Polysynthetic Twinning Characterization and Crystallographic Refinement in NaBa2M2+2M3+O6 (M=Ni, Cu)

Crystals of NaBa2M2+2M3+O6 (M = Ni, Cu) were prepared by both electrosynthesis in molten NaOH and controlled cooling in a NaOH flux. They crystallize in an F-centered orthorhombic unit cell, space group Fmmm, and are formed from the stacking along c of compact [NaM3O6]4- blocks separated by Barium cations. It appears that, due to a perfect coincidence between the (a, a , c) unit cell constants and the related primitive hexagonal lattice (a, a, c, GAMMA=120°), a recurrent twinning characterized by a three fold axis from one block to the subsequent one occurs in the crystals studied. Thus, the coexistence of three independent domains yields a misleading C-centered orthorhombic false symmetry. After the stacking fault detection from electron diffraction and HREM pictures a special attention was given to introduce the observed phenomenon in the X-ray data refinement process, leading to the volume ratio of three independent domains and to good reliability factors. We also investigated the composition plane located at the barium layers level. It was shown that the barium environment at the twin interface adopts a viewable oxygen coordination in agreement with the full coherence of domains boundaries observed in HREM pictures. Therefore, valence bond calculations seem to indicate a D1–D1 interface more stable than that of D1–D2 and D1–D3, comfirming the large size of superimposed monodomains. The compounds progressively decompose from 300°C, leading to a mixture of NiO, BaCO3, and BaNiO3. The magnetic susceptibility measured for NaBa2Ni3O6 follows a Curie–Weiss law that involves high-spin Ni2+ and Ni3+ cations.