Dielectric constant and ac conductivity of the layered cobalt oxide Bi₂Sr₂CoO_6+δ: A possible metal-dielectric composite made by self-organization of Co²⁺ and Co³⁺ ions

The dielectric constant and ac conductivity of a single-crystal sample of the layered cobalt oxide Bi₂Sr₂CoO_6+δ were measured along the c axis from 10² to 10⁹ Hz from 110 to 300 K, the frequency dependence of which is found to be similar to that for metal-dielectric composites. The ac conductivity obeys the same scaling relation as that of disordered solids, which is quantitatively consistent with a theoretical calculation for spatially inhomogeneous conductive medium. This implies that the charge carrier on the Co²⁺ site is electronically segregated in the CoO₂ plane. This compound can work as a metal-dielectric composite even in a form of single crystal, a possible mechanism of which is suggested on the basis of the electronic configuration of Co ions.