Layered Cobaltite Research Articles

Synthesis of Ca–Co hydroxides and their use in facile fabrication of textured CayCoO2 thermoelectric ceramics

A brucite-type hydroxide of CazCo1−z(OH)2 with a high Ca content was synthesized and utilized as a single-source precursor for the fabrication of bulk ceramics of a semiconducting layered calcium cobaltite CayCoO2. Plate-like CazCo1−z(OH)2 particles with z up to 0.22 were reproducibly obtained by a reverse co-precipitation method using KOH as an alkaline source. Uniaxial pressing of the plate-like CazCo1−z(OH)2 particles (z = 0.22), which was calcined at 373 K beforehand, at a pressure as high as 500 MPa produced a highly dense green pellet where the plate-like particles were stacked along the pressing direction. The green pellet was then converted by a heat treatment at 923 K into an oxide ceramic of preferentially (010)-oriented CayCoO2 via a topotactic-like pyrolytic reaction maintaining edge-sharing CoO6 octahedra. Although the ceramic also contained a minor insulating Co3O4 phase (16.7 ± 0.7 mol% estimated from thermal analysis), the electrical measurements proved its p-type semiconducting behavior resulting from the CayCoO2 phase with enhanced Seebeck coefficient exceeding 160 μV/K at room temperature.

Structure, magnetic and magnetic transport properties of layered cobaltite Sr0.9Y0.1CoO2.63

The structure, magnetic and magnetotransport properties of perovskite Sr0.9Y0.1CoO2.63are studied. It is shown that the sample is structurally two-phase. The main phase has a tetragonal-distorted unit cell and is described by the spatial group I4/mmm. The broadening of the reflexes with indexes corresponding to a doubling of the parameter from the unit cell indicates that there is no strict translational symmetry along the с-axis. The presence of a broadened superstructural reflex observed at small angles on X-ray diffraction patterns at temperatures below 400 K is due to the monoclinic phase, whose content is much smaller than the tetragonal phase. Spontaneous magnetization appears during the formation of the monoclinic phase. The magnetic structure is predominantly an antiferromagnetic G-type structure with magnetic moments of 1.5µBin the layers of CoO6octahedra and 2µBin the anion-deficient CoO4+γlayers. The electrical conductivity of Sr0.9Y0.1CoO2.63has a semiconductor in character. The magnetoresistance reaches 57 % in a field of 14 T at a temperature of 5 K and decreases strongly with increasing temperature.

Magnetic Properties of a Layered Cobaltite Sr1–xYxCoO3–δ (x = 0.1)

The structure, the magnetic and magnetotransport properties of perovskite Sr0.9Y0.1CoO2.63 have been studied. The sample is shown to have a two-phase structure. The main phase has a tetragonally distorted unit cell and is described by space group I4/mmm. The broadening of the reflections with indices corresponding to doubling unit cell parameter c indicates the absence of the rigorous translation symmetry along axis c. The existence of the broadened superstructure reflection observed in the diffraction pattern at small angles at temperature lower than 400 K is explained by the existence of the monoclinic phase whose content is significantly lower than that of the tetragonal phase, but is dominant in the Sr0.8Y0.2CoO3–δ composition. The spontaneous magnetization appears as the monoclinic phase forms. The magnetic structure is mainly G-type antiferromagnetic with magnetic moments 1.5μB in the layers of CoO6 octahedra and 2μB in the anion-deficit CoO4 + γ layers. The conduction of the Sr0.9Y0.1CoO2.63 composition has a semiconducting character. The magnetoresistance is 57% in a field of 14 T at a temperature of 5 K and strongly decreases with the temperature increase.

Quantifying Anharmonic Vibrations in Thermoelectric Layered Cobaltites and Their Role in Suppressing Thermal Conductivity

Optimizing multiple materials properties which are simultaneously in competition with each other is one of the chief challenges in thermoelectric materials research. Introducing greater anharmonicity to vibrational modes is one strategy for suppressing phonon thermal transport in crystalline oxides without detrimentally affecting electronic conductivity, so that the overall thermoelectric efficiency can be improved. Based on perturbed molecular dynamics and associated numerical analyses, we show that CoO2 layers in layered cobaltite thermoelectrics NaxCoO2 and Ca3Co4O9 are responsible for most of the in-plane heat transport in these materials, and that the non-conducting intermediate layers in the two materials exhibit different kinds of anharmonicity. More importantly, thermal conduction is shown to be altered by modifying the structure of the intermediate layers. The simulation methods developed to quantify the effect of anharmonic atomic vibrations on thermal conductivity provide a new tool for the rational design of thermoelectric materials, and the insights gained should hasten the attainment of higher conversion efficiencies so that thermoelectrics can be put to widespread practical use.

Synthesis and Properties of Ceramics Based on a Layered Bismuth Calcium Cobaltite

Bi2Ca2Co x O y (x = 0.5, 1.7, 2.8) ceramics have been prepared by solid-state reactions and their thermophysical, electrical-transport, and thermoelectric (functional) properties have been studied. The results demonstrate that increasing the cobalt oxide concentration in the ceramics leads to an increase in their thermal conductivity, electrical conductivity, and thermal diffusivity and a reduction in their apparent density. The largest Seebeck coefficient is offered by a single-phase ceramic with the composition Bi2Ca2Co1.7Oy, which in addition has the highest power factor (P300 = 26.0 μW/(m K2)) and thermoelectric figure of merit (ZT1000 = 0.019) and the smallest linear thermal expansion coefficient (α = 9.72 × 10–6 K–1).

Mictomagnetic State in an EuBaCo2–xO5.5–δ Single Crystal

The magnetic properties of an EuBaCo1.9O5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo2O5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 A, b = 7.833 A, and c = 7.551 A. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below TC = 242 K. At T < 300 K, all three spin states of the Co3+ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co3+ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co3+ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo1.9O5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo1.9O5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.

Theoretical investigation of excitonic magnetism in LaSrCoO4

We use the LDA+U approach to search for possible ordered ground states of LaSrCoO4. We find a staggered arrangement of magnetic multipoles to be stable over a broad range of Co 3d interaction parameters. This ordered state can be described as a spin-density-wave-type condensate of excitons carrying spin S = 1. Further, we construct an effective strong-coupling model, calculate the exciton dispersion and investigate closing of the exciton gap, which marks the exciton condensation instability. Comparing the layered LaSrCoO4 with its pseudo cubic analog LaCoO3, we find that for the same interaction parameters the excitonic gap is smaller (possibly vanishing) in the layered cobaltite.

Magnetic Relaxation Phenomena above the Freezing Temperature in the Single Layered Cobaltite La0.75Sr1.25CoO4

The magnetic relaxation properties of the single layered perovskite La0.75Sr1.25CoO4 are investigated, in which spin glass‐like and cluster glass behaviors coexist at low temperature. Not only below but also above the freezing temperature Tf, all magnetization measurements as a function of time display a cooperative relaxation. Above Tf, the ZFC (zero‐field‐cooled) relaxation follows the stretched exponential form while the FC (field‐cooled) relaxation rate can be well described by a power law. These magnetic relaxation phenomena can be attributed to the local interactions between ferromagnetic clusters in La0.75Sr1.25CoO4.

The investigation of magnetic phase transition in cobaltite perovskites by high-resolution neutron powder diffraction under 14 T magnetic field

We carried out neutron powder diffraction (NPD) experiments on the layered perovskite cobaltite PrBaCo2O5.5+x (x = 0.24) at Super High Resolution Neutron Powder Diffractometer (SuperHRPD) with a newly installed magnet. At low temperatures, the antiferromagnetic (AFM) reflections of the NPD profile are suppressed whilst ferromagnetic (FM) reflections are enhanced under 14 T magnetic field. The magnetic-field-induced AFM-FM transition reveals the ground-state energy of FM is close to that of AFM.

Affinity of YBaCo4O7+δ-based layered cobaltites with protonic conductors of cerate-zirconate family

Abstract This work highlights the possibility of the application of YBaCo4O7+δ-modified materials as cathodes for solid oxide fuel cells based on protonic electrolytes. To reach this goal, the basic YBaCo4O7+δ oxide material and its YBaCo3.5Zn0.5O7+δ (YBCZ) and YBaCo3.5Fe0.5O7+δ (YBCF) substituted members were successfully synthesized and their structural, thermal and electrochemical properties were evaluated for their suitability for application. It was found that the substitution of cobalt with zinc results in the structural stabilization of such “114” phases and the convergence of the thermal expansion coefficient with that of the selected BaCe0.5Zr0.3Dy0.2O3–δ (BCZD) electrolyte as well as superior electrochemical activity below 700 °C. The Ni-BCZD|BCZD|YBCZ cell yields relatively high power density (200 mW cm–2 at 600 °C) and exhibits low polarization resistance (0.51 Ω cm2 at 600 °C). This, combined with the low activation energy values found for electrode polarization conductivity of both symmetrical and fuel cells allow assuming that the Zn-substituted YBaCo4O7+δ material could be successfully used in electrochemical devices based on proton-conducting electrolytes.

Glassy magnetic behaviors in the single layered cobaltite Gd0.8Sr1.2CoO4

A comprehensive investigation of magnetic properties of single layered perovskite Gd0.8Sr1.2CoO4 has been carried out by means of dc susceptibility, ac susceptibility and magnetic relaxation. The cusp in χZFC(T) as well as a considerable frequency dependence of a shoulder in χ′(T) and a peak in χ″(T) indicates the appearance of a glassy behavior. The dynamic scaling analysis of ac susceptibility data reveals a spin glass phase below 9K. The aging effect and long-time relaxation phenomenon are also observed in this compound. Relative to those RE2−xSrxCoO4 compounds, the large structural distortion and the stability of the lower spin state in the cobalt ions can be considered as possible reasons for the existence of only one spin glass state in Gd0.8Sr1.2CoO4.

Thermoelectric functionality of Ca3Co4O9 epitaxial thin films on yttria-stabilized zirconia crystalline substrate

Abstract Layered cobaltite Ca3Co4O9 is one of the most interesting materials for energy conversion due to its excellent thermoelectric properties and stability at high temperatures. However, the best performance of this material is achieved for single crystals and epitaxial films. Within this framework, an important role is played by the substrate, which should promote epitaxial growth and that should have suitable thermal properties for its integration in devices. In this work, we report on the preparation of epitaxial thin films of Ca3Co4O9 by sputtering using low cost ceramic target on cubic yttria-stabilized zirconia (YSZ(100)) that is typically the standard for low thermal conductivity material and on hexagonal sapphire (Al2O3(0001)). The quality of the epitaxial films and its microstructure has been studied to explain the thermoelectric properties obtained for both substrates. YSZ(100) substrate provides Ca3Co4O9 films with enhanced Seebeck coefficient and Al2O3(0001) allows improved conductive Ca3Co4O9 films that finally, at 200 °C, results in a substrate-modified Z′T figure of merit one order of magnitude higher for Ca3Co4O9 on YSZ(100) than on Al2O3(0001) substrate.

Spin state of Co3+ ions in EuBaCo2–x O5.5–δ in the paramagnetic temperature range

The magnetic studies of EuBaCo2O5.50, EuBaCo1.93O5.40, and EuBaCo1.90O5.36 have been performed. Layered cobaltites EuBaCo2–x O5.5–δ demonstrated the ferrimagnetic ordering in the temperature range 200 K < T < 650 K. The ferromagnetic Curie temperature T C almost did not change as the number of Co3+–O–Co3+ bonds decreased. The contributions of the ferro- and antiferromagnetic interactions were nearly the same and determine the relationship T C > T N and T C < T N. At T > T C, the antiferromagnetic order was retained in the EuBaCo1.90O5.36. As temperature increased, EuBaCo2–xO5.5–δ conserved the IS state in pyramids, and gradual LS → IS → HS transitions occurred in octahedron sites. The increase in the effective magnetic moment μeff in EuBaCo1.90O5.36 at T > 580 K demonstrated the transition of Co3+ ions to the HS state at lower temperatures as compared to the EuBaCo2O5.50 stoichiometric composition, and the structural distortions and the increase in the unit cell volume favored to this transition.

Thermoelectric properties of layered calcium cobaltiteCa3Co4O9from hybrid functional first-principles calculations

Using a combination of first-principles calculations based on density functional theory and Boltzmann semiclassical transport theory, we compute and study the properties of pristine layered calcium cobaltite ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$. We model the system with the B1WC hybrid functional. Two supercells of increasing size which approximate the incommensurate crystallographic structure of the compound are studied and we determine their structural, magnetic, and electronic properties. It is found that the B1WC hybrid functional is appropriate to reproduce the structural, electronic, and magnetic properties, which are then extensively discussed. From the electronic band structure, the Seebeck $(S)$ and electrical resistivity $(\ensuremath{\rho})$ tensors are computed using Boltzmann transport theory within the constant relaxation-time approximation. The differences between the diagonal components are detailed and reveal a strong in-plane anisotropy of the properties. The qualitative behavior of the averaged in-plane properties, ${S}_{//}$ and ${\ensuremath{\rho}}_{//}$, is consistent with the measurements reported in the literature. Our calculation clarifies and provides a broad picture of the evolution of the thermoelectric properties with both carrier density and temperature, and suggests that the change in ${S}_{//}$ and ${\ensuremath{\rho}}_{//}$ around 100 K is not necessarily related to the magnetic transitions occurring around 100 K.

Original positively charged nanoflakes by liquid exfoliation of layered oxybromide cobaltites

For the synthesis of novel positively charged 2D-elementary blocks, we have focused on the exfoliation of recent mixed valent Co3+/4+ oxy-bromides, namely the 14H-Ba7Co6BrO17 and 18R-Ba6Co5BrO14 phases built on hexagonal-perovskite blocks separated by [Ba2O2Br]− spacers. Here, we show that the exfoliation in butanol leads to positive nanoblocks in colloidal suspensions, as confirmed Zetametry measurements. Full structural characterization was performed by TEM, EDS, Raman and IR spectroscopies, AFM, X-ray absorption spectroscopy (XANES and DANES) and synchrotron X-ray diffraction (in-plane and out of plane). Combination of all these techniques results into a detailed description of crystalline flat particles (∼100 nm large and ∼10 nm thick) where the Co3+/4+ mixed oxidation state, as well as the local Co environment, are preserved after the exfoliation process. In-plane and out of plane synchrotron X-ray diffraction confirms a strong preferred deposition (001) planes. The original bulk materials showing intrinsic ferromagnetism in the elementary blocks, the magnetic properties of the delaminated nanoparticles are also presented here. Finally, we have prepared new positive nanometric bricks based on hexagonal perovskite structure, rare objects in condensed matter.

Pressure-induced abnormal insulating state in triangular layered cobaltite LixCoO2 (x = 0.9)

Lithium cobaltite oxides (LixCoO2) have been serving as an important rechargeable battery material with reversible extraction and insertion of lithium ions.

Growth and characterization of textured YBaCo2O5 + δ thin films grown on (001)-SrTiO3 via DC magnetron sputtering

Thin films of the layered cobaltite YBaCo2O5+δ were successfully grown on (001)-oriented SrTiO3 single-crystal substrates by means of DC magnetron sputtering. The 112 phase of the compound was stabilized by choosing an adequate Co reactant and through careful thermal treatment of the target. The results demonstrate the strong influence of these variables on the final phase of the compound. A substrate temperature of 1053K and an oxygen pressure of 300Pa seemed to be appropriate growing conditions for depositing (00ℓ)-textured YBaCo2O5+δ thin films onto the chosen substrate. In like fashion to the polycrystalline YBaCo2O5+δ, the films showed a clear sequence of antiferromagnetic–ferromagnetic–paramagnetic transitions within a narrow temperature range. Well-defined hysteresis loops were observed at temperatures as high as 270K, which supports the existence of a FM order in the films. In turn, the dependence of the resistivity on the temperature shows a semiconductor-like behavior, without any distinguishable structure, within the temperature range measured (50–350K). The analysis of the experimental data showed that the transport mechanism in the films is well described by using the Mott variable range hopping (VRH) conduction model.

Synthesis of Layered Calcium Cobaltites Intended for Thermolectric Application

Layered misfit cobaltites exhibit adequate combination of electrical and thermal properties under the high temperatures. Cobaltites are due to these properties and appropriate costs optimal candidates for thermoelectric industrial applications [1]. Calcium cobaltites were prepared via conventional and less conventional synthesizing methods. Wet citric and glycine/nitrate combustion syntheses were compared with conventional solid state reaction. Prepared powders were studied in terms of phase and chemical composition, morphology, size of particles/agglomerates and thermoelectric properties. Particle size of prepared powders was varied from nano to micro size with different level of agglomeration. The lowest values of particle size (~ 160 nm) with low agglomeration were obtained with the citric combustion method. The influence of the preparation method on the properties of final bulk product was evaluated within this work.

Thermal properties of the Nd1−xCaxBaCo2O5.5 compositions (0 ≤ x ≤ 0.2)

Layered cobaltites RBaCo2O5.5, considered for application as cathodes of fuel cells, exhibit a rich spectrum of magnetic and electronic properties. Taking advantage of the fact that Nd3+ and Ca2+ ions have nearly identical ionic radii, by synthesizing the Nd1−xCaxBaCo2O5.5 compounds (for x=0, 0.02, 0.06, 0.08, 0.16, and 0.2), a hole doping was realized, without significant disturbing the crystalline structure and the ordering of oxygen vacancies. In order to study the influence of the hole doping on thermal properties of these compositions, specific heat studies were performed over the temperature range from 2 to 395 K. The main, i.e., lattice, magnon and Schottky, contributions to the specific heat were separated from the total specific heat measured and described theoretically. In particular, the lattice contribution was described by combining the Debye and the Einstein models, whereas the magnon specific heat was described in frames of a model developed for anisotropic magnetic materials (A. I. Akhiezer et al., Sov. Phys. Usp. 3 (1961) 567). Changes of width and height of a specific heat anomaly accompanying the insulator-metal phase transition, appearing unmonotonously as a function of x, were ascribed to small deviations of the oxygen content from the assumed stoichiometry and to non-uniformity of the oxygen content over the sample volume. Smearing of specific heat anomalies related to magnetic phase transitions was found and attributed tentatively to disorder introduced by nonuniform distribution of the calcium ions within (Nd,Ca)–O planes. An impact of the calcium substitution on the position of the Schottky anomaly, related to thermal excitations of the Nd3+ ions, was found and interpreted qualitatively as a side effect of the hole doping, which causes also deformations of the crystalline structure and affects the magnetic structure of the Co sublattice.

Effect of cobalt deficiency on the structural phase transition in EuBaCo2–x O6–δ

The effect of cobalt deficiency on the structural phase transition in the double layered cobaltite EuBaCo2–xO6–δ has been investigated using differential scanning calorimetry and high-temperature X-ray diffraction. It has been shown that, upon introduction of 5% cobalt vacancies, the energy and temperature of the phase transition from the orthorhombic unit cell (space group Pmmm) to the tetragonal unit cell (space group P4/mmm) decrease by 6% and 11°C, respectively, as compared to the corresponding values typical of stoichiometric EuBaCo2O6–δ. A model of the formation of cobalt–oxygen vacancies has been proposed.