Disorder-order transitions in NaxCoO2 (x∼0.58)

First-order structural transitions have been discovered for a gamma-NaxCoO(2) (x similar to 0.58) sample at T-1=235 K and T-2=288 K. Based on the combined results of electron and neutron-diffraction experiments, the compound has been revealed to exhibit a root 7a(0) x root 7a(0)-type (a(0) represents the a-axis length of the hexagonal basic structure) ordered superstructure with the space group of P6(3)/m, being isostructural with K4Co7O14, due to the ordering of Na atoms in this temperature range. The Na ordering triggers a cooperative shift of Co-atom trimers parallel to the c axis, leading to an undulated arrangement of the CoO2 conduction paths. Anomalous change in the magnetic susceptibility, electrical resistivity, and lattice parameters at T-1 and T-2 can be explained by the slight distortion of the CoO2 layer. Above T-2, in contrast, the compound turns out to possess the gamma-NaxCoO2-type structure, in which the Na1 and Na2 atoms are randomly distributed.