Cation Distribution and Anion Transport in the La 3 Ga 5- x Ge 1+ x O 14+0.5 x 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 La3 Ga5- x Ge1+ x O14+0.5 x composition. Ultrahigh field 71 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 GaO4 , four-connected GaO4 and GaO6 polyhedra are probed for the parent La3 Ga5 GeO14 structure, while one additional spectral feature corresponding to the key (Ga,Ge)2 O8 structural unit which forms to accommodate the interstitial oxide ions is detected for the Ge4+ -doped La3 Ga3.5 Ge2.5 O14.75 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 17 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.