The title materials belong to a new class of Li storage materials with high Li ion exchange capacities. They are ismorphous, having an orthorhombic structure with the space group Pmn21 and the unit cell parameters approximately a = 6.27, b = 5.33 and c = 5.01 Å. This can be recognized as an ordered superstructure of a wurtzite protoype, where all cations occupy tetrahedral sites in a heaxgonal closest packing of oxygen anions.
No matter of which syntehsis procedure is used (variations of the Pechini sol-gel method and hydrothermal method were tried) the products are of low crystallinity, having broad peaks in the diffraction patterns [1]. This can be signifinicantly improved by high-temperature-high-pressure (HP-HT) treatment (performed on the Fe analogue), which gives a material with very sharp diffraction peaks. However, the intensities of the diffraction peaks of the as-made and and the HP-HT treated samples differ significantly and the subject of this contribution is to discuss this phenomenon.
Rietveld refinement, using various models on both patterns, showed that the difference is due to a disorder of cations in the as-made sample. While the HP-HT treated material is crystallographically well ordered, the scattering power, located in various interstitial sites of the oxygen packing, lead to the conclusion that two types of disorder are present in the as-made samples: a) exchange of Li and Mn over their primary sites and b) partial migration of Li, Mn and Si to alternate tetrahedral sites in the distorted HCP of oxygen atoms, just over the basal plane of the tetrahedron to the vicinal tetrahedral position, which is empty in the ordered model. The octahedral interstices are empty in all cases – no scattering power was located there.
These structural features play an important role in the electrochemical performance of the title materials. As the octahedral interstices in the HCP are face-sharing, they form channels through which the lithium ions can migrate to and from their tetrahedral sites during the electrochemical discharge-charge process. The presence of the cation disorder also facilitates the extraction-insertion of lithium ions over the empty sites and stabilizes the lithium-deficient state of the material, occurring when the battery is charged.

[1] R. Dominko, M. Bele, M. Gaberšček,  M. Remškar, A. Meden, J. Jamnik, Electrochem. commun., 8 (2006)  217-222.

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