Effects of partial substitution of Fe for Co on the low-temperature thermoelectric and magnetic properties of Ca2.7Bi0.3Co4-xFexO9+δ (0 ≤ x ≤ 0.15)

Polycrystalline samples of Ca2.7Bi0.3Co4-xFexO9+δ (x = 0.00, 0.025, 0.05, 0.10 and 0.15) have been prepared by conventional solid-state synthesis and their thermoelectric and magnetic properties measured. 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 thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity and thermopower were increased with increasing Fe content. Among the samples, Ca2.95Bi0.10Co3.95Fe0.05O9+δ had the highest dimensionless figure of merit of 0.102 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.