Abstract
Rietveld refinements were performed on synchrotron X-ray powder diffraction patterns of Li 1.05 Mn 2 O 4− δ , Ni- and Ti- doped Li 1.05 Mn 2 O 4− δ to investigate the structural changes in conjunction with the non-adiabatic small polaron-hopping behavior. Samples with and without substitution were confirmed to belong to the Fd 3 ¯ m space group of the cubic system. The lattice parameters a and the oxygen positional parameters u decreased with increasing Ni content, and increased with increasing Ti content. In Ni-doped samples, M−O bond lengths (M: transition metal at the 16 d site) were shorter and O-M-O angles were larger than those of Li 1.05 Mn 2 O 4− δ . The local distortion in MO 6 octahedra decreased as a result of Ni substitution. The less distorted MO 6 octahedra enhanced the overlap of the Mn 3 d and O 2 p wave functions. This may explain why the polaron-hopping energies W H of Li 1.05 Mn 2− x Ni x O 4− δ ( x = 0.2 and 0.3) are smaller than that of Li 1.05 Mn 2 O 4− δ . Ti substitution kept the increase of M−O bond length small. The O-M-O angle decreased slightly compared to that of Li 1.05 Mn 2 O 4− δ , but barely changed with increasing Ti content. Structural changes in MO 6 octahedra by Ti substitution were too small to affect the overlap of the Mn 3 d and O 2 p wave functions. This explains why the hopping energies W H of Li 1.05 Mn 2− x Ti x O 4− δ remained unchanged. Ni and Ti have different ionic radii from Mn. Thus, substitution of Ni and Ti modify the lattice distortion, and consequently induced a gradual increase in the self-trapped polaron’s energy, i.e., the activation energy of thermopower E S . MO 6 (M: transition metal) octahedral distortion, O-M-O angle, was connected with the polaron-hopping energy W H . The figures on the left are (a) O-M-O angle, and (b) activation energies E σ and E S , and hopping energy W H vs. Ni content for Li 1.05 Mn 2− x Ni x O 4− δ , and the figures on the right are (c) O-M-O angle, and (d) activation energies E σ and E S , and hopping energy W H vs. Ti content for Li 1.05 Mn 2− x Ti x O 4− δ . Circles correspond to undoped Li 1.05 Mn 2 O 4− δ . • Structural changes of Ni- and Ti- doped lithium manganese spinels were investigated. • Ni substitution decreased MO 6 (M: transition metal) octahedral distortion because of the decrease of Mn 3+ . • The increase of the distortion by Ti substitution was too small to change the environment around M. • MO 6 octahedral distortion, O-M-O angle, affected small-polaron hopping conduction.
Published Version
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