Interrelationship between Li+diffusion, charge, and magnetism inLi7Mn2O4andLi71.1Mn1.9O4spinels: Elastic, inelastic, and quasielastic neutron scattering

Abstract

Using quasielastic neutron scattering (QENS), we have investigated a self-diffusive behavior of Li${}^{+}$ ions for both $^{7}\mathrm{Li}$Mn${}_{2}$O${}_{4}$ and $^{7}\mathrm{Li}$${}_{1.1}$Mn${}_{1.9}$O${}_{4}$ spinels. In addition, we have carried out elastic and inelastic neutron scattering measurements using the same samples, to study the interrelationship between Li${}^{+}$ self-diffusion, magnetism, and charge distribution in the lattice. From the QENS results, the self-diffusion of Li${}^{+}$ was observable above 280 K, and a self-diffusion coefficient (${D}_{s}^{\mathrm{Li}}$) for $^{7}\mathrm{Li}$Mn${}_{2}$O${}_{4}$ was estimated as $~{10}^{\ensuremath{-}8}$ cm${}^{2}$/s at 400 K. ${D}_{s}^{\mathrm{Li}}$ for $^{7}\mathrm{Li}$${}_{1.1}$Mn${}_{1.9}$O${}_{4}$ was comparable to that for $^{7}\mathrm{Li}$Mn${}_{2}$O${}_{4}$. Furthermore, combining with the results of elastic and inelastic measurements, it was found that $^{7}\mathrm{Li}$${}_{1.1}$Mn${}_{1.9}$O${}_{4}$ undergoes a transition from a low-temperature ($T$) short-range charge-ordered (SRCO) phase to a high-$T$ charge-disordered (CDO) phase at 280 K. The structure of the SRCO was determined as a hexagon, because the formation of hexagon spin clusters was deduced from a magnetic diffuse scattering at low $T$. Assuming the presence of the SRCO-CDO transition at 280 K, both the anomaly of the diffusive behavior at 280 K and the local lattice distortion below 280 K are reasonably explained, despite the absence of long-range CO for $^{7}\mathrm{Li}$${}_{1.1}$Mn${}_{1.9}$O${}_{4}$.