Cation Distribution and Anion Transport in the La₃Ga_5–xGe_1+xO_14+0.5x Langasite Structure

Abstract

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 3+ /Ge 4+ cation distribution and oxygen anion transport in a family of solid electrolytes with langasite structure and La 3 Ga 5−x Ge 1+x O 14+0.5x 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 GaO 4 , four-connected GaO 4 and GaO 6 polyhedra are probed for the parent La 3 Ga 5 GeO 14 structure, while one additional spectral feature corresponding to the key (Ga,Ge) 2 O 8 structural unit which forms to accommodate the interstitial oxide ions is detected for the Ge 4+ -doped La 3 Ga 3.5 Ge 2.5 O 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 Ga 3+ /Ge 4+ 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.