High-temperature phase transition in the three-layered sodium cobaltite P′3-NaxCoO2 (x∼0.62)

The high temperature phase transition in the three-layered P'3-NaxCoO(2) (x similar to 0.62) has been investigated by means of heat capacity measurement, x-ray diffraction, and Na-23 magic angle spinning (MAS)-NMR spectroscopy in the 300-550 K range. The phase transition occurs nearby T-S=350 K. Below T-S, the unit cell is monoclinic (space group C2/m). Above TS, the monoclinic cell is reversibly converted into a rhombohedral cell (space group R3m). The crystallographic change mainly manifests into Na rearrangement in the interslab from a low symmetry position to a higher symmetry position. A global picture for both systems of the (x,y,z) off-center position of Na could be understood as a balance between on-site Na+-Co3+/4+ electrostatic repulsions (z shift) and in-plane Na+-Na+ electrostatic repulsions (xy shift). We suggest that Na+ interlayer redistribution is the driving force of the phase transition. Na-23 MAS-NMR spectroscopy has been used to investigate changes in the environment and in the distribution of the sodium cations occurring by raising the temperature. The gradual suppression of the second-order quadrupolar, interactions and the resulting new resonance is consistent with the sodium site exchange mechanism. Changes in the resistivity at TS suggest a strong coupling between the Na+ and CoO2 layers.