Publication date: 10th April 2024
Nuclear Magnetic Resonance (NMR) plays a critical role in understanding structural and dynamical disorder, and ionic transport diffusion pathways in solids, complementing diffraction or impedance spectroscopy methods. Here, we present a recently published approach[1] combining experimental and computational Magic Angle Spinning (MAS) NMR aimed at elucidating the local configurational disorder and oxide ion diffusion mechanism in promising structural families of fast oxide ion conductors having the melilite,[1] [2] langasite[3] and perovskite[4] structures.
Layered tetrahedral network melilite possessing the La1.54Sr0.46Ga3O7.27 composition exhibits the flexibility required to accommodate interstitial oxide anions leading to excellent ionic transport properties at moderate temperatures[2]. 17O and 71Ga MAS NMR spectra display complex spectral line shapes that could be accurately predicted using a computational ensemble-based approach to model site disorder across multiple cationic and anionic sites, thereby enabling the assignment of bridging/non-bridging oxygens (Figure 1) and the identification of distinct gallium coordination environments. The 17O and 71Ga MAS NMR spectra of La1.54Sr0.46Ga3O7.27 display additional features that are not observed for the parent LaSrGa3O7 phase[submitted] and that can only be detected at the most powerful NMR magnet in the world operating at 35.2 T.[5] These features are attributed to interstitial oxide ions incorporated upon cation doping and stabilised by the formation of five-coordinate Ga centres conferring framework flexibility.[2]
17O high temperature (HT) MAS NMR experiments capture exchange within the bridging oxygens at 130 °C and reveal coalescence of all oxygen signals in La1.54Sr0.46Ga3O7.27 at approximately 300 °C (Figure 1), indicative of the participation of both interstitial and framework oxide ions in the transport process. These results, further supported by the coalescence of the 71Ga resonances in the 71Ga HT MAS NMR spectra of La1.54Sr0.46Ga3O7.27, unequivocally provide evidence for the conduction mechanism in this melilite phase.
Other related examples on langasite[3] and perovskite[4] highlight the potential of MAS NMR spectroscopy to enhance the understanding of ionic motion in solids.
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