Cation Distribution and Anion Transport in the La₃Ga_5-Ge₁₊O_14+0.5 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 Ga³⁺/Ge⁴⁺ cation distribution and oxygen anion transport in a family of solid electrolytes with langasite structure and La₃Ga_5-Ge₁₊O_14+0.5 composition. Ultrahigh field ⁷¹Ga 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 GaO₄, four-connected GaO₄ and GaO₆ polyhedra are probed for the parent La₃Ga₅GeO₁₄ structure, while one additional spectral feature corresponding to the key (Ga,Ge)₂O₈ structural unit which forms to accommodate the interstitial oxide ions is detected for the Ge⁴⁺-doped La₃Ga_3.5Ge_2.5O_14.75 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 Ga³⁺/Ge⁴⁺ distribution across all Ga/Ge sites that is controlled by a kinetically governed cation diffusion process. Variable temperature ¹⁷O 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.