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89 Y MAS NMR of Pyrochlores

89 Y MAS NMR of Pyrochlores. Sharon Ashbrook. Department of Earth Sciences University of Cambridge. Why Pyrochlores?. Pyrochlores have been proposed as host phases for Ac and Ln radioactive waste Over last 50 years 1400 MT of Pu produced 300 MT weapons programs 200 MT MOX fuel

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89 Y MAS NMR of Pyrochlores

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  1. 89Y MAS NMR of Pyrochlores Sharon Ashbrook Department of Earth SciencesUniversity of Cambridge

  2. Why Pyrochlores? • Pyrochlores have been proposed as host phases for Ac and Ln radioactive waste • Over last 50 years 1400 MT of Pu produced 300 MT weapons programs 200 MT MOX fuel 900 MT stored at nuclear power stations • 70-80 MT added per year • In addition to Pu/U minor actinides Np, Am, Cm • Long lived isotopes 239Pu (24,100 y), 237Np (2.1 million y) and 233U (160,000 y) • Most important contributors to human exposure Ewing et al. 2004

  3. a recoil Why Pyrochlores? • High crystal chemical flexibility (> 500 synthetic compositions) • Tolerant of defects/substitutions and variable oxidation states • Component of the ceramic wasteform SYNROC • Titanates have very high chemical durability • High waste loading (naturally up to 30% UO2 and 10% ThO2) • Zirconate pyrochlores shown to be resistant to radiation damage (a-decay) • Natural analogues enable study of long-term behaviour and durability • Low leach rates (100 times less than borosilicate glass)

  4. Pyrochlores • A2B2O6Y Y = O, OH, F • Ordered superstructure of fluorite (AX2) with 1/8 O removed in an ordered manner (Fd–3m) • 2 cation sites VIIIA 2+, 3+ VIB 5+, 4+ • 3 anion sites 8a 8b(vacant) 48f (x, 1/8, 1/8) • Structure defined by 48f (x) and a

  5. 1.46 1.78 defect fluorite monoclinic Pyrochlore Stability rA/rB • A/B cation disorder • 48f x = 3/8 • Anion disorder (1 O with 7/8 occupancy) • Fm–3m • Complex structure • Layers of perovskite-like slabs • 4 A cations, 4 B cations • VIB but VI-XIIA

  6. 89Y NMR • Y as a non-radioactive model for actinide substitution • Use 89Y solid-state NMR to study local structure and ordering (Y, La)2 (Ti, Sn, Zr, Hf)2 O7 Type of structure Position of Y (A/B) Local environment (NNN) and coordination Ordering Phase transitions • Low gyromagnetic ratio (n0 = 24.5 MHz at 11.7 T) • Long T1 relaxation times • 89Y background in ZrO2 rotor • Acoustic probe ringing • Spin I = 1/2 • 100% natural abundance • High resolution by MAS • Chemical shift very sensitive to local environment 11.7 T (24.5 MHz for 89Y) Varian Infinity Plus with 7.5 mm MAS probe and low-g adaptor

  7. Single short flip angle (p/5) pulse • Shorter recycle intervals possible • Long (~100 ms) ringdown delay • Use of Si3N4 rotor to reduce background • Allows faster spinning (7.5 mm up to 8 kHz) • Use of Carr-Purcell-Meiboom-Gill (CPMG) echo train • Accurate p/2 and p pulses • More signal per recycle interval • Minimisation of ringing effects Add echoes FT FT 89Y NMR

  8. Simulation • DCSA = –390 ppm and h = 0 (x 48f = 0.422082) • Fairly long T1 of ~300 s Experiment rA/rB = 1.684 1500 °C for 6 days Y2Ti2O7 • Y2Ti2O7 is a model ordered pyrochlore • Single sharp resonance (70 Hz FWHH) at ~65 ppm with spinning sidebands • Y on VIII A site with 6 A and 6 B next nearest neighbours (NNN)

  9. rA/rB = 1.415 1600 °C for 4 days Y2Zr2O7 • Defect fluorite • Two broad Y resonances at 77 ppm and 186 ppm • VIIIY and VIY : cation disorder • Chemical shift increases as CN decreases • Broad resonances (1.5 kHz FWHH) reflect disorder • Ratio of A : B is 33% : 66% • Although T1 is long, similar relative relaxation B A Simulation Experiment

  10. (I) Y2Ti2–xSnxO7 1.477 < rA/rB < 1.684 1500 °C for 6 days

  11. Y2Ti2–xSnxO7 • Pyrochlore throughout solid solution • Y on VIIIA (150 ppm at x = 2 and 65 ppm at x = 0) • No Y on VIB • Differing NNN environments resolved • Relative intensities can be compared to theory

  12. * 1 3 10 30 100 300 1000 3000 10000 s * * tau / s Y2Ti2–xSnxO7 • Although T1 is long, little differential relaxation is observed Simulation Simulation Saturation recovery x = 1.6 1 3 10 30 100 300 1000 3000 10000

  13. 6 B sites Probability of n Sn NNN number of Sn NNN x/2 number of permutations Y2Ti2–xSnxO7 • Assume A site contains Y • Assume B site contains mixture of Sn/Ti • Random distribution of 6 B NNN • Probability of Sn on B, p = x/2 • No account of topology P(n Sn) = W (p)n (1 – p)6 – n

  14. 3 6 2 1 4 0 5 1 5 6 2 0 4 4 3 3 4 5 0 6 3 2 Y2Ti2–xSnxO7

  15. Y2Ti2–xSnxO7 2 • D Sn NNN is ~14-16 ppm • Suggests splitting of n = 3 resonance Y2Ti1.2Sn0.8O7 3 1 4 0 5 250 200 150 100 50 0 d (ppm)

  16. 6 Sn NNN 5 Sn NNN Theory 4 Sn NNN 3 Sn NNN Experiment 2 Sn NNN 1 Sn NNN 0 Sn NNN n (Sn NNN) Y2Ti2–xSnxO7

  17. Summary: Y2Ti2–xSnxO7 • Pyrochlore throughout solid solution • Y on VIIIA only (150 - 65 ppm) • Different Sn/Ti NNN environments can be distinguished • Results match those generated by a simple statistical model assuming a random distribution of Sn/Ti (no clustering/avoidance) • Appear to resolve two different types of environment with 3 Sn/Ti NNN • 119Sn NMR (possibly 17O) • Higher field measurements- better resolution? • Ab initio calculations of the chemical shift expected for the differing environments

  18. (II) Y2–xLaxTi2O7 1.684 < rA/rB < 1.917 1500 °C for 2 days

  19. Pyrochlore Pyrochlore + Monoclinic? Pyrochlore + Monoclinic Pyrochlore + Monoclinic Monoclinic Y2–xLaxTi2O7 • Change from pyrochlore at x = 0 to monoclinic at x = 2 • By radius ratio rules, predicted to occur at x = 0.84 • Broad 2 phase region observed?

  20. 6 Y 5 Y 5 Y 6 Y 4 Y? Y2–xLaxTi2O7 • Appearance of monoclinic phase difficult to detect (broad resonance) • Effect of 1 and probably 2 La NNN 6 Y

  21. Theory Experiment Y2–xLaxTi2O7 6 Y 5 Y 6 Y 5 Y 4 Y

  22. * * * • 2 phases are clear at x = 1.65, 1.7 and 1.75 • Not apparent at x = 1.8 Y2–xLaxTi2O7 • Broad, complex resonance for monoclinic spectrum • 4 Y species • Range of coordination numbers • Broadens as more Y is substituted as a result of disorder in NNN etc.

  23. 1 2 3 4 % 2 – x Y2–xLaxTi2O7 • Spectra are consistent with 4 Y species • Two sharper peaks and two broader peaks • Preferential substitution at low Y content

  24. Y1 Y2 “Perovskite-like” 2.6 Å - 2.7 Å Y3 Y4 2.45 Å - 2.65 Å “Inter-layer” Y2–xLaxTi2O7 Monoclinic

  25. Y3 Y4 “Inter-layer” Y1 Y2 “Perovskite-like” Y2–xLaxTi2O7 • Y is smaller than La • Preferentially substitutes into smaller inter-layer sites? • These must give rise to the broad resonances whilst the larger perovskite-like species give rise to sharper peaks • Consistent with general literature observations that d tends to decrease as CN increases

  26. Summary: Y2–xLaxTi2O7 • Change from pyrochlore to monoclinic as x increases • Boundaries of 2 phase region appear consistent with diffraction • At low x, A site ordering appears random • 4 resonances (2 sharp, 2 broad) in monoclinic phase • Preferential (non-uniform) substitution at low Y content • Suggest this occurs into the smaller inter-layer sites rather than the perovskite-like blocks • Further characterization of monoclinic cation sites and ordering (139La, 47/49Ti NMR) • Bond valence analysis • Bond distances from diffraction refinements (neutron and X-ray) • Ab initio calculations for Y on each A site

  27. Complex crystal chemistry • Need to know tolerance of disorder, limits of substitution, radiation resistance etc., for safe immobilization of actinide in ceramic wasteforms Amorphization of La2Zr2O7 Conclusions • 89Y NMR is a useful probe of the cation environment in pyrochlores • Pyrochlore, defect fluorite and monoclinic structure types can be distinguished • Large changes in chemical shift with coordination number • Smaller chemical shift change with NNN - used to consider cation ordering models • Linewidth reflects the disorder

  28. Acknowledgements • Dr Karl Whittle and Elizabeth Harvey • Dr Ian Farnan • Dr Gregory Lumpkin and Dr Simon Redfern • Professor Claire Grey (Stony Brook, NY) Cambridge Centre for Ceramic Immobilisation Royal Society for the award of a Dorothy Hodgkin Fellowship

  29. W = 6 W = 2 W = 6 + 6 • Also true for n = 2, 4 • 15 possible ways W = 3 W = 6 W = 6 Y2Ti2–xSnxO7 • 20 possible ways of arranging 3 Sn on 6 B sites • Not all may have the same effect upon the shift

  30. Y2Ti2–xSnxO7 • Other possible ordering models?? x Ordered avoidance 0 Sn 1 Sn Ordered clustering x 0 Sn 6 Sn

  31. Y2Ti2–xZrxO7 • Also fluorite at x = 1.6 • A : B = 33% : 66% • 2 phase at x = 1.2? • A : B = 43% : 56% • At x = 0.8 and x = 0.4 additional peak appears

  32. Y2Ti2–xZrxO7 8 CN • 8 CN peak ~65 ppm pyrochlore shifts to ~77 ppm fluorite • Resonance progressively broader until x = 1.2 6 CN • 6 CN peak ~164 ppm x = 0.4/0.8, ~189 ppm fluorite • 2 phase at x = 1.2?

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