X-ray powder diffraction study of cation distribution and the Fd3 m→ P4 132 symmetry reduction in Li 0.5Fe 2.5O 4/LiMn 2O 4 spinel solid solutions

Abstract

Structural changes in solid solutions formed between the cubic normal spinel, LiMn 2O 4, and ordered inverse spinel, Li 0.5Fe 2.5O 4, have been investigated 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 constant from 8.21Å to 8.34Å for the Fe/(Fe+Mn) mole ratio of 0 and 1, respectively. A distinct departure from Vegard’s law appears for Fe/(Fe+Mn)≥0.6. The additional ‘superstructure’ reflections on the X-ray patterns confirm an ordering of lithium ions in octahedral sites, bringing about the lowering of Fd3 m symmetry to the P4 132/ P4 332 space group. Li + ions are distributed over both octahedral and tetrahedral cationic positions of spinel lattice, although the end-members of the solid solution series, LiMn 2O 4 and Li 0.5Fe 2.5O 4, contain Li + coordinated tetrahedrally and octahedrally, respectively. The ordering of Li + ions in the 4 b Wyckoff’s positions of a cubic primitive unit cell ( P4 132/ P4 332), observed for the increasing Fe 3+ content, has been confirmed for that system with infrared spectroscopy. The number of infrared active vibrations increases significantly with the lowering of crystal symmetry caused by the 1:3 ordering of Li + in octahedral sites.