Magnetic and Metal–Insulator Transitions in β-Na_0.5CoO₂ and γ-K_0.5CoO₂ —NMR and Neutron Diffraction Studies—

Abstract

Co-oxides β-Na_0.5CoO₂ and γ-K_0.5CoO₂ have been prepared by the Na de-intercalation from α-NaCoO₂ and by the floating-zone method, respectively. It has been found that successive phase transitions take place at temperatures T_c1 and T_c2 in both systems. The appearance of the internal magnetic field at T_c1 with decreasing temperature T indicates that the antiferromagnetic order exists at T≤T_c1, as in γ-Na_0.5CoO₂. For β-Na_0.5CoO₂, the transition temperatures and the NMR parameters determined from the data taken for magnetically ordered state are similar to those of γ-Na_0.5CoO₂, indicating that the difference of the stacking ways of the CoO₂ layers between these systems do not significantly affect their physical properties. For γ-K_0.5CoO₂, the quantitative difference of the physical quantities are found from those of β- and γ-Na_0.5CoO₂. The difference between the values of T_ci (i=1 and 2) of these systems might be explained by considering the distance between CoO₂ layers.