Cr⁶⁺ ion doping effects in layered Ca₃Co₄O₉ system

Highlights: • The XPS and XRD results indicate the valence state of doped Cr ions is +6. • Magnetic/electrical/thermal transports below 350 K are investigated. • The room-temperature thermopower of Ca₃Co_3.7Cr_0.3O₉ reaches up to 140 μV/K. • As Cr doping increasing, carrier localization enhances and electronic correction weakens. - Abstract: The effects of Cr doping on structural, magnetic, electrical and thermal transport properties of Ca₃Co_4−xCr_xO₉ (0 ⩽ x ⩽ 0.3) have been investigated systematically. Based on the analysis of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) data, the Cr ions are suggested to be doped at Co⁴⁺-sites in CoO₂ layers in the form of Cr⁶⁺. The introduction of the nonmagnetic Cr⁶⁺ ions results in the decrease of the average effective magnetic moment, which confirming that the ferrimagnetic state of Ca₃Co₄O₉ system can be well suppressed by Cr doping. The magnitude of resistivity and the metal–insulator-transition temperature increase monotonously with increasing x, indicating the enhanced carrier localization and the more stable spin-density-wave state in these Cr-doped samples. As Cr⁶⁺ ions are doped into system, the thermopower increases obviously. Correspondingly, the room-temperature thermopower reaches the maximum value of 140 μV/K for x = 0.3 sample. These results of magnetism and electrical/thermal transport are mainly originated from the variations of carrier concentration and electronic correlation resulted from the partial substitution of Cr⁶⁺ for Co ions