Effects of nickel doping on structural and optical properties of spinel lithium manganate thin films

Spinel LiNixMn2−xO4 ( x ≤ 0.9 ) thin films were synthesized by a sol–gel method employing spin-coating. The Ni-doped films were found to maintain cubic structure at low x but to exhibit a phase transition to tetragonal structure for x ≥ 0.6 . Such cubic–tetragonal phase transition can be explained in terms of Ni3+(d7) ions with low-spin (t2g6,eg1) configuration occupying the octahedral sites of the compound, thus being subject to the Jahn–Teller effect. By X-ray photoelectron spectroscopy both Ni3+ and Ni2+ ions were detected where Ni2+ is more populated than Ni3+. Optical properties of the LiNixMn2−xO4 films were investigated by spectroscopic ellipsometry in the visible–ultraviolet range. The measured dielectric function spectra mainly consist of broad absorption structures attributed to charge-transfer transitions, O2−(2p)→Mn4+(3d) for 1.9 (t2g) and 2.8–3.0 eV (eg) structures and O2−(2p)→Mn3+(3d) for 2.3 (t2g) and 3.4–3.6 eV (eg) structures. Also, sharp absorption structures were observed at about 1.6, 1.7, and 1.9 eV, interpreted as being due to d–d crystal-field transitions within the octahedral Mn3+ ion. In terms of these transitions, the evolution of the optical absorption spectrum of LiMn2O4 by Ni doping could be explained and the related electronic structure parameters were obtained.