Crystals of NaBa2M2+2M3+O6 (M = Ni, Cu) were prepared by electrosynthesis in molten NaOH or controlled cooling in a NaOH flux. They crystallized in an F-centered orthorhombic unit cell (Fmmm) and formed by the stacking, along c, of compact [NaM3O6]4- blocks; separated by Ba cations. It appeared that, due to a perfect coincidence between the (a, a√3, c) unit-cell constants and the related primitive hexagonal lattice (a, a, c, γ =120°), a recurrent twinning characterized by a 3-fold axis from one block to the subsequent one occurred in the crystals. The coexistence of 3 independent domains yielded a misleading C-centered false orthorhombic symmetry. It was shown that the Ba environment at the twin interface adopted a visible O coordination; in agreement with the full coherence of domain boundaries observed in high-resolution electron microscopy.

Polysynthetic Twinning Characterization and Crystallographic Refinement in NaBa2M22+M3+O6 (M = Ni, Cu). E.Quarez, M.Huve, P.Roussel, O.Mentré: Journal of Solid State Chemistry, 2002, 165[2], 214-27