Electrical transport behavior of Ca3MnxCo4-xO9 (0≤x≤1.28) at low temperatures

Electrical transport behavior of Ca3MnxCo4-xO9 (0 <= x <= 1.28), prepared by sol-gel process, was investigated at temperatures from 300 K down to 5 K. The results indicate that dc conductivity sigma and carrier concentration decay exponentially with decreasing temperature below similar to 75 K and similar to 100 K for Ca3MnxCo4-xO9 with x=0 and 0.03, respectively. The increase of resistivity (T less than or similar to 100 K) of Ca3Mn0.03Co3.97O9 with decreasing temperature originates mainly from reduction in its carrier concentration due to weakening of thermal activation. Nevertheless, the temperature behavior of resistivity at T less than or similar to 75 K for Ca3Co4O9 is mainly governed by two factors: (1) reduction in its carrier concentration; and (2) rise of carrier mobility resulting presumably from reduced optical phonon scattering. In contrast, the temperature dependence of the resistivity for the heavily substituted Ca3MnxCo4-xO9 (x=0.57,0.9,1.28) displays semiconductor-like behavior in the whole temperature range investigated, but does not obey an exponential law. Instead, a relationship ln sigma proportional to-(T-0/T)(1/3) is found to hold for these oxides, in agreement with Mott's two-dimensional variable range hopping model, which suggests that Ca3Co4O9 is a quasi-two-dimensional transport system, and the hopping conduction comes from the disorder introduced by the substitution of Mn for Co in the conduction plane.