Carrier type change induced by fluorine doping in spin-chain compound Ca3Co2O6

Abstract

The structural, magnetic, electrical, and thermoelectric properties of fluorine (F)-doped Ca₃Co₂O₆ were investigated systematically. Based on the use of X-ray diffraction, X-ray photoelectron spectroscopy spectra, and magnetic data, it was concluded that part of the Co³⁺ ions at octahedral sites would transfer into Co²⁺ because of the substitution of F⁻ for O²⁻. The induced Co²⁺ ions are antiferromagnetically coupled with the nearest neighboring Co³⁺. The room-temperature resistivity decreases monotonously with the increase of fluorine (F)-doping content, x, and it is suggested that this is related to the increased carrier mobility. The resistivity curves of the samples with x ≥ 0.4 show an anomaly at T* ∼ 320 K, which is considered to be related to the change of activation energy (E_a). The room temperature thermopower S_{300 K} changed from a positive value of 686.6 μV K⁻¹ to a negative value of −333.1 μV K⁻¹ as x increases from 0 to 0.4 and considering this with the Hall coefficient result, it is concluded that suitable F-doping in Ca₃Co₂O₆ can change the type of the majority of carriers from p to n with a decreased resistivity. The thermal conductivity, κ of the n-type samples is much larger than that of the p-type, and the κ of both the p-type and n-type samples decreases monotonously with increasing x. The anomaly of κ near x = 0.4 is suggested to originate from the decreased grain boundary scattering and the sudden decrease of E_a at the critical point of the carrier type change.