Ga For Co Research Articles

Tuning electrochemical potential of LiCoO2 with cation substitution: first-principles predictions and electronic origin

With a goal to improve the performance of LiCoO2 as a cathode material in Li-ion batteries, we simulate substitution of various elements (X = Be, Mg, Al, Ga, Si and Ti) for Co using first-principles density functional theory and predict changes in its electrochemical potential. While the electrochemical potential of LiCoO2 is enhanced with substitution of Be, Mg, Al and Ga for Co, an opposite effect is predicted of Si and Ti substitution. We determine the electronic origin of these changes in electrochemical potential using (a) Bader method of topological analysis of charge density, (b) partial density of electronic states to estimate oxidation states of metal and oxygen, and charge re-distribution upon lithiation. We find that the distribution of electronic charge donated by Li is influenced by the nature of the X–O bond. A larger electron transfer to O (in XO6 octahedron) upon lithiation leads to stronger Li intercalation and thereby higher electrochemical voltage. Our findings provide a platform for a rational design of cathode materials in Li batteries with enhanced voltage.

High-Temperature Thermoelectric and Microstructural Characteristics of Cobalt-Based Oxides with Ga Substituted on the Co-Site

The effects of Ga substitution on the Co-site on the high-temperature thermoelectric properties and microstructure are investigated for the misfitlayered Ca3Co4O9 and the complex perovskite-related Sr3RECo4O10.5 (RE = rare earth) cobalt-based oxides. For both systems, substitution of Ga for Co results in a simultaneous increase in the Seebeck coefficient (S) and the electrical conductivity (σ), and the influence is more significant in the high temperature region. The power factor (S 2 σ) is thereby remarkably improved by Ga substitution, particularly at high temperatures. Texture factor calculations using x-ray diffraction pattern data for pressed and powder samples reveal that the Ga-doped samples are highly textured. Microstructure observed by scanning electron microscopy shows very well-crystallized grains for the samples with Ga substitution for Co. Among the Ga-doped samples, Ca3Co3.95Ga0.05O9 shows the best ZT value of 0.45 at 1200 K, which is about 87.5% higher than the nondoped one, a considerable improvement.

Effects of Ga substitution on the high temperature properties of the n = 3 Ruddlesden Popper system LaSr 3Fe 1.5 − x/2Co 1.5 − x/2Ga xO 10 − δ (0 ≤ x ≤ 0.8)

The effects of the Ga incorporation on the crystal chemistry and high temperature properties of the n = 3 Ruddlesden Popper system LaSr 3Fe 1.5 − x/2Co 1.5 − x/2Ga xO 10 − δ (0 ≤ x ≤ 0.8) have been investigated. As the Ga content increases the symmetry of the unit cell varies from tetragonal for the samples with x = 0 and 0.3 to orthorhombic for the x = 0.8 sample. Due to the fixed Ga + 3 oxidation state, the substitution of Ga for Fe and Co increases the oxygen vacancy concentration in the samples at room temperature while it decreases the variation of the oxygen content with temperature. The total expansion coefficient of the LaSr 3Fe 1.5 − x/2Co 1.5 − x/2Ga xO 10 − δ specimen in the temperature range 80 ≤ T ≤ 900 °C decreases with increasing Ga content due to the suppression of the chemical expansion at high temperature. Similarly, the electrical conductivity also decreases as the Ga content increases. The electrochemical performances of the LaSr 3Fe 1.5 − x/2Co 1.5 − x/2Ga xO 10 − δ samples as cathode materials in SOFC have been tested with electrolyte-supported single cells using La 0.8Sr 0.2Ga 0.8Mg 0.2O 2.8 (LSGM) as an electrolyte. Although the maximum power density of the cells decreases with increasing Ga content, the LaSr 3Fe 1.5 − x/2Co 1.5 − x/2Ga xO 10 − δ cathode provides an important advantage of decrease in total expansion with increasing Ga content.

Effect of metal substitution for cobalt on the oxygen adsorption properties of YBaCo4O7

YBaCo4O7 compound is capable to intake and release a large amount of oxygen in the temperature range of 200–400°C. In the present study, the effect of Zn, Ga and Fe substitution for Co on the oxygen adsorption/desorption properties of YBaCo4O7 were investigated by thermogravimetry (TG) method. Due to fixed oxidation state of Zn2+ ions, the substitution of Zn2+ for Co2+ suppresses the oxygen adsorption of YBaCo4−xZnxO7. The substitution of Ga3+ for Co3+ also decreases the oxygen absorption capacity of YBaCo4−xGaxO7. This can be explained by the strong affinity of Ga3+ ions towards the GaO4 tetrahedron. Compared with Zn- and Ga-substituted samples, the drop of oxygen adsorption capacity is smallest for Fe-substituted samples because of the similar changeability of oxidation states of Co and Fe ions.

Anomalous transport properties of Co-site impurity doped NaxCoO2

The effect of substitution of Cu, Zn, Ga, Mn, and Ni for Co in layered NaxCoO2 (x∼0.85) on its resistivity (ρ), magnetization (M), and thermopower (S) has been investigated. At low temperature, ρ remains metallic (dρ/dT>0) for Cu, Zn, and Ga impurities up to a maximum doping of 15%, while for Mn and Ni a metal-insulator transition occurs at 3% and 6% doping, respectively. For the Cu-doped samples, ρ(T) shows a strong anomaly and a hysteresis around 280 K where a first-order structural phase transition occurs due to long-range ordering of the Na-vacancy clusters. Well above the antiferromagnetic transition temperature, the magnetic susceptibility exhibits the Curie–Weiss law. The Neel temperature is insensitive to impurity while both the Curie constant and Weiss constant (θ) depend on the nature of impurity. A small enhancement in magnetization above 280 K and an increase of θ with Cu doping are the signatures of superior Na-vacancy ordering in the Cu-doped samples. In contrast to ρ and M, S changes systematically with doping and temperature, and does not show any anomalous behavior around 280 K.

Neutron powder diffraction and magnetic phase diagram of RCo4Ga compounds (R=Y and Pr)

The crystallographic and magnetic properties of hexagonal RCo4Ga are presented (R=Y and Pr). X-ray diffraction investigations exhibit a CaCu5-type single phase for both compounds. Neutron powder diffraction experiments demonstrate that the Ga atoms preferentially occupy the 3g crystallographic site of the CaCu5 structure. Substitution of Ga for Co substantially decreases the mean Co magnetic moment, the Curie temperature, and the saturation magnetization. The magnetocrystalline anisotropy is also drastically changed upon Ga substitution. A.c. susceptibility measurements have evidenced a spin reorientation transition in PrCo4Ga. A detailed investigation of this phenomenon occurring in PrCo4Ga is presented. Neutron and X-ray powder diffraction experiments as well as magnetic measurements are combined in order to clarify the effects of the Ga substitution on the magnetocrystalline anisotropy and the spin reorientation transition. The magnetic phase diagram of RCo4Ga (R=Y and Pr) has been determined in the whole ordered temperature range.

Electronic structure and magnetic properties of Gd(Co1−xMx)2 compounds, M=Ga, Si, Ge

We report the results of measurements of the magnetic susceptibility and the crystal and electronic structures of the polycrystalline intermetallic compounds Gd(Co1−xMx)2, M=Ga, Si, Ge, and the results of the electrical resistivity measurements for these systems when M=Ga and Si. The dependence of the Curie temperature, TC, and the lattice constant versus concentration, x, of the M element was established for all investigated systems. The influence of the partial substitution of Ga for Co is reflected in a drastic change in TC with concentration x. This result differs very much from that when M=Si, where we do not detect a change in the TC(x) and also when M=Ge, where we observe a very weak dependence of the Curie temperature on x. The electronic structure of all systems was studied by using X-ray photoelectron spectroscopy (XPS). From the XPS spectra in the valence band region we derive that there is hybridization between the Co d states and the M p states. This effect changes the shape of the valence band in the vicinity of the Fermi level. The observed changes in the electronic structure correlate with the change in magnetic properties of the investigated systems.

Magnetic properties of Sm2Co17−xGax (x=0–7) compounds

The study of the effect of Ga substitution on the magnetic properties of arc-melted Sm2Co17−xGax (x=0–7) compounds has been carried out by means of x-ray diffraction (XRD) and magnetic measurements. XRD patterns show that all samples of Sm2Co17−xGax (x=0–7) crystallize in the rhombohedral Th2Zn17-type structure. The Ga substitution for Co leads to an approximately linear decrease in the Curie temperature and a rapid decrease in the saturation magnetic moment which is faster than that in the case of magnetic dilution. Spin-reorientation transition is observed for Sm2Co11Ga6 compound at 32 K. In order to determine the room-temperature easy magnetization direction (EMD), XRD measurements are performed on magnetically aligned samples with x⩽5. The result shows that the EMD of these compounds corresponds to the c axis. The anisotropy constants K1 and K2 of Sm2Co17−xGax compounds were derived by fitting the hard-direction magnetization recoil curves measured on magnetically aligned powder samples as well as by fitting the magnetization recoil curves measured on polycrystalline bulk samples. The fitting results indicate that the substitution of Ga for Co in Sm2Co17 compound decreases the anisotropy constants, thus indicating the weakening of the easy-axis anisotropy.

Magnetic properties of Pr 2 Co 17- x M x ( M = Ga, Si) compounds

The influence of Ga or Si substitution for Co on the structural and magnetic properties of Pr2Co17 compounds is investigated. All samples studied here are single phase and have the rhombohedral Th2Zn17-type structure. The unit-cell volume is found to increase linearly by the substitution of Ga for Co, but reduce by the substitution of Si for Co in Pr2Co17 compounds. In Pr2Co17-xMx, the Curie temperature decreases monotonically with increasing x at an approximate rate of 153K per Ga atom and 175K per Si atom. The saturation magnetic moment of Pr2Co17-xMx (M = Ga, Si) decreases with increasing x. The rates of the decrease are larger than that expected as a simple dilution. For Pr2Co17-xSix, the spin-reorientation transition is observed above room temperature. The spin-reorientation temperature Tsr first decreases with increasing Si content and then increases at higher x values (x > 2). The spin-reorientation behavior is interpreted by the competition between the Pr and Co sublattice anisotropies. The easy magnetization direction in Pr2Co17-xGax compounds is perpendicular to the c-axis, and no spin reorientation transition is observed.

Magnetovolume effects in metamagnetic itinerant-electron systems and

High-field magnetization and magnetostriction measurements have been performed for typical metamagnetic itinerant-electron systems and with at 4.2 K in pulsed magnetic fields up to 40 T. The substitution of Al or Ga for Co significantly decreases the critical field of the metamagnetic transition. The magnetovolume coupling constant in the paramagnetic state is nearly constant up to about x = 0.05 and then decreases with increasing x. Moreover, the value of is considerably reduced by the metamagnetic transition. The variation of produced by the change of x or the metamagnetic transition is considered to originate from the change of the spin fluctuation spectrum.

The structure and magnetic properties of (x = 0-8) compounds with and Tb

X-ray diffraction (XRD) and magnetic measurements were employed to investigate the effect of Ga substitution for Co on the structure and magnetic properties of and compounds (x = 0, 1, 2, 3, 4, 5, 6, 7 and 8). XRD patterns show that all samples are single phase with the hexagonal -type structure or the rhombohedral -type structure except the samples with x = 7 and 8 which contain a small amount of Co. The Ga substitution for Co in and compounds leads to a monotonic increase in unit cell volume and an approximately linear decrease in saturation magnetization. The Curie temperature is found to decrease rapidly with increasing Ga concentration. The compensation points are observed for the samples with x = 4, 5 and 6. The intersublattice-molecular-field coefficients, , have been deduced on the basis of magnetization curves at the compensation temperature. The exchange-coupling constants and have been calculated by analysing both the field dependence and the temperature dependence of the magnetization. It has been found that the substitution of Ga for Co in these compounds reduces the Co-Co exchange interaction, while it has little influence on the R-Co exchange interaction. It is noteworthy that the Ga substitution has not changed the room-temperature easy magnetization direction for both and compounds.

Structure and magnetic properties of Dy 2Co 17− xGa x ( x=0–6) compounds

The structure and magnetic properties of arc-melted Dy 2Co 17− x Ga x compounds were studied. These compounds have the hexagonal Th 2Ni 17-type structure for 0≤ x≤3 and the rhombohedral Th 2Zn 17-type structure for 3≤ x≤6. The unit cell volume shows an approximately linear increase at a rate of about 7.7 Å 3 per Ga atom. The Curie temperature was found to decrease with increasing Ga concentration. The saturation magnetization decreases linearly with increasing Ga content from 7.0 μ B f.u. −1 for x=0 to −7.9 μ B f.u. −1 for x=6. The exchange coupling constants J DyCo and J CoCo have been derived by analyzing both the field dependence and the temperature dependence of the magnetization. An approximately constant value of J DyCo indicates that the substitution of Ga for Co has only a small influence on the exchange coupling between the Dy and Co atoms. The effect of Ga substitution on J CoCo is strong, leading to a linear decrease of T C in the Dy 2Co 17− x Ga x compounds.

The magnetic properties of Gd2Co17−xGax compounds

X-ray diffraction (XRD) and magnetization measurements were performed to investigate the effect of Ga substitution for Co on the structural and magnetic properties of Gd2Co17−xGax compounds (x=0, 1, 2, 3, 4, 5, 6, 7, and 8). Crystal-structure studies indicate that all samples are single phase with the rhombohedral Th2Zn17-type structure except for the samples with x=7 and 8, which contain a small amount of Co. The Ga substitution for Co in Gd2Co17 compounds leads to a monotonic increase in the unit cell volume and an approximately linear decrease in the saturation magnetization. The Curie temperature TC is found to decrease monotonically from 1210 K for x=0 to 30 K for x=8. The compensation points are observed for the Gd2Co17−xGax samples with x=5 and 6. The value of the compensation temperature, determined from the temperature dependence of the magnetization measured in a magnetic field of 1000 Oe, is 120 and 152 K for x=5 and 6, respectively. It is noteworthy that the substitution of Ga for Co in the Gd2Co17 compounds has a remarkable influence on the magnetocrystalline anisotropy. XRD studies on magnetic-field oriented Gd2Co17−xGax powders show that the sample with x=0 exhibits an easy-plane anisotropy, while the samples with x⩾1 exhibit an easy c-axis anisotropy at room temperature. The change of the easy magnetization direction of Gd2Co17−xGax compounds from a basal plane to c axis indicates that the substitution of Ga for Co leads to the decrease of the planar anisotropy of the Co sublattice.