Transferred hyperfine interaction and structure inLiMn2O4andLi2MnO3coexisting phases:mA XRD andLi7NMR-MAS study

Abstract

X-ray diffraction (XRD) and ${}_{}{}^7{}_{}{}^{}\mathrm{MAS}$ measurements have been performed on ${\mathrm{LiMn}}_2$ ${\mathrm{O}}_4$ (lithium cationic fraction, x=0.333), ${\mathrm{Li}}_2$ ${\mathrm{MnO}}_3$ (x=0.667), and the intermediate composition with x=0.40. The use of magic angle spinning produces complex manifolds of the spinning sidebands. The predominant interaction affecting the NMR spectrum is the transferred hyperfine coupling (TFHI) between the Mn ions and the Li spins. TFHI parameters are extracted from MAS-NMR spectra of oxides containing a large quantity of manganese. We obtained a full spectral assignment for the spinel ${\mathrm{LiMn}}_2$ ${\mathrm{O}}_4$, for the rocksalt ${\mathrm{Li}}_2$ ${\mathrm{MnO}}_3$, and for the x=0.40 sample, which contains the rocksalt and both stoichiometric and nonstoichiometric spinel phases. According to XRD data, the rocksalt phase contains three nonequivalent lithium sites whose NMR peaks are shifted upfield from x=0.667 to 0.40 because of the cell volume expansion.mThe ${\mathrm{Li}}^{+}$ ions belonging to the two spinel phases of x=0.40 are quite mobile and give origin by motional narrowing to a single peak, which is slightly shifted upfield with respect to that of ${\mathrm{LiMn}}_2$ ${\mathrm{O}}_4$.