Physical properties, strontium ordering and structural modulation in layered hexagonal Sr0.35CoO2

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Layered Sr0.35CoO2 has been synthesized by means of an ion exchange reaction from Na0.7CoO2. Resistivity measurements show that this material can be either metallic or semiconducting depending on the annealing conditions. Magnetic susceptibility, following the Curie-Weiss law, increases with lowering temperature and shows a small anomalous kink at around 30 K. Transmission-electron-microscopy observations reveal the presence of two superstructures arising respectively from the intercalated Sr-ordering (a compositional modulation) with q1 = a* / 3 + b* / 3 and a periodic structural distortion (a transverse structural modulation) with q2 = a* / 2.