Cation Disorder and Anionic Transport in New Complex Oxides from Experimental and Computational NMR Spectroscopy

Melilite La_1.54Sr_0.46Ga₃O_7.27 (<i>Nat. Mat.</i> <b>2008</b>, 498) and langasite La₃Ga_5-xGe_1+xO_14+0.5x (x &lt; 1.5) oxides (<i>Chem. Mater</i>. <b>2019</b>, 5742) have attracted significant research interests as next generation solid electrolytes owing to their high oxide ion conductivity in the 600°C and 800°C temperature range. Multinuclear, multidimensional NMR data aimed at elucidating cation disorder and oxygen dynamics in melilite (<i>JACS</i> <b>2023</b>, 21817; <i>ChemPhysChem</i> <b>2024</b>, e202300934) and langasite (<i>JACS</i> <b>2024</b>, 14022) have established the oxygen transport mechanism of these highly conductive phases.<br/>Structural information is provided by multinuclear magic angle spinning (MAS) NMR experiments up to 1.5 GHz, 35.2 T at the NHMFL and is aided by first principles computation using an ensemble-based approach to comprehensively model the complex sites disordered. Powerful insights into the local dynamics are also gained from a range of ¹⁷O and ⁷¹Ga high temperature (HT) MAS NMR approaches and obtained with exciting probe capabilities enabling experiments up to 700°C under MAS at 850 MHz, 20 T to be recorded.<br/>Strongly deshielded signals in the ¹⁷O MAS NMR spectra are observed in the doped phases, which are not observed for the pristine materials, and are assigned to defective oxygen interstitials associated with La³⁺ or Ge⁴⁺ doping in melilite and langasite, respectively. Five-coordinate Ga sites accommodating the interstitial defects are also clearly identified from ⁷¹Ga NMR spectra at 35.2 T. Coalescence of all the signals in the ¹⁷O HT MAS NMR spectra of melilite and langasite above 300 °C and 400 °C, respectively, indicates chemical exchange between all signals implying the unexpected participation of all oxide ions in the diffusion mechanism. This work addresses profound debates in the literature and suggests design rules for next generation fast oxide ion conductors.