Rock Salt-type Layers Research Articles

Exploring the High-Temperature Electrical Performance of Ca3−xLaxCo4O9 Thermoelectric Ceramics for Moderate and Low Substitution Levels

Aliovalent substitutions in Ca3Co4O9 often result in complex effects on the electrical properties and the solubility, and impact of the substituting cation also depends largely on the preparation and processing method. It is also well-known that the monoclinic symmetry of this material’s composite crystal structure allows for a significant hole transfer from the rock salt-type Ca2CoO3 buffer layers to the hexagonal CoO2 ones, increasing the concentration of holes and breaking the electron–hole symmetry from the latter layers. This work explored the relevant effects of relatively low La-for-Ca substitutions, for samples prepared and processed through a conventional ceramic route, chosen for its simplicity. The obtained results show that the actual substitution level does not exceed 0.03 (x < 0.03) in Ca3−xLaxCo4O9 samples with x = 0.01, 0.03, 0.05 and 0.07 and that further introduction of lanthanum results in simultaneous Ca3Co4O9 phase decomposition and secondary Ca3Co2O6 and (La,Ca)CoO3 phase formation. The microstructural effects promoted by this phase evolution have a moderate influence on the electronic transport. The electrical measurements and determined average oxidation state of cobalt at room temperature suggest that the present La substitutions might only have a minor effect on the concentration of charge carriers and/or their mobility. The electrical resistivity values of the Ca3−xLaxCo4O9 samples with x = 0.01, 0.03 and 0.05 were found to be ~1.3 times (or 24%) lower (considering mean values) than those measured for the pristine Ca3Co4O9 samples, while the changes in Seebeck coefficient values were only moderate. The highest power factor value calculated for Ca2.99La0.01Co4O9 (~0.28 mW/K2m at 800 °C) is among the best found in the literature for similar materials. The obtained results suggest that low rare-earth substitutions in the rock salt-type layers can be a promising pathway in designing and improving these p-type thermoelectric oxides, provided by the strong interplay between the mobility of charge carriers and their concentration, capable of breaking the electron–hole symmetry from the conductive layers.

Hydration effect on properties of the La2-xAxNi1-yFeyO4+δ (A=Ca, Sr) cathode materials for H+-SOFCs

The effect of air humidity on the transport properties of the La2-xAxNi1-yFeyO4+δ (A=Ca, Sr; x = 0.5, 0.6 and y = 0.4, 0.5) oxides was studied in the temperature range of 25–950 °C. Total conductivity of the samples decreased with increasing relative humidity (RH), most significantly in the range of 250–550 °C. Thermogravimetric analysis in dry and wet air indicated that the observed changes in conductivity could be attributed to the hydration of the samples. The high-temperature X-ray powder diffraction (HT-XRPD) results for La1.4Sr0.6Ni0.5Fe0.5O4+δ (LSNF0605) suggested that the hydration led to the insertion of hydroxide ions into the interstitial sites resulting in the expansion of the rock-salt type layers and shrinkage of the perovskite layers. The Electrochemical Impedance Spectroscopy (EIS) measurements for the LSNF0605/Ce0.8Sm0.2O2-δ (SDC) and LSNF0605/La28−zW4+zO54+1.5z (LWO) symmetrical cells showed that the hydration affected the surface exchange process in LSNF0605. The activation energy of surface exchange decreased with the increase in RH for the LSNF0605/LWO cell indicating that water could participate in the surface exchange. The activation energy of the charge transfer – ionic diffusion process in the LSNF0605 electrode for the LSNF0605/LWO cell possessed smaller values compared to that for the LSNF0605/SDC cell in wet air, suggesting the presence of proton conduction.

The fourfold superstructure in Li3Sb11S18

Li3Sb11S18 was synthesized by a solid-state reaction of stoichiometric amounts of Li2S and Sb2S3. The crystal structure was determined from dark-gray single crystals at 123 K. Li3Sb11S18 crystallizes in the triclinic space group $$ P\bar{1} $$ with a = 8.871(1) A, b = 11.993(1) A, c = 15.407(1) A, α = 80.014(5)°, β = 73.599(5)°, γ = 77.443(5)°, V = 1523.9(2) A3, and Z = 2 (data at 123 K). Both metals have a distorted octahedral coordination by sulfur. These octahedra are arranged in two different types of alternating layers. Strands of trans edge sharing octahedra LiS6 and octahedra SbS6 form heterocubane units LiSb3S4, which are condensed to a rock salt type layer one. Layer two consists mainly of SbS6-units also packed in a rock salt motif. Some of the antimony atoms are replaced by lithium in a systematic way. In total, a deficient rock salt type structure results for Li3Sb11S18.

Thermoelectric Properties of Misfit Layered Bismuth-Based Rhodium Oxides, (Bi,Pb)<sub>2</sub>Sr<sub>2</sub>Rh<sub>2</sub>O<sub>y</sub>

Rhodium oxides, including a misfitlayered structure with alternate stacking of a rock salttype layer and a hexagonal RhO2 layer, are expected to have good thermoelectric properties. Among them, the thermoelectric properties (electrical conductivity (σ), Seebeck coefficient (S), Figure of merit (ZT) and calculated thermal conductivity (κ) by S, σ, ZT, and absolute temperature (T)) of bismuth-based rhodium oxides ((Bi1-x,Pbx) 2Sr2Rh2Oy, x = 0 and 0.02, hereafter BSR and BPSR, respectively) were investigated. In comparison with Bi2Sr2Co2Oy (BSC) at 700°C, S and κ enhanced (increased S, 110 (BSR) and 105 μV K-1 (BPSR) from 85 μV K-1 (BSC) and decreased κ, 0.32 (BSR) and 0.50 W m-1 K-1 (BPSR) from 1.75 W m-1 K-1 (BSC)), whereas σ decreased (15 (BSR) and 31 S cm-1 (BPSR) from 70 S cm-1 (BSC)). BPSR reached the highest ZT value of 0.067 at 700°C, compared to those of 0.056 (BSR) and 0.027 (BSC).

Structural Transition Behavior of LMO-NMC Composite Used for Cathode of Li Ion Battery at High Voltage Operation

LiMn2O4 (LMO)-LiNi1-x-yCoxMnyO2 (NCM) composite has been considered as one of the best cathode materials for Electric Vehicles (EVs) or Plug-in Hybrid Electric Vehicles (PHEVs) because of its long-life, high capacity and so on. However, its degradation mechanism has not been understood enough and researchers have attempted to clarify the mechanism. We clarified that degradation stored under a high-temperature condition forms some crystalline layer attributable to rock-salt type structure on the particle surface of active cathode material and this layer relates to the output fading.1 In addition, it is considered that this layer influences on the reaction between active cathode material and electrolyte because this layer exits on the surface of active cathode material. Therefore, in this study, we investigated the influence at high voltage operation to accelerate the reaction. For sample preparation, laminate type cells were assembled with a Li foil anode, an electrolyte (1 M LiPF6 ethyl carbonate/dimethyl carbonate/ethyl methyl carbonate=3/3/4) and the LMO-NCM cathode sampled from commercial LiB cell of EVs before and after degradation test, called “initial sample” and “after degradation sample”, respectively hereafter, (storage under 45° C and SOC100% for 900 days). While the laminate type cells were charged up to 6 V by CC-CV (CV time: 5 hr.), in-situ time-resolved X-ray diffraction measurement was carried out at synchrotron radiation facility SPring-8 (BL28XU, Hyogo, Japan) to observe electrochemically induced change of their crystal structure. The relationship between SOC and lattice constants of LMO and NCM is shown in Figure 1. For LMO, the lattice constant change was almost same in “initial sample” and “after degradation sample”. On the other hand, for NCM, the lattice constant change was different in “initial sample” and “after degradation sample”. That is to say, the lattice constant change of “initial sample” was larger than that of “after degradation sample” for NCM in more than SOC 600% particularly. However, in comparison with each current integration, the lattice constant of NCM remain nearly unchanged before and after the degradation. These results propose that the lattice constant of NCM has strong dependency on the current integration. In addition, gas chromatograph-mass spectrometer analysis of the gas and electrolyte revealed that decomposition of the used electrolyte occurred at 6 V. In order to understand the difference of reaction between “initial sample” and “after degradation sample”, the surface state of NCM particle and LMO-NCM composite electrode were characterized by transmission electron microscope and soft X-ray absorption spectroscopy. Interestingly, some crystalline layer attributable to rock-salt type structure was observed on the surface of the NCM particle of “after degradation sample” before and after charge to 6 V. Based on these experimental evidences, we can conclude that the charge current at high voltage operation is mainly consumed by the decomposition of the electrolyte and the structure change of NCM. In addition, the rock-salt type layer on the surface of NCM particle formed by degradation disturbs the reaction with electrolyte. This work was supported by Research and Development Initiative for Scientific Innovation of New Generation Batteries project from New Energy and Industrial Technology Development Organization in Japan. (1) T. Fujimoto, K. Kitada et al, Transactions of Society of Automotive Engineers of Japan, 45, 297(2014). Figure 1

Lattice Dynamics Modified by Excess Oxygen in Nd2NiO4+δ: Triggering Low-Temperature Oxygen Diffusion

Nonstoichiometric Nd2NiO4+δ shows oxygen ion mobility already at room temperature and as such is a promising material in the field of solid state ionic conductors. The present work aims to understand the impact of oxygen overstoichiometry on the lattice dynamics of the Nd2NiO4 framework and to correlate dynamic instabilities to the anionic mobility at room temperature. We performed neutron time-of-flight spectroscopy experiments coupled with DFT-based molecular dynamics calculations on the phases Nd2NiO4.0, Nd2NiO4.10, and Nd2NiO4.25. Specific signatures in the phonon density of states gave evidence that excess oxygen on interstitial lattice sites activates large displacements of the apical oxygen atoms along the [110] direction, thus favoring their diffusion toward interstitial sites within the rock salt-type layer already at ambient temperature. We implemented a position recurrence method to analyze apical oxygen displacements during the 40 ps time scale of molecular dynamics simulations. The results hi...

Resolution of a modulated structure by electron and powder X-ray diffraction

These past few years, many new structures have been solved using electron diffraction methods: zone axis precession electron diffraction (PED) and tomography in reciprocal space. Both methods enable to reduce importantly the multiple scattering, so that the reflection intensities can be used for structure determination by direct methods. The ferrite Sr25Fe30O77 belongs to a family of phases whose structures consist of an intergrowth of m perovskite layers with complex rocksalt type layers [1,2]. The compound of interest is the member m = 4 of this family and its structure has been solved by combining both electron diffraction methods cited above. This oxide crystallizes in an orthorhombic system with the sub-cell parameters a ≍ b ≍ 5.4 Å and c ≍ 42 Å in a F type lattice. The structure exhibits modulation along a axis with a modulation vector q = 2/5 a. The commensurate nature of the modulation enables to describe the structure in a supercell with the cell parameters a ≍ 27 Å, b ≍ 5.4 Å and c ≍ 42 Å. PED patterns were recorded in zone axis with a Spinning Star unit using a precession angle of 20. The intensities were extracted with CRISP software and the resulting dataset was then implemented in SIR2008 for structure solution. The tomography data collection, recorded by tilting manually every 0.5 degree from -30 to +30 degrees, was inserted in a "3D Electron Diffraction Tomography" software, which reconstructs the 3D reciprocal space and extracts automatically the reflection intensities. The intensity file was then used on SIR2008 for structure determination. In order to confirm and refine the structural model, a powder X-ray diffraction pattern was performed on a laboratory diffractometer with Cu Ka1 radiation. Cell parameters were refined with the WinPlotr and FullProf Softwares using both LeBail and Rietveld methods [3]. The structural model obtained with electron diffraction data was tried and confirmed as the correct structure by the Rietveld refinements.

Local structure determination of substitutional elements in Ca3Co4−xMxO9(M = Fe, Cr, Ga) using X-ray absorption spectroscopy

Ca3Co4O9 is one of the most promising thermoelectric oxide materials at high temperature. Its structure consists of two misfit layers: the CaOCoOCaO rocksalt-type (RS) layer and the CdI2-type CoO2 layer. In this paper, we reported the synthesis of single phase Ca3Co4−xMxO9 (where M = Fe, Cr, and Ga). Thermoelectric measurements showed the variation of thermoelectric properties depending on the charge state of the substitutional elements. We showed the direct evidence for the location of the substitutional elements in the system using an X-ray absorption spectroscopy technique. The extended X-ray absorption fine structure spectra were fitted with the models. Also, the X-ray absorption near edge structure spectra were compared with the simulated spectra from the first principle calculation. The data analysis show that for Fe and Cr substitution, the Fe and Cr atoms are more likely to be located in the RS layer rather than in the CoO2 as presumed previously. For the Ga case, Ga atoms accommodate both sites; still the majority stays in the RS layer. This is supported by the calculation of the total energy of the system, which showed that the total energy is lower when the substituted elements were located in the RS layer.

ChemInform Abstract: (Li/Ag)CoO2: A New Intergrowth Cobalt Oxide Composed of Rock Salt and Delafossite Layers.

AbstractA new lamellar (Li/Ag)CoO2 phase of alternating rock salt and delafossite type layers is obtained during (Li+,Na+)/Ag+ ion exchange of the OP4‐(Li/Na)CoO2 precursor as an intermediate to the final D4‐AgCoO2 delafossite.

Transition-Metal Oxides with Triangular Lattices: Generation of New Magnetic and Electronic Properties

The search for multifunctional materials as multiferroics to be applied in microelectronic or for new, chemically stable and nontoxic, thermoelectric materials to recover waste heat is showing a common interest in the oxides whose structures contain a triangular network of transition-metal cations. To illustrate this point, two ternary systems, Ba-Co-O and Ca-Co-O, have been chosen. It is shown that new phases with a complex triangular structure can be discovered, for instance, by introduction of Ga (3+) into the Ba-Co-O system to stabilize Ba 6Ga 2Co 11O 26 and Ba 2GaCo 8O 14, which both belong to a large family of compounds with formula [Ba(Co,Ga)O 3-delta] n [BaCo 8O 11]. In the latter, both sublattices contain triangular networks derived from the hexagonal perovskite and the spinel structure. Among the hexagonal perovskite, the Ca 3Co 2O 6 crystals give clear evidence where the coupling of charges and spins is at the origin of a magnetocapacitance effect. In particular, the ferrimagnetic to ferromagnetic transition, with a one-third plateau on the M( H) curve characteristic of triangular magnetism, is accompanied by a peak in the dielectric constant. A second class of cobaltites is the focus of much interest. Their 2D structure, containing CoO 2 planes isostructural to a CdI 2 slice that are stacked in an incommensurate way with rock salt type layers, is referred to misfit cobaltite. The 2D triangular network of edge-shared CoO 6 octahedra is believed to be responsible for large values of the Seebeck coefficient and low electrical resistivity. A clear relationship between the structuresincommensurability ratiosand the electronic properties is evidenced, showing that the charge carrier concentration can be tuned via the control of the ionic radius of the cations in the separating layers.

Ｒｕｄｄｌｅｓｄｅｎ‐Ｐｏｐｐｅｒ Ｃａｎ＋１（Ｍｎ１−ｘＶｘ）ｎＯ３ｎ＋１（ｎ＝２）化合物粉体の調製と焼結体の熱電特性

Ruddlesden-Popper type compounds, being presented as (ABO3)n-AO or An+1BnO3n+1 (n=1, 2, 3…∞), have a layer structure where perovskite layer ABO3 and rock salt type layer AO are alternately in place; Can+1(Mn1-xVx)nO3n+1 (0≤x≤1) belongs also to this compound. As CaO layer shows low thermal conductivity whereas CaMnO3 layer reveals high electrical conductivity and high Seebeck coefficient, therefore, Can+1(Mn1-xVx)nO3n+1 seems to become a promising module as a thermoelectric conversion material. However, there was no report concerning about the thermoelectric conversion characteristic of this compound. In this study we have investigated electrical conductivity, Seebeck coefficient, and thermal conductivity of Can+1(Mn1-xVx)nO3n+1 and discussed about the possibility of the application as thermoelectric conversion elements from dimensionless performance index ZT.

Double Modulation and Microstructure of the Thermoelectric Misfit Compound [Ca2-yLnyCu0.7+yCo1.3-yO4][CoO2]b1/b2 (Ln = Pr, Y and 0 ≤ y ≤ 1/3)

In the present paper, we present one route for increasing the copper content in the misfit cobaltites by studying the [Ca 2-y Ln y Cu 0.7+y Co 1.3-y O 4 ][CoO 2 ] b1/b2 (Ln = Pr,Y) series (Ln = magnetic Pr and nonmagnetic Y). We pay special attention to the modulated character of these compounds. The electron diffraction study shows a double modulation of the structure. The parameters of the two systems (rock salt and CdI 2 types) are a 1 = a 2 = 4.805(9) A, b 1 = 4.482(9) A, b 2 = 2.804(9) A, c 1 = c 2 = 12.75(3) A, and β 1 = β 2 = 93.7(2)°, v = b 1 /b 2 = 1.599(4). The first modulation vector q 1 * = 0.63b 1 * is associated with the misfit character of the compound, and the second one, q 2 * = 0.44a 1 * + 0.43c 1 *, is an intrinsic modulation of the rock salt layers. Accounting for this second modulation, the reconstruction of the reciprocal space leads the super-space group C2/m(1q0,α0γ)mm, with q 1 * = 0.63b 1 * and q 2 * = 0.44a* + 0.43c*. The intrinsic modulation in the double [(Co,Cu)O] 2 rock salt-type layer of a misfit cobaltite is shown for the first time by electron microscopy. Different twinning mechanisms are also characterized. The relationships with the strontium ferrites built up from four rock salt-related layers are discussed. An increase of the thermopower is obtained, as well with a magnetic (Pr) as a nonmagnetic (Y) cation, which shows that the enhancement is due to a valence effect rather than to a magnetic effect.

Thermoelectric Power In Misfit Cobaltites Ceramics: Optimization By Chemical Substitutions

The ceramics of misfit cobaltites form a large family of layer oxides which exhibit large values of Seebeck coefficient (S 300K ≥ 90μV.K -1 ). They can be divided in two categories depending on the number of the rocksalt-type (RS) layers, n = 3 or n = 4, separating the [CoO 2 ] conducting layers. From the comparison of their structural features, it is found that the S values are governed by the incommensurability ratio and/or the charge of the RS block. This is clearly evidenced by making chemical substitutions at the level of the RS block: isovalent but with cations of different ionic radius (r Ba 2+ > r Sr 2+ > r Ca 2+ ) modifying the incommensurability ratio or aliovalent substitutions for cobalt (Pb 4+ , Ti 4+ ) which keep unchanged the structural incommensurability but make the RS charge changing.

Defect Formation in La<sub>2</sub>Ni(M)O<sub>4+δ</sub> (M= Co, Cu): Modelling and Coulometric Titration Study

The p(O2)-T- diagrams of La2Ni1-xMxO4+ (M=Co and Cu, x= 0-0.20), determined by the coulometric titration technique at 923-1223 K in the oxygen partial pressure range 10-4 to 0.6 atm, can be adequately described by equilibrium processes of oxygen intercalation into the rock-salt type layers and hole localization on B-site cations forming 3+ oxidation states. For the hole activity, a non-ideal solution model taking into account the repulsion of p-type electronic charge carriers can be used. The electrostatic repulsion excludes occupation of nearest neighboring sites and leads to splitting of the energy levels for more distant sites. The affinity of Ni and Cu cations with respect to the hole localization is similar and cannot be statistically separated analyzing the oxygen nonstoichiometry data only. On the contrary, cobalt cations tend to remain in the trivalent state and Co3+ should be treated as a separate type of charged point defect. Oxygen vacancies formed in the perovskite-like layers due to intrinsic Frenkel disorder have no essential effect on the oxygen thermodynamics. As expected, the thermodynamic functions governing the intercalation-related processes are independent of defect concentrations.

Thermoelectric [Sr2(Bi,Co)n−2On]RS[CoO2]1.8 layer cobaltites: The role of the intergrowth between the n=3 and n=4 terms

The Bi–Sr–Co–O system has been investigated for small bismuth contents in order to study the formation of the misfit oxide series built up from the stacking of three rocksalt-type layers (n=3) with a single CdI2-type CoO2 hexagonal layer. Starting from nominal compositions, written as [Sr2Co1−xBixO3]RS[CoO2]1.8 (x=0.1, 0.2, 0.3, 0.4, and 0.5), samples have been synthesized and their structure has been analyzed by combining transmission electron microscopy techniques and powder x-ray diffraction data. For x≈0.5, intergrowths are clearly observed, made of a majority of triple (n=3) and a few quadruple (n=4) rocksalt-type layer-based misfit cobaltites. Transport measurements (2.5 up to 320 K) reveal that the Bi introduction reduces the resistivity, especially at room temperature. Furthermore, the thermopower (Seebeck coefficient S) of such intergrowth (S≈120μVK−1 at 300 K) is found to be of the same order as the one measured for the n=4 Bi–Sr cobaltite while the resistivity (ρ≈20mΩcm at 300 K) is significantly reduced in comparison with the same compound.

A new layered cobaltite (Ga 1/3Co 2/3) 2Sr 2CoO 6+δ with high spin Co 3+: modulated structure and physical properties

Investigations in the Sr–Co–Ga–O system by means of transmission electron microscopy techniques have allowed the detection of a new layered cobaltite. Its structure has been obtained by combining high-resolution images and powder X-ray diffraction. The modulated structure of this novel oxide, (Ga 1/3Co 2/3) 2Sr 2CoO 6+ δ is derived from the perovskite with a = a p √ 2 , b = a p √ 2 , c = 19.2 Å and a modulation vector q * = q 1 a * + q 2 c * . For the as-prepared samples, the δ value is close to 0.4 and the modulated structure ( q 1 ≈ 1 / 3 and q 2 = 1 ) can be described in an ideal orthorhombic 3D supercell Bb 2 b with the unit cell parameters a = 3 a p √ 2 = 16.3030 ( 5 ) Å , b = 5.4725 ( 2 ) Å and c = 19.2034 ( 4 ) Å . The layer stacking consists in an intergrowth between a [SrCoO 3] perovskite-type block and a block of triple layers, [(SrO)(Co 2/3Ga 1/3O 1+ δ /2)(Co 2/3Ga 1/3O 1+ δ /2)] derived from [AO] rock-salt type layers. The magnetic study shows that the Co 3+ cations exhibit a high spin state in this structure. The θ p value, −570 K, suggests that antiferromagnetic interactions dominate in this compound as in the brownmillerite cobaltites. Nonetheless, its resistivity ρ 300 K = 1 0 - 1 Ω cm is lower than that of the brownmillerite compounds. Since the positive Seebeck coefficient indicates the presence of holes (Co 4+) in the CoO 2 conducting layers, the existence of weak ferromagnetic Co 3+–O–Co 4+ interactions developing below ≈150 K is proposed to explain the electronic properties of this new oxide.

A New Thermoelectric Misfit Cobaltite: [Sr2CoO3][CoO2]1.8.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A new thermoelectric misfit cobaltite: [Sr 2CoO 3][CoO 2] 1.8

The study of the SrCoO system has allowed to isolate a new layered cobaltite [Sr 2CoO 3][CoO 2] 1.8. The structural analysis by transmission electron microscopy shows a composite structure consisting in two substructures related to triple rock–salt-type layers and single CdI 2-type layer, respectively. The structure of this compound is similar to that of the well-known misfit cobaltite [Ca 2CoO 3][CoO 2] 1.62, also called “Ca 3Co 4O 9”. The latter is intensively studied for thermoelectric power application to convert waste heat energy into electricity at temperature well above 300 K. A large thermoelectric power is also found for this new cobaltite [Sr 2CoO 3][CoO 2] 1.8, which exhibits a Seebeck coefficient such as S 300 K ≈110 μV K −1. This value, lower than that of [Ca 2CoO 3][CoO 2] 1.62 ( S 300 K ≈125 μV K −1), shows that the misfit character, quantified by the ratio b 1/ b 2=1.8 and 1.62 for the “Sr” and “Ca” based misfit cobaltites, respectively, is directly connected to the thermoelectric properties in these materials.

Fabrication of textured thermoelectric layered cobaltites with various rock salt-type layers by using β-Co(OH)2platelets as reactive templates

Textured bulk ceramics have been prepared of various cobaltites including the common CdI2-type CoO2 layer (i.e., [Ca2(Co0.65Cu0.35)2O4]0.624[CoO2]: CCO-4-layer, [Bi2M2−xO4]p[CoO2] (M2 = Sr2: BSCO, M2 = Ca1Sr1: B(SC)CO, M2 = Ca2: BCCO)) by using β-Co(OH)2 (CdI2-type) platelets as reactive templates, and their thermoelectric (TE) properties have been examined. All the specimens were found to be highly c-axis aligned ceramics. Especially for CCO-4-layer, B(SC)CO and BCCO, the present study is the first report on the fabrication of textured ceramics. Our textured ceramic specimens exhibited larger values of electrical conductivity (σ) than the ceramics prepared by a conventional solid-state reaction (SSR specimens): e.g., σab = 0.46 × 104 S m−1 for the textured specimen and σSSR = ∼0.26 × 104 S m−1 at 1060 K for the SSR specimen for BCCO. This study indicated that our preparation method using β-Co(OH)2 templates is a versatile and effective technique to fabricate cobaltites, including CoO2 layers, with a high degree of orientation and improved TE properties.

Metallicity and thermopower of the misfit cobaltite[Bi2Ba1.8Co0.2O4]RS[CoO2]2

The misfit cobaltite, [Bi 2 Ba 1 . 8 Co 0 . 2 O 4 ] R S [CoO 2 ] 2 , built up from the stacking of rocksalt-type layers (RS) and single hexagonal [CoO 2 ] layers (H), has been synthesized. This complex structure, described in a commensurate monoclinic supercell, exhibits a double commensurate modulation: a misfit modulation with b R S /b H =2 and an intrinsic modulation of the RS layers [q i =0.2(a R S +b R S )]. This oxide is characterized by an enhanced metallic behavior with respect to the misfit Bi(Pb)/Sr/Co/O cobaltites and in contrast to these compounds by a positive magnetoresistance and significantly smaller values of the thermoelectric power (<100 μV K - 1 ). A comparison of its behavior with those of Sr 2 RuO 4 and of the heavy-fermion-like oxide LiV 2 O 4 is presented.