Experimental evidence for the coupling of Li-motion and structural distortions in LiMnPO$_4$

TL;DR

This study uses NMR and x-ray scattering to investigate how lithium motion in LiMnPO₄ couples with structural distortions, revealing disorder effects that impact lithium mobility and battery performance.

Contribution

It provides the first detailed experimental evidence linking Li diffusion to structural distortions and Mn-disorder in LiMnPO₄, combining NMR and diffuse x-ray scattering data.

Findings

Li-diffusion couples with structural distortions.

Mn-disorder affects hyperfine coupling measurements.

Structural distortions influence Li-mobility and battery performance.

Abstract

We present a detailed $^7$Li- and $^{31}$P-NMR study on single crystalline LiMnPO$_4$ in the paramagnetic and antiferromagnetic phase (AFM, $T_N \sim$ 34 K). This allows us to determine the spin directions in the field-induced spin-flop phase. In addition, the anisotropic dipolar hyperfine coupling tensor of the $^7$Li- and $^{31}$P-nuclei is also fully determined by orientation and temperature dependent NMR experiments and compared to the calculated values from crystal structure data. Deviations of the experimental values from the theoretical ones are discussed in terms of Mn disorder which is induced by Li-disorder. In fact, the disorder in the Mn-sublattice is directly revealed by our diffuse x-ray scattering data. The present results provide experimental evidence for the Li-diffusion strongly coupling to structural distortions within the MnPO$_4$ host, which is expected to significantly affect the Li-mobility as well as the performance of batteries based on this material.