Low-temperature thermoelectric and magnetic characteristics of Ca_2.9Bi_0.1Co_4−xFe_xO_9+δ (0 ≤ x ≤ 0.10)

The effect of Fe ion doping on the low-temperature thermoelectric and magnetic properties of Ca_2.9Bi_0.1Co_4−xFe_xO_9+δ (x = 0.00, 0.025, 0.05 and 0.10) have been investigated. The samples were prepared by conventional solid-state synthesis. The X-ray diffraction patterns revealed that all the samples are single phase. The electrical resistivity results indicated that all the samples obey the variable range hopping in the low temperature regime. The T^* (transition temperature from Fermi liquid metal to incoherent metal) was increased and the slope of A value (Fermi-liquid transport coefficient) was decreased with increasing Fe content. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity, thermopower and total thermal conductivity were decreased with increasing Fe content. Among the doped samples, Ca_2.95Bi_0.10Co_3.90Fe_0.10O_9+δ had the highest dimensionless figure of merit of 0.056 at 300 K. Magnetic measurements indicated that all the samples exhibit a low-spin state of cobalt ion. The effective magnetic moments were decreased with increasing Fe content.