•The doped Cu2+ ions are determined to be located at trigonal prism sites.•The magnetization steps are sensitive to magnetic field during cooling process.•The electrical transport properties can be well described by 1D-VRH model.•Compared to un-doped sample, the ZT of sample with x = 0.6 can be enhanced 60 times.The effect of Cu doping on the structural, magnetic, electrical and thermal transport properties of Ca3Co2-xCuxO6 single crystals has been investigated systematically. Based on the analysis of the structural parameter and x-ray photoelectron spectroscopy spectra, the valence state of Cu is considered to be +2. All samples undergo a long-range spin density wave (SDW) transition at TN and a glass-like magnetic transition at Tf with decreasing temperature. Both TN and Tf decrease monotonously with increasing Cu-doping content. A series of magnetization steps can be observed in the M(H) curve of Ca3Co2O6, and the magnetization steps are sensitive to the sample cooling magnetic field and Cu doping content x. With the increase of x, the resistivity along c-axis decreases while the thermopower increases. As a result, the figure of merit (ZT) increases considerably and the room-temperature ZT value of the sample with x = 0.6 is nearly 60 times larger than that of the un-doped crystal.Download high-res image (287KB)Download full-size image

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 (Ea). The room temperature thermopower S300 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 Ea at the critical point of the carrier type change.

The structural, magnetic, electrical and dielectric properties of an Ir-based double perovskite compound, La2CoIrO6, have been investigated. The sample undergoes a paramagnetic–ferromagnetic transition at TC, followed by a reentrant spin-glass transition at lower temperatures. The reentrant spin glass state in La2CoIrO6 is associated with the competitions of the antiferromagnetic coupling between Ir4+ and Co2+ ions and the ferromagnetic clusters. La2CoIrO6 shows a semiconducting transport behavior in the temperature range 65 to 360 K and the transport behavior can be well described by the three-dimensional Mott variable range hopping conduction mechanism. Moreover, a strong frequency dependence of dielectric constant behavior for La2CoIrO6 is observed and the dielectric relaxation can be ascribed to the electron hopping between different transition metal ions. In addition, the isothermal magnetic field dependent dielectric constant measurements show that a clear magnetodielectric coupling effect exists in La2CoIrO6 at low temperatures.

Thermoelectric properties of the Ca3Co4O9 system have been enhanced through an exotic route: Na doping at Co site, namely misplaced substitution. To compare, we have also performed the research of Na doping at Ca site and Co vacancy. In view of the analysis of XRD, XPS, and Raman data, Na+ ions could be suggested to enter into Co sites of [CoO2] layers in Ca3Co4–xNaxO9 (x = 0, 0.05, 0.10) and Ca sites of [Ca2CoO3] layers in Ca2.90Na0.10Co4O9, respectively. And Co ions are omitted from [CoO2] layers in Ca3Co3.90O9. Among all samples, Ca3Co3.90Na0.10O9 shows the maximum ZT value, which at room temperature reaches to ∼0.0117. Such a value is about 150% larger than that of Ca3Co4O9 and 63% larger than that of Ca2.90Na0.10Co4O9. The results indicate that the misplaced substitution is more beneficial to enhance the thermoelectric performance of Ca3Co4O9 system, compared with the traditional idea with Na doping at Ca site. Such an enhancement is mainly attributed to the combined action of electronic correlation, locally modified band structure near Fermi level, and carrier concentration.

•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 Ca3Co3.7Cr0.3O9 reaches up to 140 μV/K.•As Cr doping increasing, carrier localization enhances and electronic correction weakens.The effects of Cr doping on structural, magnetic, electrical and thermal transport properties of Ca3Co4−xCrxO9 (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.

•Valence state of doped Sb ions is variational as Sb-doping level increases.•The spin-density-wave state is more stable in the Sb-doped samples.•The ZT value of Ca3Co3.90Sb0.10O9 is two times larger than that of Ca3Co4O9.•Sb-doping may be effective to improve thermoelectric performance of Ca3Co4O9.The structure, magnetic, electrical and thermal transport properties of Ca3Co4−xSbxO9 (0 ⩽ x ⩽ 0.2) have been investigated systematically. Base on the analysis of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the valence state of the doped Sb ions is suggested to be +3 in the x = 0.05 sample, and its content increases monotonously along with the gradual introduction of Sb5+ ions as x further increases. The resistivity and the metal–insulator-transition temperature increase with increasing x, indicating the more stable spin-density-wave state in the Sb-doped samples. The thermopower decreases first as x = 0.05, and then increases monotonously with increasing x till to 0.15. The thermal conductivity decreases obviously due to Sb-doping. As a result, the Ca3Co3.90Sb0.10O9 sample has the largest thermoelectric figure-of-merit ZT value of 0.012 at room temperature, which is about two times larger than that of Ca3Co4O9. The results are suggested to originate from the variations of carrier concentration and electronic correlation via Sb-doping with the different valence states.

The structural, magnetic, electrical and thermal transport properties of the Cl-doped Ca3Co4O9−xClx (x = 0, 0.225, 0.45, 0.675, 0.9) ceramics have been investigated systematically. The degree of orientation of grains increases with increasing Cl-doping content. Obvious shoulder is observed in the dχ−1(T)/dT versus T curve for the Cl-doped samples and the shoulder shifts to lower temperature as Cl-doping content increases, indicating the long-range spin-density-wave state becomes unstable in these Cl-doped samples. Cl-doping also has a suppression effect on the ferrimagnetic state. The resistivity ρ of the Cl-doped samples is lower than that of Ca3Co4O9 in whole measured temperature range, which is suggested to be related to the increased carrier mobility induced by Cl-doping. The power factor P of the x = 0.9 sample is about 1.6 times larger and ZT is 1.3 times larger than that of the un-doped sample Ca3Co4O9.Highlights► Present a new way to investigate the thermoelectric properties of layered cobaltite. ► Obvious shoulder is observed in the dχ−1(T)/dT versus T curve for the Cl-doped samples. ► Cl-doping has a negative effect on the ferrimagnetic state. ► The resistivity ρ of the Cl-doped samples is lower than that of Ca3Co4O9 in whole measured temperature range. ► Both the power factor P and the dimensionless figure of merit ZT increase as Cl doped into the system.

The effects of Rh doping on the structural, magnetic, electrical, and thermal transport properties of Ca3Co4–xRhxO9 (0 ≤ x ≤ 0.4) samples have been investigated systematically. XRD and XPS results show that the doped Rh ions are in the form of Rh3+. Only a metal–insulator transition (MIT) and an anomaly of the slope related to the transition from a Fermi liquid to an incoherent metal at low temperatures were observed in the resistivity curve for the undoped sample. As Rh ions were doped into the samples, an additional anomaly and MIT occurred in the resistivity curve near room temperature, which are suggested to originate from the spin-state transition (SST) of Co ions. The low-temperature MIT temperature increased with increasing Rh-doping content, indicating that the spin-density-wave state became stable as a result of the enhanced random Coulomb potential in CoO2 octahedral block layers induced by Rh substitution. Based on an analysis of the thermopower and XPS data, Rh3+ ions are suggested to substitute at the Co3+ sites of CoO2 layers. The substitution induced a partial SST of Co4+ ions from the low-spin to the high-spin state, leading to the formation of a spin-state polaron. The evolution of the electrical and magnetic properties with Rh doping is summarized in a single phase diagram for Ca3Co4–xRhxO9. It should be noted that the thermopower of the system did not change obviously with Rh doping, but the thermal conductivity decreased significantly. As a result, the ZT value increased markedly with increasing Rh-doping content. The ZT value at room temperature for Ca3Co3.6Rh0.4O9 reached 0.014, which is about 2.4 times larger than that of Ca3Co4O9. The results show that Rh doping might be an effective route to improving the thermoelectric performance of the Ca3Co4O9 system.

The structure, anisotropic magnetic, electrical and thermal transport properties for single crystals of Ca3Co4−xCuxO9 (x = 0, 0.2, 0.4, 0.6 and 0.8) have been investigated systematically. The Cu-doping with x = 0.2 at Co-site is sufficient to drive the low-temperature spin-glass state in the Ca3Co4O9 system. The value of resistivity along ab-plane decreases monotonously with increasing x in the whole temperature range studied, and around room temperature, the in-plane resistivity of Ca3Co3.2Cu0.8O9 is about 71% smaller than that of the undoped sample. The temperature region where the Fermi-liquid transport mechanism dominates becomes remarkably narrowed due to the Cu-doping while the electronic correlation in the system is enhanced. With further addition of Cu in the Ca3Co4O9 system, the in-plane thermopower (Sab) increases slowly and the room-temperature Sab for Ca3Co3.2Cu0.8O9 is about 17% larger than that of the undoped sample. As a result, the power factor along the ab-plane is enhanced by about 3.8 times compared to the undoped sample. The results are suggested to originate from the variations of carrier concentration and electronic correlation in this system via the different Cu-doping states: Cu3+/Cu2+ (Cu3+ major) into the CoO2 layer for x ≤ 0.4, while Cu2+/Cu3+ (Cu2+ major) into the Ca2CoO3 layers for x > 0.4.

The influence of mono-valence-metal (Li, Na, and K) doping effect on the structural, resistivity, magnetic and magnetocaloric properties of La0.7Ca0.3MnO3 polycrystalline samples is studied for a fixed (5% at Ca site) dopant concentration. All the samples crystallize in orthorhombic structure and the lattice parameters increase continuously as the dopant atoms changes from Li to Na and then K. Paramagnetic–ferromagnetic phase transition at TC and insulator–metal phase transition at Tp are observed for all studied samples. The transition temperature decreases as Ca atoms is replaced by Li, while the transition temperature shifts to higher values as Ca is substituted by Na or K. In addition, the maximum magnetic entropy change of the K-doped sample is much smaller than that of the free- and Na-doped samples. The results are discussed according to the change of A-site-disorder effect caused by the systematic variations of A-site average ionic radius 〈rA〉 and A-site-cation mismatch σ2.Highlights► All studied samples have been prepared by sol–gel method. ► The Li-doped sample has the lowest Curie temperature. ► The K-doped sample has the smallest maximum magnetic entropy change.

Structural, magnetic, resistivity and thermal transport measurements have been performed to study the Mo-doping effect on a layered cobaltite Ca3Co4−xMoxO9(0≤x≤0.4) system. The results indicate that the low-temperature magnetic behavior of the system changes from a ferrimagnetic state to a spin-glass-like state upon Mo doping, which is due to the decrease in the average valence of Co ions. Moreover, all the Mo-doped samples have a higher resistivity and larger thermopower S compared with the Mo-free sample. The variation in the resistivity and thermopower between the Mo-doped and the Mo-free samples is dominated by the change in the carrier concentration of the samples. In the Mo-doped samples with x≥0.1, both the resistivity and thermopower decrease gradually with increasing Mo-doping level, which is suggested to mainly originate from the variation in the carrier mobility of the samples. In addition, an obvious thermopower upturn is observed in the S(T) curve of all the Mo-doped samples, which can be explained by the enhancement of spin-fluctuation induced by Mo-doping.Highlights► The electron-type-doping effect in the Ca3Co4O9 system is systematically studied. ► Spin-glass behavior in the Mo-doped Ca3Co4O9 samples has been observed. ► Thermopower upturn related to spin fluctuation is observed in Mo-doped samples.

•The highly textured Bi2Sr2Co2Oy samples were prepared via high-magnetic-field sintering.•The high-magnetic-field sintering induces spin state transition of the partial Co4+ ions.•The in-plane ZT value of the sample sintered under 8 T is about 100% larger than that of pristine.•The high-magnetic-field sintering can influence crystal nucleation and spin fluctuation.•The high-magnetic-field sintering may improve thermoelectric performance of Bi2Sr2Co2Oy.Herein, we report the effect of the high-magnetic-field (HMF) sintering on the structural, electrical and thermal transport properties of the layered Bi2Sr2Co2Oy. Based on the results of x-ray diffraction and microtopography, we find the grains prefer to the oriented growth along the direction of applied magnetic field, leading to a higher textured degree and anisotropy. Correspondingly, the in-plane conductivity increases while the out-of-plane one reduces with increasing sintering magnetic field. At the same time, the in-plane thermopower shows an obvious enhancement after the HMF sintering, which mainly originates in the spin state transition of Co4+ ions. As a result, a remarkable enhancement of the in-plane thermoelectric figure-of-merit is obtained in the sample after the HMF sintering (8 T).

The structure, anisotropic magnetic, electrical and thermal transport properties for single crystals of Ca3Co4−xCuxO9 (x = 0, 0.2, 0.4, 0.6 and 0.8) have been investigated systematically. The Cu-doping with x = 0.2 at Co-site is sufficient to drive the low-temperature spin-glass state in the Ca3Co4O9 system. The value of resistivity along ab-plane decreases monotonously with increasing x in the whole temperature range studied, and around room temperature, the in-plane resistivity of Ca3Co3.2Cu0.8O9 is about 71% smaller than that of the undoped sample. The temperature region where the Fermi-liquid transport mechanism dominates becomes remarkably narrowed due to the Cu-doping while the electronic correlation in the system is enhanced. With further addition of Cu in the Ca3Co4O9 system, the in-plane thermopower (Sab) increases slowly and the room-temperature Sab for Ca3Co3.2Cu0.8O9 is about 17% larger than that of the undoped sample. As a result, the power factor along the ab-plane is enhanced by about 3.8 times compared to the undoped sample. The results are suggested to originate from the variations of carrier concentration and electronic correlation in this system via the different Cu-doping states: Cu3+/Cu2+ (Cu3+ major) into the CoO2 layer for x ≤ 0.4, while Cu2+/Cu3+ (Cu2+ major) into the Ca2CoO3 layers for x > 0.4.