Disorder and Oxide Ion Diffusion Mechanism in La1.54Sr0.46Ga3O7.27 Melilite from Nuclear Magnetic Resonance

Layered tetrahedral network melilite is a promising structural family of fast ion conductors that exhibits the flexibility required to accommodate interstitial oxide anions leading to excellent ionic transport properties at moderate temperatures. Here, we present a combined experimental and computational Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) approach which aims at elucidating the local configurational disorder and oxide ion diffusion mechanism in a key member of this structural family possessing the La1.54Sr0.46Ga3O7.27 composition. 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 and the identification of distinct gallium coordination environments. The 17O and 71Ga MAS NMR spectra of La1.54Sr0.46Ga3O7.27 display additional features not observed for the parent LaSrGa3O7 phase which are attributed to interstitial oxide ions incorporated upon cation doping and stabilised by the formation of five-coordinate Ga centres conferring framework flexibility. 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, 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 and highlight the potential of MAS NMR spectroscopy to enhance the understanding of ionic motion in solid electrolytes.

层状四面体网络黄长石（melilite）是一类极具潜力的快离子导体结构家族，其具备容纳间隙氧阴离子所需的结构柔性，可在中等温度下展现优异的离子传输性能。本研究结合实验与计算方法，采用魔角旋转（Magic Angle Spinning, MAS）核磁共振（Nuclear Magnetic Resonance, NMR）技术手段，旨在阐明该结构家族中组成为La₁.₅₄Sr₀.₄₆Ga₃O₇.₂₇的关键成员的局部构型无序性与氧离子扩散机制。17O与71Ga的MAS NMR谱图呈现复杂的谱线形貌，可通过基于系综的计算建模方法精准模拟多阳离子与阴离子位点的无序性，由此实现桥氧/非桥氧的信号归属，并区分不同的镓配位环境。La₁.₅₄Sr₀.₄₆Ga₃O₇.₂₇的17O与71Ga MAS NMR谱图存在母体相LaSrGa₃O₇未观测到的额外特征，这些特征源于阳离子掺杂引入的间隙氧离子，而五配位镓中心的形成稳定了该骨架并赋予其结构柔性。17O高温（high temperature, HT）MAS NMR实验在130 ℃下捕捉到桥氧内部的交换过程，并观测到La₁.₅₄Sr₀.₄₆Ga₃O₇.₂₇中所有氧信号在约300 ℃时发生峰合并，表明间隙氧离子与骨架氧离子均参与了传输过程。这些结果辅以La₁.₅₄Sr₀.₄₆Ga₃O₇.₂₇的71Ga HT MAS NMR谱图中镓共振峰的合并现象，明确证实了该黄长石相的传导机制，并凸显了MAS NMR光谱技术在加深固体电解质中离子运动认知方面的应用潜力。