Small polaron transport in Li7MnN4 containing isolated MnN4 tetrahedra

The electrical transport properties related to the Mn 3d-band in Li7MnN4, which has been considered as an electrode material of Li ion secondary battery, are studied by combining with the optical absorption and electron paramagnetic resonance (EPR). The optical absorption studies show that the empty Mn 3d-band attributed to isolated MnN4 tetrahedra in Li7MnN4 lies at about 1.15 eV above the top of the valence band. An EPR signal with g -value = 1.9848 is observed at 77 K, suggesting the presence of Mn5+ in the isolated MnN4 tetrahedra like that in Li3MnO4. The temperature dependence of electrical resistivity shows the hopping conduction with the activation energy of 112 meV at higher temperature above 180 K and the tunneling conduction at lower temperature below 60 K, suggesting the small polaron motion in the Mn-3d band as predicted by Holstein.