NATURE OF THE STACKING-FAULTS IN ORTHORHOMBIC LIMNO2

The synthesis of orthorhombic LiMnO2 (O-LiMnO2) with very small crysta ls (diameter approximate to 0.3 mu m) leads to peculiar X-ray diffract ion patterns. Some reflections (with k even) remain thin allowing for cell parameter refinements, showing that, compared to phases with bigg er crystals, b and c remain unchanged, whereas an important increase o f the a parameter is observed. Other reflections (with k = 2n + 1 and h not equal 0) are widened substantially, while the remnant peaks (k = 2n + 1 and h = 0) undergo a strong asymmetrization. These features ha ve been related successfully to faults corresponding to a b/2 translat ion of a basic unit constituting O-LiMnO2. A simulation made with the Diffax program allowed good reproduction of the experimental X-ray dif fraction data, showing a statistic distribution of the faults, at leas t for the low fault concentrations corresponding to the samples under study. The insertion of the fault corresponds to the insertion of a mo noclinic cell between two blocks of orthorhombic symmetry. This cell ( a approximate to 5.53 Angstrom, b approximate to 2.80 Angstrom, c appr oximate to 5.30 Angstrom) corresponds to a newly obtained monoclinic L iMnO2 phase obtained through a topotactic deintercalation of alpha-NaM nO2. The fault percentage of the compounds studied goes from 1 to 6% a nd is well correlated to the substitution ratio between lithium and ma nganese when the fault occurrence is treated as a cationic disorder (o nly 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)n(0) + x (m)a(m)sin gamma, where a(0) and a(m) are the parameters of the orthor hombic 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 refi ned from the fault-unaffected thin reflection peaks.