Disorder-order transitions in Na_xCoO₂ (x∼0.58)

First-order structural transitions have been discovered for a γ -Nax CoO2 (x∼0.58) sample at T1 =235 K and T2 =288 K. Based on the combined results of electron and neutron-diffraction experiments, the compound has been revealed to exhibit a 7 a0 ×7 a0 -type (a0 represents the a -axis length of the hexagonal basic structure) ordered superstructure with the space group of P 63 /m, being isostructural with K4 Co7 O14, 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 T1 and T2 can be explained by the slight distortion of the CoO2 layer. Above T2, in contrast, the compound turns out to possess the γ -Nax CoO2 -type structure, in which the Na1 and Na2 atoms are randomly distributed.