Nature of the stacking faults in orthorhombic LiMnO2

Abstract

The synthesis of orthorhombic LiMnO ₂ (O-LiMnO ₂ ) with very small crystals (diameter≈0.3 µm) leads to peculiar X-ray diffraction patterns. Some reflections (with k even) remain thin allowing for cell parameter refinements, showing that, compared to phases with bigger crystals, b and c remain unchanged, whereas an important increase of the a parameter is observed. Other reflections (with k=2n+1 and h≠0) are widened substantially, while the remnant peaks (k=2n+1 and h=0) undergo a strong asymmetrization. These features have been related successfully to faults corresponding to a b /2 translation of a basic unit constituting O-LiMnO ₂ . A simulation made with the Diffax program allowed good reproduction of the experimental X-ray diffraction data, showing a statistic distribution of the faults, at least for the low fault concentrations corresponding to the samples under study. The insertion of the fault corresponds to the insertion of a monoclinic cell between two blocks of orthorhombic symmetry. This cell (a≈5.53 Å, b≈2.80 Å, c≈5.30 Å) corresponds to a newly obtained monoclinic LiMnO ₂ phase obtained through a topotactic deintercalation of α-NaMnO ₂ . The fault percentage of the compounds studied goes from 1 to 6% and is well correlated to the substitution ratio between lithium and manganese when the fault occurrence is treated as a cationic disorder (only in the case of small disorder for which the lines remain treatable with the Rietveld refinement program). The fault percentage can also be determined easily from the cell parameter relation a=x ₀ a ₀ +x _m a _m sinγ, where a ₀ and a _m are the parameters of the orthorhombic and monoclinic cell of the pure phases and x ₀ and x _m their relative fractions, a being the parameter of the faulted phase as refined from the fault-unaffected thin reflection peaks.