Th<sub>0.45</sub>Nd<sub>0.37</sub>Ce<sub>0.18</sub>O<sub>1.815</sub> 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 <i>Ia</i>3̅ 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 L<sub>3</sub>-edge revealed that Ce incorporates as Ce<sup>4+</sup> into the structures but involves local distortion that resembles cluster behavior and loss of nearest-neighbors. In comparison, HERFD-XANES measurements on the Nd L<sub>3</sub>-edge supported by electronic structure calculations reveal that Nd<sup>3+</sup> adopts a local coordination environment similar to the long-range C-type structure while providing charge balancing for the formation of oxygen defects. Th L<sub>3</sub>-edge EXAFS analysis reveals shorter average Th-O distances in the title compounds in comparison to pristine ThO<sub>2</sub> in addition to shorter Th-O and Th-Ce distances compared to Th-Th or Ce-Ce in the corresponding F-type binary oxides (ThO<sub>2</sub> and CeO<sub>2</sub>). 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 Nd<sup>3+</sup> incorporation, induces local bond contraction and enhanced Th cation valence, leading to the observed increased lattice expansion with progressive Nd<sup>3+</sup> 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. Bixbyite ; XANES ; Extended X-ray absorption fine structure ; Crystallography ; Powder diffraction ; Chemistry ; Absorption spectroscopy ; Spectroscopy ; Materials science ; Analytical Chemistry (journal) ; Oxide ; Physics Nuclear Materials and Properties ; Nuclear materials and radiation effects ; Radioactive element chemistry and processing