Enhanced thermoelectric properties induced by chemical pressure in Ca3Co4O9

We report the investigation of boron substitution on structural, electrical, thermal, and thermoelectric properties of Ca3−xBxCo4O9 (x=0, 0.5, 0.75, and 1) in the temperature range between 300 K and 5 K. X-ray diffraction studies show that the Ca3Co4O9 phase is successfully preserved as the majority phase in the x=0.5 sample despite the small size of boron ions. Electrical transport measurements confirm that B3+ substitution for Ca2+ causes an increase in resistivity due to the decrease in carrier concentration. x=0.5 sample is found to have a Seebeck coefficient of 181 μV/K at room temperature which is ~1.5 times higher than that of the pure Ca3Co4O9. Our results indicate that the chemical pressure due to the large ionic radii difference between B3+ (0.27 Å) and Ca2+ (1 Å) enhances the thermoelectric properties as long as the unique crystal structure of Ca3Co4O9 is preserved.