Cation Distribution and Anion Transport in the LaGaGeO Langasite Structure.

Exploration of compositional disorder using conventional diffraction-based techniques is challenging for systems containing isoelectronic ions possessing similar coherent neutron scattering lengths. Here, we show that a multinuclear solid-state Nuclear Magnetic Resonance (NMR) approach provides compelling insight into the Ga/Ge cation distribution and oxygen anion transport in a family of solid electrolytes with langasite structure and LaGaGeO composition. Ultrahigh field Ga Magic Angle Spinning (MAS) NMR experiments acquired at 35.2 T offer striking resolution enhancement, thereby enabling clear detection of Ga sites in different coordination environments. Three-connected GaO, four-connected GaO and GaO polyhedra are probed for the parent LaGaGeO structure, while one additional spectral feature corresponding to the key (Ga,Ge)O structural unit which forms to accommodate the interstitial oxide ions is detected for the Ge-doped LaGaGeO phase. The complex spectral line shapes observed in the MAS NMR spectra are reproduced very accurately by the NMR parameters computed for a symmetry-adapted configurational ensemble that comprehensively models site disorder. This approach further reveals a Ga/Ge distribution across all Ga/Ge sites that is controlled by a kinetically governed cation diffusion process. Variable temperature O MAS NMR experiments up to 700 °C importantly indicate that the presence of interstitial oxide ions triggers chemical exchange between all oxygen sites, thereby enabling atomic-scale understanding of the anion diffusion mechanism underpinning the transport properties of these materials.