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+(d(7)) ions with low-spin (t(2g)(6), e(g)(1)) 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 (t(2g)) and 2.8-3.0 eV (eg) structures and O2- (2p) -> Mn3+ (3d) for 2.3 (t(2g)) 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. (c) 2006 Elsevier Ltd. All rights reserved.