Cation Disorder in Pyrochlore Ceramics: ⁸⁹Y MAS NMR and First-Principles Calculations

Abstract

The use of first-principles DFT calculations to interpret solid-state 89Y MAS NMR spectra of Y2Ti2-x Sn x O7 pyrochlores, materials with applications for the encapsulation of actinide-bearing radioactive waste, is investigated. Although NMR is a sensitive probe of local structure, which does not rely on the presence of long-range order, spectra of disordered materials are often complex and difficult to interpret. We show how calculations can be used alongside experiment to confirm that Y occupies only the eight-coordinate pyrochlore A site in these materials and that the 89Y isotropic chemical shift is primarily affected by the number of Sn/Ti on the neighboring B sites. Small changes in local geometry and more distant B-site cation substitutions are shown to have a smaller effect on the chemical shift, and will result in broadening, shoulders, and small splittings in the NMR spectrum. In general, the results confirm the validity of the assumptions made in the previous spectral analysis, although in a very small minority of cases, chemical shifts are calculated which lie outside the expected ranges. However, these are shown to result from significant deviations in local geometry (O−Y−O bond angles and Y−O bond distances) and are thought to arise from the periodicity (and, therefore, long-range order) which is imposed in the calculations. Using our calculated results we can confirm that there is a random distribution of Sn/Ti on the six-coordinated pyrochlore B sites in Y2Ti2-x Sn x O7, and also demonstrate that an equilibrium structure has been achieved by studying materials which have been annealed for different durations.

Color and Brightness Tuning in Heteronuclear Lanthanide Terephthalate Coordination Polymers

“…[31] Recent papers proved that 89 Y NMR spectroscopy is a useful probe for local ordering in solid-state compounds. [38][39][40][41] Consequently, we performed 13 (Figures 2 and 3) show no evidence of local ordering and support the interpretation of randomly distributed yttrium and lanthanum atoms. Figure 2 clearly shows that regardless of the lanthanum doping rate, the 89 Y NMR signal remains almost identical.…”

Recent technique developments and applications of solid state NMR in characterising inorganic materials

“…This work was recently extended to use first principles DFT calculations. The shift values calculated and the effects of the Ti,Sn ordering confirmed that the yttrium does remain eight-fold coordinated and that B site occupancy is random [53]. 109 Ag NMR is another low-nucleus and an extensive study of 20 simple inorganic silver salts added a large amount of new 109 Ag data to the literature [54].…”

Exploiting Periodic First-Principles Calculations in NMR Spectroscopy of Disordered Solids

“…When considering B-site substitution, it is impractical to perform calculations on truly disordered solids (as an unfeasibly large number of atoms would be involved), and a simplified approach is required. 16 The pyrochlores. 19 The combination of high-resolution NMR and GIPAW DFT calculations enables detailed information to be obtained about the nature of the B-site cation disorder in pyrochlores and provides detailed insight into the local geometry -information it is difficult to obtain from diffraction-based methods.…”