Layer-type oxide NaxMx/2IITi1-x/2IVO2 (M = Co, Ni; x = 0.60 to 1.0) was prepared by solid-state reaction. In both series, two structural variants of type α-NaFeO2 (O3) and β-RbScO2 (P2) were obtained consecutively as x decreased with a borderline composition around xc~0.7. With the decrease of x, the ionic conductivity was found to increase up to 8.4 x 10-2S/cm at 770K (Na0.67Co0.33Ti0.67O2). Compositions of P2 were found to exhibit the conductivity values two to five times greater than those of O3, primarily due to the larger rectangular threshold available for the diffusion of Na+ ions. Such a structural effect had also been considered to depend upon the polarizability of alkali ion. HT-XRD and 23Na-NMR data of Na0.67Co0.33Ti0.67O2 strongly suggested that the diffusion of Na+ ion was deeply related with the local distortion of trigonal prismatic sites, leading to the change of activation energy around 430K.

Ionic Conduction Properties of Layer-Type Oxides NaxMx/2IITi1-x/2IVO2 (M = Ni, Co; 0.60 < x < 1.0). Y.J.Shin, M.H.Park, J.H.Kwak, H.Namgoong, O.H.Han: Solid State Ionics, 2002, 150[3-4], 363-72