Hydrolytic Stability of Mesoporous Zirconium Titanate Frameworks Containing Coordinating Organic Functionalities

The hydrolytic stability of lanthanide and actinide selective mono- and polyphosphonate-functionalized mesoporous zirconium titanium oxide adsorbents has been investigated in nitric acid solutions. Hydrolytic degradation of the surfaces, as measured through the fractional loss of phosphorus and elements of the oxide framework, increased by more than an order of magnitude as the nitric acid concentration was increased from 0 to 2 mol/L. The unfunctionalized parent oxide suffered considerable dissolution in 2 mol/L acid over a period of 72 h. Under identical conditions, the fractional Zr and Ti release was reduced to 1 × 10^–2 for monophosphonate functionalized hybrids and reached as low as 1 × 10^–6 for trisphosphonate functionalized variants. The bisphosphonates showed intermediate values. The leaching of P, Zr and Ti was found to be incongruent with the Zr leaching to a lesser extent implying enhanced stability of the Zr–O–P bond. Quantitative analysis of the dissolution kinetics indicated a parabolic dissolution model with a rate constant in the range of 0.5–1.5 mg g^–1 min^–1/2 for the elemental leaching of P, Ti, and Zr. The leaching of Zr from the mesoporous matrix was relatively more complex than for the other elements with evidence of a leaching mechanism involving two processes. ToF-SIMS and DRIFT analysis demonstrated that after leaching in 2 M HNO₃ for 24 h, a significant proportion of grafted ligands remained on the surface. The oxide functionalized with amino trismethylenephosphonic acid, which had previously shown excellent ¹⁵³Gd³⁺ selectivity, was demonstrated to have outstanding stability, with low fractional elemental losses and preservation of mesoporous texture even after leaching for 24 h in 2 M HNO₃. This suggests this particular hybrid to be worthy of additional study

Hydrolytic Stability of Mesoporous Zirconium Titanate Frameworks Containing Coordinating Organic Functionalities

The hydrolytic stability of lanthanide and actinide selective mono- and polyphosphonate-functionalized mesoporous zirconium titanium oxide adsorbents has been investigated in nitric acid solutions. Hydrolytic degradation of the surfaces, as measured through the fractional loss of phosphorus and elements of the oxide framework, increased by more than an order of magnitude as the nitric acid concentration was increased from 0 to 2 mol/L. The unfunctionalized parent oxide suffered considerable dissolution in 2 mol/L acid over a period of 72 h. Under identical conditions, the fractional Zr and Ti release was reduced to 1 × 10^–2 for monophosphonate functionalized hybrids and reached as low as 1 × 10^–6 for trisphosphonate functionalized variants. The bisphosphonates showed intermediate values. The leaching of P, Zr and Ti was found to be incongruent with the Zr leaching to a lesser extent implying enhanced stability of the Zr–O–P bond. Quantitative analysis of the dissolution kinetics indicated a parabolic dissolution model with a rate constant in the range of 0.5–1.5 mg g^–1 min^–1/2 for the elemental leaching of P, Ti, and Zr. The leaching of Zr from the mesoporous matrix was relatively more complex than for the other elements with evidence of a leaching mechanism involving two processes. ToF-SIMS and DRIFT analysis demonstrated that after leaching in 2 M HNO₃ for 24 h, a significant proportion of grafted ligands remained on the surface. The oxide functionalized with amino trismethylenephosphonic acid, which had previously shown excellent ¹⁵³Gd³⁺ selectivity, was demonstrated to have outstanding stability, with low fractional elemental losses and preservation of mesoporous texture even after leaching for 24 h in 2 M HNO₃. This suggests this particular hybrid to be worthy of additional study

Gradual Structural Evolution from Pyrochlore to Defect-Fluorite in Y₂Sn_2–<i>x</i>Zr_<i>x</i>O₇: Average vs Local Structure

We have studied the long-range average and local structures in Y₂Sn_2–<i>x</i>Zr_<i>x</i>O₇ (<i>x</i> = 0–2.0) using synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, respectively, and by theoretical methods. While the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defect-fluorite at <i>x</i> ∼ 1.0–1.2, X-ray absorption near-edge structure (XANES) results at the Zr L₃- and Y L₂-edges reveal a gradual structural evolution across the whole compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary, as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defect-fluorite region. The Zr and Y L-edge spectra are very sensitive to changes in the local structure; such sensitivity enables us to reveal the progressive nature of the phase transition. Experimental results are supported by <i>ab initio</i> atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures are predicted, and the trends agree well with the experimental XANES results. The calculations predict that the coordination of cations in the Y₂Zr₂O₇ defect-fluorite (normally considered to be 7 for all cations) varies depending on the species with the average coordination of Y and Zr being 7.2 and 6.8, respectively

Probing Long- and Short-Range Disorder in Y₂Ti_2–<i>x</i>Hf_<i>x</i>O₇ by Diffraction and Spectroscopy Techniques

We studied the long-range average and short-range local structures in Y₂Ti_2–<i>x</i>Hf<i>_x</i>O₇ (<i>x</i> = 0–2.0) using diffraction and spectroscopy techniques, respectively. Both neutron and synchrotron X-ray powder diffraction data show a clear phase transition of the average structure from ordered pyrochlore to disordered defect-fluorite at <i>x</i> ≈ 1.6; the long-range anion disorder appears to develop gradually throughout the entire pyrochlore region in contrast to the rapid loss of cation ordering from <i>x</i> = 1.4 to 1.6. The commonly observed two-phase region around the pyrochlore/defect-fluorite phase boundary is absent in this system, demonstrating high sample quality. X-ray absorption near-edge structure (XANES) results at the Y L₂-, Ti K- and L_3,2-, Hf L₃-, and O K-edges indicate a gradual local structural evolution across the whole compositional range; the Y coordination number (CN) decreases and the CN around Ti and Hf increases with increasing Hf content (<i>x</i>). The spectroscopic results suggest that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary as determined by diffraction, and this disorder evolves continuously from short- to medium- and eventually to long-range detectable by diffraction. This study highlights the complex disordering process in pyrochlore oxides and the importance of a multitechnique approach to tackle disorder over different length scales and in the anion and cation sublattices, respectively. The results are important in the context of potential applications of these oxides such as ionic conductors and radiation-resistant nuclear waste forms