Lattice and local-mode vibrations in anhydrous and protonized LiMn2O4 spinels from first-principles theory

Abstract

The phonon dispersion relations and density of states of anhydrous LiMn₂O₄ spinels have been studied using first-principles theory. Above the acoustic phonon branches, the heavy Mn ions and the light Li ions separately contribute primarily to the optic phonons of low (∼180 to 280 cm⁻¹) and high (>400 cm⁻¹) energies, respectively, whereas the O ions partake in lattice vibrations over a wide range of frequencies (180–644 cm⁻¹). The measured phonon density of states of a LiMn₂O₄ powder corroborates the calculated results. Symmetry analysis of the calculated Brillouin-zone center modes allows a detailed comparison with the Raman- and infrared-active modes reported in the literature. First-principles calculations were also performed on protonized LiMn₂O₄ spinels. Each proton on the (001) surface of (Li,H)_xMn₂O₄ spinels, expected near an ‘8a’ site, is bonded to one oxygen neighbor to form one (–OH) unit, similar to those in the bulk. Local structural relaxation leads to shifting of the surface proton inwards to the subsurface Mn–O layer. The vibrational frequencies of the protons are almost constant, other than a slight dependence on the Li/H concentration, regardless of the protons being in the bulk or on the surface.