17O solid-state NMR spectroscopy of A2B2O7 oxides: quantitative isotopic enrichment and spectral acquisition?

Arantxa Fernandes,David Mckay,Giulia P M Bignami,Sharon E Ashbrook,Daniel M Dawson,Karl R Whittle,Frédéric Blanc,Scott Sneddon,Robert F Moran

doi:10.1039/c8ra00596f

Abstract

The potential of 17O NMR spectroscopy for the investigation of A2B2O7 ceramic oxides important in the encapsulation of radioactive waste is demonstrated, with post-synthetic enrichment by exchange with 17O2 gas. For Y2Sn2O7, Y2Ti2O7 and La2Sn2O7 pyrochlores, enrichment of the two distinct O species is clearly non quantitative at lower temperatures (∼700 °C and below) and at shorter times, despite these being used in prior work, with preferential enrichment of OA2B2 favoured over that of OA4. At higher temperatures, the 17O NMR spectra suggest that quantitative enrichment has been achieved, but the integrated signal intensities do not reflect the crystallographic 1 : 6 (O1 : O2) ratio until corrected for differences in T1 relaxation rates and, more importantly, the contribution of the satellite transitions. 17O NMR spectra of Y2Zr2O7 and Y2Hf2O7 defect fluorites showed little difference with any variation in enrichment temperature or time, although an increase in the absolute level of enrichment (up to ∼7.5%) was observed at higher temperature. DFT calculations show that the six distinct resonances observed cannot be assigned unambiguously, as each has contributions from more than one of the five possible next nearest neighbour environments. For La2Ti2O7, which adopts a layered perovskite-like structure, little difference in the spectral intensities is observed with enrichment time or temperature, although the highest absolute levels of enrichment (∼13%) were obtained at higher temperature. This work demonstrates that 17O NMR has the potential to be a powerful probe of local structure and disorder in oxides, but that considerable care must be taken both in choosing the conditions for 17O enrichment and the experimental acquisition parameters if the necessary quantitative measurements are to be obtained for more complex systems.

Highlights

The fundamental importance of oxide-based systems in technology, the energy arena, geochemistry and catalysis, and the presence of oxygen in a number of biomaterials, should have resulted in oxygen nuclear magnetic resonance (NMR) spectroscopy emerging as a vital tool for the characterisation of solid materials
Isotopic enrichment in 17O is required to enable NMR spectra to be acquired on a reasonable timescale, for multidimensional experiments.[2,5,6,7]
A 17O magic-angle spinning (MAS) NMR spectrum of Y2Zr2O7 has been acquired in previous work,[27] which contained a broadened lineshape proposed to result from the overlap of lines from the ve different O species, all of which were thought to have small CQ values, these were not resolved directly

Summary

Introduction

The fundamental importance of oxide-based systems in technology, the energy arena, geochemistry and catalysis, and the presence of oxygen in a number of biomaterials, should have resulted in oxygen nuclear magnetic resonance (NMR) spectroscopy emerging as a vital tool for the characterisation of solid materials. The relative intensities of the resonances for the O1 and O2 species in a pyrochlore would be expected to be 1 : 6 (Wykoff positions 8a and 48f, respectively).[23,24] the spectra in Fig. 2 for Y2Sn2O7, Y2Ti2O7 and La2Sn2O7 have ratios of 1 : 1.7, 1 : 3.1 and 1 : 1.7, respectively, indicating that enrichment is non uniform at the temperature used (700 C), or that the spectral acquisition was not quantitative.

Results

Conclusion