Local Structure in Disordered Melilite Revealed by Ultrahigh Field ⁷¹Ga and ¹³⁹La Solid‐State Nuclear Magnetic Resonance Spectroscopy

Abstract

Multinuclear Nuclear Magnetic Resonance (NMR) spectroscopy of quadrupolar nuclei at ultrahigh magnetic field provides compelling insight into the short‐range structure in a family of fast oxide ion electrolytes with La1+xSr1–xGa3O7+0.5x melilite structure. The striking resolution enhancement in the solid‐state 71Ga NMR spectra measured with the world’s unique series connected hybrid magnet operating at 35.2 T distinctly resolves Ga sites in four‐ and five‐fold coordination environments. Detection of five‐coordinate Ga centers in the site‐disordered La1.54Sr0.46Ga3O7.27 melilite is critical given that the GaO5 unit accommodates interstitial oxide ions and provides excellent transport properties. This work highlights the importance of ultrahigh magnetic fields for the detection of otherwise broad spectral features in systems containing quadrupolar nuclei and the potential of ensemble‐based computational approaches for the interpretation of NMR data acquired for site‐disordered materials.

Cation Distribution and Anion Transport in the La3Ga5–xGe1+xO14+0.5x Langasite Structure

“…Ultrahigh field NMR spectroscopy is particularly critical for the detection of half-integer quadrupolar nuclei such as 71 Ga because strong quadrupolar interactions result in a fourth-rank second-order quadrupolar broadening of the NMR resonances (in Hz) that remains even under MAS but is inversely proportional to the external magnetic field strength B 0 . While the 71 Ga MAS NMR spectra at 23.5 T are dominated by broad, overlapped resonances and show limited gain in resolution with respect to data acquired at 20 T under ν r = 65 kHz, further increasing the external magnetic field strength by 50% considerably enhances the spectral resolution, thereby enabling the detection of several distinct 71 Ga resonances at 35.2 T for both La 3 Ga 5 GeO 14 and La 3 Ga 3.5 Ge 2.5 O 14.75 , as previously observed for the related La 1+ x Sr 1– x Ga 3 O 7+0.5 x melilite family of fast oxide ion conductors …”