X-ray powder diffraction study of cation distribution and the Fd3m -> P4(1)32 symmetry reduction in Li0.5Fe2.5O4/LiMn2O4 spinel solid solutions

Structural changes in solid solutions formed between the cubic normal spine l, LiMn2O4, and ordered inverse spinel, Li0.5Fe2.5O4, have been investigate d by X-ray powder diffraction and infrared spectroscopic techniques. By refining the X-ray diffraction patterns with Rietveld profile analysis, the lattice parameters, interatomic distances and cation distribution were determined. The Fe-for-Mn substitution increases the spinel unit-cell const ant from 8.21 Angstrom to 8.34 Angstrom for the Fe/(Fe+Mn) mole ratio of 0 and 1, respectively. A distinct departure from Vegard's law appears for Fe/ (Fe+Mn)greater than or equal to 0.6. The additional 'superstructure' reflec tions on the X-ray patterns confirm an ordering of lithium ions in octahedr al sites, bringing about the lowering of Fd3m symmetry to the P4(1)32/P4(3) 32 space group. Li+ ions are distributed over both octahedral and tetrahedr al cationic positions of spinel lattice, although the end-members of the so lid solution series, LiMn2O4 and Li0.5Fe2.5O4, contain Li+ coordinated tetr ahedrally and octahedrally, respectively. The ordering of Li+ ions in the 4b Wyckoff's positions of a cubic primitive unit cell (P4(1)32/P4(3)32), observed for the increasing Fe3+ content, has been confirmed for that system with infrared spectroscopy. The number of i nfrared active vibrations increases significantly with the lowering of crys tal symmetry caused by the 1:3 ordering of Li+ in octahedral sites. (C) 199 9 Elsevier Science S.A. All rights reserved.