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1.
The effect of Fe substitution for Co on direct current (DC) electrical and thermal conductivity and thermopower of Ca3(Co1−xFex)4O9 (x = 0, 0.05, 0.08), prepared by a sol–gel process, was investigated in the temperature range from 380 down to 5K. The results indicate that the substitution of Fe for Co results in an increase in thermopower and DC electrical resistivity and substantial (14.9–20.4% at 300K) decrease in lattice thermal conductivity. Experiments also indicated that the temperature dependence of electrical resistivity ρ for heavily substituted compounds Ca3(Co1−xFex)4O9 (x = 0.08) obeyed the relation lnρ ∝ T−1/3 at low temperatures, T < ~55K, in agreement with Mott’s two-dimensional (2D) variable range hopping model. The enhancement of thermopower and electrical resistivity was mainly ascribed to a decrease in hole carrier concentration caused by Fe substitution, while the decrease of thermal conductivity can be explained as phonon scattering caused by the impurity. The thermoelectric performance of Ca3Co4O9 was not improved in the temperature range investigated by Fe substitution largely due to great increase in electrical resistivity after Fe substitution. 相似文献
2.
C.S. ZhangL. Yang 《Journal of magnetism and magnetic materials》2012,324(8):1469-1472
Core-shell Co(1−x)NixFe2O4/polyaniline nanoparticles, where the core was Co(1−x)NixFe2O4 and the shell was polyaniline, were prepared by the combination of sol-gel process and in-situ polymerization methods. Nanoparticles were investigated by Fourier transform spectrometer, X-ray diffraction diffractometer, Scanning electron microscope, Differential thermal analysis and Superconductor quantum interference device. The results showed that the saturation magnetization of pure Co(1−x)NixFe2O4 nanoparticles were 57.57 emu/g, but Co(1−x)NixFe2O4/polyaniline composites were 37.36 emu/g. It was attributed to the lower content (15 wt%), smaller size and their uneven distribution of Co(1−x)NixFe2O4 nanoparticles in the final microsphere composites. Both Co(1−x)NixFe2O4 and PANI/Co(1−x)NixFe2O4 showed superparamagnetism. 相似文献
3.
Iduru Shigeta Osamu Murayama Alexander Brinkman Yukio Tanaka Hans Hilgenkamp 《Journal of Physics and Chemistry of Solids》2011,72(5):604-607
We report spin polarization P of Ru2−xFexCrSi Heusler alloys by the Andreev reflection technique. Ru2−xFexCrSi with L21-type structure and saturation magnetic moment of per formula unit is theoretically predicted to be half-metals in the wide range of the composition x. We had clarified that the experimental results of saturation magnetic moment in Fe-rich compounds had coincided with the theoretical prediction. Therefore, we have measured the differential conductance of Ru2−xFexCrSi/Pb planar-type junctions. The P value of Ru2−xFexCrSi was determined by fitting the differential conductance with the modified Blonder-Tinkham-Klapwijk theory. We have found that the behavior of P for Ru2−xFexCrSi was independent of the composition x in the Fe-rich region; P=0.53 for both of x=1.5 and 1.7. The spin polarization is the similar value to Co-based Heusler alloys. 相似文献
4.
D. Ruiz-Molina Lev N. Zakharov D.N. Hendrickson 《Journal of Physics and Chemistry of Solids》2004,65(4):831-837
Double oxidation of [CoIII(3,5-DTBCat)(3,5-DTBSQ)(bpy)] (1,ls-Co(III)) by AgBF4 and of [CoII(3,5-DTBSQ)2(bpy)] (1,hs-Co(II)) by a mixture of HClO4/H2O2 yielded [CoIII(3,5-DTBSQ) (bpy)2]X2, where X−=BF4− (4) and ClO4− (5), respectively. The mechanism for the double-oxidation process that leads to a loss of one of the quinone ligands and in some cases to a redistribution of the electronic charge is discussed here. 相似文献
5.
R. Martínez-García G. Goya F.M. Romero 《Journal of Physics and Chemistry of Solids》2006,67(11):2289-2299
The heating of Co(2+) ferricyanide above 80 °C induces an inner charge transfer from Co(2+) towards Fe(III) to form the mixed valence system Co(2+)Co(III) ferri- ferro-cyanide. This charge transfer takes place preserving the material framework and forming a solid solution of the initial and final species. The cell edge of the cubic cell (Fm-3m) of this solid solution follows a regular variation with the material composition. This mixed valence system was characterized using X-ray diffraction, infrared, thermo-gravimetric, Mössbauer and magnetic measurements. Its formation is easily detected by the appearance of an intermediate ν(CN) absorption band in the infrared spectra at around 2120 cm−1, 40 cm−1 below and above the observed frequency for this vibration in Co(2+) ferri- and ferro-cyanide, respectively. 相似文献
6.
A. Veres 《Journal of Physics and Chemistry of Solids》2006,67(4):822-827
Structural, electric and magnetic properties of Ba3Mg1−xCoxNb2O9 based dielectric ceramic compounds have been studied. The samples, prepared by a solid state reaction method, were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM), dielectric (ε(T)) and magnetic measurements (χ−1(T)). The XRPD analyses showed that the crystal structure of these compounds does change by the increase of substitution degree, passing from a superstructure hexagonal-type, (no. 164), space group (SG) to a simple structure cubic-type, (no. 221), SG. However, the evolution of the elementary unit cell lattice parameter can be followed and it exhibit a linear increasing tendency with increase in the substitution, indicating the existence of a solid solution through out the investigated range of substitution (0-1). The microstructure analysis shows a variation in the grain size and also the porosity of the samples with the degree of substitution. The results are in good agreement with that of dielectric measurements, which also showed that the dielectric constant (ε) increases with the increase of cobalt content. The magnetic characterization of cobalt substituted samples showed an antiferromagnetic type super-exchange interaction between these magnetic ions. At the same time, the values of effective magnetic momentum (μeff) are close to the value that corresponds to Co2+ free ions. The study highlights the possibility of modelling these materials by substitutions, in order to improve properties of negative-positive-zero (NPO) type dielectric applications. 相似文献
7.
Chunlin Ni Xiaoping Liu Linliang Yu Lemin Yang 《Journal of Physics and Chemistry of Solids》2007,68(1):59-65
A new molecular solid, [1-(4′-bromo-2′-fluorobenzyl)-4-dimetylaminopyridinium]-bis(maleonitriledithiolato)nickel(III), (BrFBzPyN(CH3)2(Ni(mnt)2)(1), has been prepared and characterized by elemental analyses, IR, ESI-MS spectra, single crystal X-ray diffraction and magnetic measurements. Compound 1 crystallizes in the orthorhombic space group Pnma, a=20.579(4) Å, b=7.078(1) Å, c=17.942(4) Å, α=β=γ=90°, V=2613.3(9) Å3, Z=4. The Ni(III) ions of 1 form a quasi-one-dimensional Zigzag magnetic chain within a Ni(mnt)2− column through Ni?S, S?S, Ni?Ni, or π?π interactions with an Ni?Ni distance of 4.227 Å. Magnetic susceptibility measurements in the temperature range 2-300 K show that 1 exhibits a spin-gap transition around 200 K, and antiferromagnetic interaction in the high-temperature phase (HT) and spin gap in the low-temperature phase (LT). The transition for 1 is second-order phase transition as determined by DSC analyses. 相似文献
8.
Xiansong Liu Pablo Hernández-Gómez Kai Huang Shixing Qiu 《Journal of magnetism and magnetic materials》2009,321(16):2421-2424
M-type strontium ferrites substituted by La3+-Co2+(Sr1−xLaxFe12−xCoxO19) were prepared by ceramic process. Effects of the substituted amount of La3+ and Co2+ on structure and magnetic properties of Sr1−xLaxFe12−xCoxO19 compounds have systematically been investigated by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and magnetic disaccommodation. In the measurement range from 80 to 500 K, the magnetic disaccommodation is represented by means of isochronal curves. It is well known that magnetic disaccommodation cannot be obviously found in the M-type of pure strontium ferrites. However, three peaks were observed in Sr1−xLaxFe12−xCoxO19, and this behavior is explained in terms of the presence of Fe2+ cation and to the site occupation by the magnetic Co2+ ionic within the hexagonal structure. 相似文献
9.
Qing Fang Li Hao Feng ZhaoXia Zhong Jun Liang Su 《Journal of magnetism and magnetic materials》2012,324(7):1463-1467
The electronic structure and magnetic properties of Co-doped Heusler alloys (Mn1−xCox)2 VGa (x=0.0, 0.25, 0.5, 0.75, 1.0) have been studied by first-principles calculations. The results show that the lattice constants decrease with increasing Co content except x=1.0. The spin polarization for x=0.5 is only 34%, much lower than the other concentrations. The compounds of x=0.0, 0.25 show nearly half-metallicity because the Fermi level slightly touches the valence bands. And the compounds of x=0.75, 1.0 exhibit the half-metallic character with 100% spin polarization. It is found the local moments of Mn(Co) basically show a linear increasing trend while the moments of V show a linear decreasing trend with increasing doping concentration. However, the local moments for x=0.5 quite depart from the linear trend. The majority-spin component at the Fermi level increases while the minority-spin component at the Fermi level decreases with the substitution of Co atoms for Mn atoms when x≤0.75. For x≥0.75, the majority-spin component remains more or less the same and the gap in the minority DOS increases with Co doping. The majority spin states are shifted to valence bands and the majority spin states around EF increase due to a leakage of charge from the unoccupied spin-up states to the occupied majority states with increasing Co content. 相似文献
10.
R. Laiho E. Lähderanta P.A. Petrenko V.N. Stamov Yu.P. Stepanov V.S. Zakhvalinskii 《Journal of Physics and Chemistry of Solids》2003,64(12):2313-2319
In a weak magnetic field LaMnO3+δ exhibits at δ=0.065 below the paramagnetic-to-ferromagnetic (FM) Curie temperature, TC, a mixed (spin-glass and FM) phase followed by a frustrated FM phase at δ between 0.100 and 0.154. The same behavior is observed in La1−xCaxMnO3 with x between 0 and 0.3. This can be understood by the similar variation of the Mn4+ concentration, c between ≈0.13 and 0.34, in both materials when x or δ is increased. On the other hand, considerable differences are found between these compounds in the values of the magnetic irreversibility, in the dependencies of TC(c) and the magnetic susceptibility, χ(c), as well as in the critical behavior of χ(T) near TC. These differences can be explained by distortions of the cubic perovskite structure, by the reduced lattice disorder and by the more homogeneous hole distribution in LaMnO3+δ than in La1−xCaxMnO3. 相似文献
11.
Xiao Lin 《Journal of Physics and Chemistry of Solids》2011,72(5):434-437
Polycrystalline Sr and Co co-doped superconducting oxypnictides Pr1−ySryFe1−xCoxAsO were synthesized by solid state reaction method, and superconductivity was investigated by measuring resistivity, magnetic susceptibility and thermopower. In the PrFe1−xCoxAsO samples with only Co doping, Tc reaches a maximum of 16 K at x=0.075-0.1, and the variation of Tc with Co content (x) is dome-shaped, consistent with the other Co-doped 1 1 1 1 phase systems such as LaFe1−xCoxAsO and SmFe1−xCoxAsO. In the Co and Sr co-doped Pr0.8Sr0.2Fe1−xCoxAsO system, Tc reaches a maximum of about 16 K at x=0.15, and the Tc‐x dome shifts to higher Co content in the phase diagram of Tc versus Co content (x). Our result indicates that Sr doping effectively induces hole-type charge carriers in the PrFeAsO system and the disorder caused by Sr and Co dopants has little effect on Tc. The thermopower data confirms that Sr dopant is hole-type and Co dopant is electron-type. The shift of Tc‐x dome to higher Co content caused by Sr doping implies that the Sr dopant can compensate for the Co doping effect. The results suggest that there could be a universal dependence of Tc on charge carrier density in both hole-type and electron-type oxypnictide superconductors. 相似文献
12.
The crystal structure, magnetism properties, and density of states for FeAs layered compound SrFe2As2 have been investigated by using the density functional theory (DFT) method. The magnetism under a checkerboard nearest neighbor anti-ferromagnetic (NN AFM) and ferromagnetic (FM) order ground-state have been analyzed with substitution for Sr with K ion in Sr1−xKxFe2As2. The results indicate that the distortion of FeAs tetrahedrons is sensitive to the electron doping concentration. The system magnetism was suppressed by K doping in NN-AFM ground state instead of FM. The density of states at Fermi level N(EF) under NN AFM ground state would be regarded as a driving force for the increased Tc of Sr1−xKxFe2As2 system as observed experimentally. Our calculation reflects that NN AFM type spin fluctuation may still exist in the Sr1−xKxFe2As2 system and it may be an origin of strong spin fluctuation in this system besides the spin density wave (SDW) states. 相似文献
13.
M. Itoi M. Enomoto N. Kojima Y. Kobayashi K. Koyama Y. Uwatoko 《Solid State Communications》2004,130(6):415-420
(n-C3H7)4N[FeIIFeIII(dto)3] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between FeII and FeIII occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P63, Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [FeIIFeIII(dto)3]∞, contracts by 0.1 Å at the charge-transfer transition temperature (TCT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 Å at TCT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around TCT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C3H7)4N[FeIIFeIII(dto)3] is regarded as spin entropy driven phase transition. 相似文献
14.
Zhida Han Zhenghe HuaDunhui Wang Chengliang ZhangBenxi Gu Youwei Du 《Journal of magnetism and magnetic materials》2006
Polycrystalline samples of Laves-phase alloys Dy(Co1−xFex)2(x=0, 0.02,0.04,0.06,0.08) have been prepared by arc-melting method. No first order phase transition was observed for samples with x≠0. With the increase of Fe content, the Curie temperature increases greatly, while the calculated magnetic entropy change, ΔSM, shows an obvious decrease with a broader peak. The origin of the magnetocaloric effect in Dy(Co1−xFex)2 alloys has been discussed. 相似文献
15.
Chang-Zheng Zhang Hong-Yan Mao Yan-Li Wang Jing-Cao Tao 《Journal of Physics and Chemistry of Solids》2007,68(2):236-242
The reaction of metal ions, flexible aliphatic dicarboxylates and pyrazine in aqueous solution afford two new metal-organic coordination polymers, {[Cu2(μ2-η2-O2C(CH2)2CO2-η2-μ2)2(H2O)2]·2H2O}n (1) and [Eu2(μ2-η2-O2CCH2CO2-η1-μ1)2(μ2-η2-O2CCH2CO2-η2-μ2)(H2O)6]n (2). Polymer 1 contains the paddle-wheel cage dicopper(II) units, forming a one-dimensional (1D) double-stranded chain structure along the a-axis, in which the copper(II) atoms are bridged by the carboxylate groups of four succinates. The intradimer Cu-Cu distance is 2.613(2) Å; the interdimer Cu?Cu distance is 6.473 Å. To our knowledge, compound 1 represents the first example of a double-stranded chain structure containing dinuclear paddle-wheel type cage. In the three-dimensional (3D) compound 2, each central Eu(III) ion have a distorted monocapped square antiprism coordination geometry. The structure is built up from two types of polymeric chains with [EuO6(H2O)3]n units as tethers, resulting in microporous framework. The magnetic behavior of 1 shows that the occurrence of a strong antiferromagnetic coupling between the copper(II) ions through the short bridges via the carboxyl groups can be obtained; the best fittings to the experimental magnetic susceptibilities gave −2J=314 cm−1. 相似文献
16.
The magnetic properties of Co-ferrite-doped hydroxyapatite (HAP) nanoparticles of composition Ca10−3xFe2xCox(PO4)6(OH)2 (where x=0, 0.1, 0.2, 0.3, 0.4 and 0.5% mole) are studied. Transmission electron microscope micrograms show that the 90 nm size nanoparticles annealed at 1250 °C have a core/shell structure. Their electron diffraction patterns show that the shell is composed of the hydroxyapatite and the core is composed of the Co-ferrite, CoFe2O4. Electron spin resonance measurements indicate that the Co2+ ions are being substituted into the Ca(1) sites in HAP lattice. X-ray diffraction studies show the formation of impurity phases as higher amounts of the Fe3+/Co2+ ions which are substituted into the HAP host matrix. The presence of two sextets (one for the A-site Fe3+ and the other for the B-site Fe3+) in the Mössbauer spectrum for all the doped samples clearly indicates that the CoFe2O4.cores are in the ferromagnetic state. Evidence of the impurity phases is seen in the appearance of doublet patterns in the Mössbauer spectrums for the heavier-doped (x=0.4 and 0.5) specimens. The decrease in the saturation magnetizations and other magnetic properties of the nanoparticles at the higher doping levels is consistent with some of the Fe3+ and Co2+ which being used to form the CoO and Fe2O3 impurity phase seen in the XRD patterns. 相似文献
17.
The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo4O7 has been investigated. It is shown that the oxides CaBaCo4O7+δ and CaBaCo4−xO7−δ (0≤x≤0.20) synthesized below 1100 °C in air exhibit phase separation, where ferrimagnetic regions with TC~56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at ~14–20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co3+ cations, with respect to the ideal formula CaBaCo22+Co23+O7. This excess Co3+ tends to destroy the ferromagnetic zig-zag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples. 相似文献
18.
Antiferromagnetic phase transition in two vanadium garnets AgCa2Co2V3O12 and AgCa2Ni2V3O12 has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature TN=6.39 K for AgCa2Co2V3O12 and 7.21 K for AgCa2Ni2V3O12, respectively. The magnetic susceptibilities exhibit broad maximum, and these TN correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χmax). The magnetic entropy changes from zero to 20 K per mol Co2+ and Ni2+ ions are 5.31 J K−1 mol-Co2+-ion−1 and 6.85 J K−1 mol-Ni2+-ion−1, indicating S=1/2 for Co2+ ion and S=1 for Ni2+ ion. The magnetic susceptibility of AgCa2Ni2V3O12 shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μeff=3.23 μB Ni2+-ion−1 and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa2Co2V3O12. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state 2E(t26e) and the first excited state is spin quartet state 4T1(t25e2) which locates extremely close to the ground state. The low spin state S=1/2 for Co2+ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity. 相似文献
19.
Novel mixed valence states have been obtained by the treatment of cobaltous ferrocyanides (Co+2FeII) and ferricyanides (Co+2FeIII) in an ozone flow. The CN stretching bands occur at 2085 cm–1 for Co+2FeII and at 2160 cm–1 for Co+2FeIII. After the ozonization process of Co+2FeII, an intense band approximately at 2125 cm–1 is detected. This intermediate band must correspond to a mixed valence state of the type: FeII–CN–Co2+–NC–FeIII Mössbauer spectra recorded in situ during the ozonization of Co+2FeII show the presence of two components: a doublet with isomer shift and quadrupole splitting values close to the cobalti ferricyanide and a very broad line for the mixed valence state. From the Mössbauer and infrared spectra of the aged samples of the Co+2FeII after ozonization, a relaxation process to the initial state of the samples is observed but the mixed valence state is stable. 相似文献
20.
Optical transitions in normal-spinel Co3O4 have been identified by investigating the variation of its optical absorption spectrum with the replacement of Co by Zn. Three optical-transition structures were located at about 1.65, 2.4, and 2.8 eV from the measured dielectric function of Co3O4 by spectroscopic ellipsometry. The variation of the absorption structures with the Zn substitution (ZnxCo3−xO4) can be explained in terms of charge-transfer transitions involving d states of Co ions. The 1.65 eV structure is assigned to a d-d charge-transfer transition between the t2g states of octahedral Co3+ ion and t2 states of tetrahedral Co2+ ion, t2g(Co3+)→t2(Co2+). The 2.4 and 2.8 eV structures are interpreted as due to charge-transfer transitions involving the p states of O2− ion: p(O2−)→t2(Co2+) for the 2.4 eV absorption and p(O2−)→eg(Co3+) for the 2.8 eV absorption. The observed gradual reduction of the 1.65 and 2.4 eV absorption strength with the increase of the Zn composition for ZnxCo3−xO4 can be explained in terms of the substitution of the tetrahedral Co2+ sites by Zn2+ ions. The crystal-field splitting ΔOh between the eg and the t2g states of the octahedral Co3+ ion is estimated to be 2 eV. 相似文献