Structural and magnetic properties of LiNi 0.5 Mn 1.5 O 4 and LiNi 0.5 Mn 1.5 O 4−δ are investigated using densityfunctional theory calculations.Results indicate that nonstoichiometric LiNi 0.5 Mn 1.5 O 4−δ and stoichiometric LiNi 0.5 Mn 1.5 O 4 exhibit two different structures, i.e., the face-centred cubic (Fd-3m) and primitive, or simple, cubic (P 4 3 32) space groups, respectively. It is found that the magnetic ground state of LiNi 0.5 Mn 1.5 O 4 (P 4 3 32 and Fd-3m) is a ferrimagnetic state in which the Ni and Mn sublattices are ferromagnetically ordered along the [110] direction whereas they are antiferromagnetic with respect to each other. We demonstrate that it is the presence of an O-vacancy in LiNi 0.5 Mn 1.5 O 4−δ with the Fd-3m space group that results in its superior electronic conductivity compared with LiNi 0.5 Mn 1.5 O 4 with the P 4 3 32 space group.
Lattice dynamical properties of LiFePO4 were studied using first principles density functional theory taking into account the on-site Coulomb interaction within the GGA+U scheme. The Born effective charge tensors, phonon frequencies at the Brillouin zone center and phonon dispersion curves were calculated and analyzed. The Born effective charge tensors exhibit anisotropy, which gives an indirect evidence for the one-dimensional Li migration tunnel along the [010]direction in LiFePO4, which has been proposed by other theoretical calculations and experimental observations. The calculated phonon frequencies at the Г point of the Brillouin zone agree well with the available experimental results.
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