Monoclinic phase of the misfit-layered cobalt oxide (Ca0.85OH)1.16CoO2

A monoclinic phase of the misfit-layered cobalt oxide (Ca_0.85OH)_1.16CoO₂ was successfully synthesized and characterized. It was found that this new material is a poly-type phase of the orthorhombic form of (CaOH)_1.14CoO₂, recently discovered by the present authors. Both the compounds consist of two interpenetrating subsystems: CdI₂-type CoO₂ layers and rock-salt-type double-atomic-layer CaOH blocks. However, these two phases exhibit a different stacking structure. By powder X-ray and electron diffraction (ED) studies, it was found that the two subsystems of (Ca_0.85OH)_1.16CoO₂ have c-centered monoclinic Bravais lattices with common a=4.898 Å, c=8.810 Å and β=95.8° lattice parameters, and different b parameters: b₁=2.820 Å and b₂=4.870 Å. Chemical analyses revealed that the monoclinic phase has a cobalt valence of +3.1-3.2. Resistivity of the monoclinic phase is approximately 10¹-10⁵ times lower than that of the orthorhombic phase. This suggests that the monoclinic phase is a hole-doped phase of the insulating orthorhombic phase. Furthermore, large positive Seebeck coefficients (∼100 μV/K) were observed near room temperature.     

Crystal structure of the parent misfit-layered cobalt oxide [Sr2O2]qCoO2

The crystal structure of our newly discovered Sr-Co-O phase is investigated in detail through high-resolution electron microscopy (HREM) techniques. Electron diffraction (ED) measurement together with energy dispersive X-ray spectroscopy (EDS) analysis show that an ampoule-synthesized sample contains an unknown Sr-Co-O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co=1. From HREM images a layered structure with a regular stacking of a CdI₂-type CoO₂ sheet and a rock-salt-type Sr₂O₂ double-layered block is observed, which confirms that the phase is the parent of the more complex “misfit-layered (ML)” cobalt oxides of [M_mA₂O_m+2]_qCoO₂ with the formula of [Sr₂O₂]_qCoO₂, i.e. m=0. It is revealed that the misfit parameter q is 0.5, i.e. the two sublattices of the CoO₂ sheet and the Sr₂O₂ block coexist to form a commensurate composite structure. We propose a structural model with monoclinic P2₁/m symmetry, which is supported by simulations of ED patterns and HREM images based on dynamical diffraction theory.     

Synthesis, structure and magnetic properties of Na6Co2O6

Na₆Co₂O₆ was synthesized via the azide/nitrate route by reaction between NaN₃, NaNO₃ and Co₃O₄. Stoichiometric mixtures of the starting materials were heated in a special regime up to 500°C and annealed at this temperature for 50 h in silver crucibles. Single crystals have been grown by subsequent annealing of the reaction product at 500°C for 500 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structure (, Z=1, a=5.7345(3), b=5.8903(3), c=6.3503(3) Å, α=64.538(2), β=89.279(2), γ=85.233(2)°, 1006 independent reflections, R₁=8.34% (all data)), cobalt is tetrahedrally coordinated by oxygen. Each two CoO₄ tetrahedra are linked through a common edge forming Co₂O₆^6- anions. Cobalt ions within the dimers, being in a high spin state (S=2), are ferromagnetically coupled (J=17 cm^-1). An intercluster spin exchange (zJ′=−4.8 cm^-1) plays a significant role below 150 K and leads to an antiferromagnetically ordered state below 30 K. Heat capacity exhibits a λ-type anomaly at this temperature and yields a value of 19.5 J/mol K for the transition entropy, which is in good agreement with the theoretical value calculated for the ordering of the ferromagnetic-coupled dimers. In order to construct a model for the spin interactions in Na₆Co₂O₆, the magnetic properties of Na₅CoO₄ have been measured. This compound features isolated CoO₄ tetrahedra and shows a Curie-Weiss behavior (μ=5.14 μ_B, Θ=−20 K) down to 15 K. An antiferromagmetic ordering is observed in this compound below 10 K.     

Structural and magnetic properties of the Kagomé antiferromagnet YbBaCo4O7

The mixed-valent compound YbBaCo₄O₇ is built up of Kagomé sheets of CoO₄ tetrahedra, linked in the third dimension by a triangular layer of CoO₄ tetrahedra in an analogous fashion to that found in the known geometrically frustrated magnets such as pyrochlores and SrCr_9xGa_12−9xO₁₉ (SCGO). We have undertaken a study of the structural and magnetic properties of this compound using combined high-resolution powder neutron and synchrotron X-ray diffraction. YbBaCo₄O₇ undergoes a first-order trigonal→orthorhombic phase transition at 175 K. We show that this transition occurs as a response to a markedly underbonded Ba²⁺ site in the high-temperature phase and does not appear to involve charge ordering of Co²⁺/Co³⁺ ions in the tetrahedra. The symmetry lowering relieves the geometric frustration of the structure, and a long-range-ordered 3-D antiferromagnetic state develops below 80 K.     

Structure and properties of a novel cobaltate La0.30CoO2

The layered cobaltate La_0.30CoO₂ was prepared from Na_xCoO₂ precursor by a solid-state ionic exchange and was characterized by means of X-ray and neutron diffraction, magnetic, thermal and electric transport measurements. The compound consists of hexagonal sheets of edge-sharing CoO₆ octahedra interleaved by lanthanum monolayers. Compared to Na⁺ in the parent system, the La³⁺ ions occupy only one-third of available sites, forming a 2-dimensional superstructure. The deviation from the ideal stoichiometry La_1/3CoO₂ introduces extra hole carriers into the diamagnetic LS Co³⁺ matrix making the sample Pauli paramagnetic. The temperature dependence of the electrical conductivity in La_0.30CoO₂ follows Mott's T^−1/3 law up to about 400 K, which is in contrast with the standard metallic behavior in the Na⁺ homolog possessing the same formal doping. The experiments are complemented by electronic structure calculations for La_0.30CoO₂ and related Na_xCoO₂ systems.     

Magnetic and thermoelectric properties of layered LixNayCoO2

The magnetic, thermoelectric, and structural properties of Li_xNa_yCoO₂, prepared by intercalation and deintercalation chemistry from the thermodynamically stable phase Li_0.41Na_0.31CoO₂, which has an alternating Li/Na sequence along the c-axis, are reported. For the high Li-Na content phases Li_0.41Na_0.31CoO₂ and Li_0.40Na_0.43CoO₂, a sudden increase in susceptibility is seen below 50 K, whereas for Li_0.21Na_0.14CoO₂ an antiferromagnetic-like transition is seen at 10 K, in spite of a change from dominantly antiferromagnetic to dominantly ferromagnetic interactions with decreasing alkali content. The Curie constant decreases linearly with decreasing alkali content, at the same time the temperature-independent contribution to the susceptibility increases, indicating that as the Co becomes more oxidized the electronic states become more delocalized. Consistent with this observation, the low alkali containing phases have metallic-like resistivities. The 300 K thermopowers fall between 30 μV/K (x+y=0.31) and 150 μV/K (x+y=0.83).     

Investigations of the magnetic properties and structures of the pillared perovskites, La5Re3MO16 (M=Co, Ni)

La₅Re₃CoO₁₆ and La₅Re₃NiO₁₆ were synthesized by solid-state reaction and studied by SQUID magnetometry, heat capacity and powder neutron diffraction measurements. These two compounds belong to a series of isostructural Re-based pillared perovskites [Chi et al. J. Solid State Chem. 170 (2003) 165]. Magnetic susceptibility measurements indicate apparent short-range ferri or ferromagnetic correlations and possible long-range antiferromagnetic order for La₅Re₃CoO₁₆ at 35 K, and at 38 and 14 K for La₅Re₃NiO₁₆. Heat capacity measurements of the Co compound show a lambda anomaly, typical of long-range magnetic order, at 32 K. In contrast, the Ni compound displays a broader, more symmetric feature at 12 K in the heat capacity data, indicative of short-range magnetic order. Low-temperature powder neutron diffraction revealed contrasting magnetic structures. While both show an ordering wave vector, k=(0,0,1/2), in La₅Re₃CoO₁₆, the Co²⁺ and Re⁵⁺ moments are ordered ferrimagnetically within the corner-shared octahedral layers, while the layers themselves are coupled antiferromagnetically along the c-axis, as also found in La₅Re₃MnO₁₆ and La₅Re₃FeO₁₆. In the case of the Ni material, the Re⁵⁺ and Ni²⁺ moments in the perovskite layers couple ferromagnetically and are canted 30° away from the c-axis, angled 45° in the ab-plane. The layers then couple antiferromagnetically at low temperature, a unique magnetic structure for this series. The properties of the La₅Re₃MO₁₆ series, with M=Mn, Fe, Co, Ni and Mg are also reviewed.     

Structure and properties of the CaFe2O4-type cobalt oxide CaCo2O4

The calcium cobalt oxide CaCo₂O₄ was synthesized for the first time and characterized from a powder X-ray diffraction study, measuring magnetic susceptibility, specific heat, electrical resistivity, and thermoelectric power. CaCo₂O₄ crystallizes in the CaFe₂O₄ (calcium ferrite)-type structure, consisting of an edge- and corner-shared CoO₆ octahedral network. The structure of CaCo₂O₄ belongs to an orthorhombic system (space group: Pnma) with lattice parameters, a=8.789(2) Å, b=2.9006(7) Å and c=10.282(3) Å. Curie-Weiss-like behavior in magnetic susceptibility with the nearly trivalent cobalt low-spin state (Co³⁺, 3d, S=0), semiconductor-like temperature dependence of resistivity (ρ=3×10⁻¹ Ω cm at 380 K) with dominant hopping conduction at low temperature, metallic-temperature-dependent large thermoelectric power (Seebeck coefficient: S=+147 μV/K at 380 K), and Schottky-type specific heat with a small Sommerfeld constant (γ=4.48(7) mJ/Co mol K²), were observed. These results suggest that the compound possesses a metallic electronic state with a small density of states at the Fermi level. The doped holes are localized at low temperatures due to disorder in the crystal. The carriers probably originate from slight off-stoichiometry of the phase. It was also found that S tends to increase even more beyond 380 K. The large S is possibly attributed to residual spin entropy and orbital degeneracy coupled with charges by strong electron correlation in the cobalt oxides.     

Electron microscopy, spectroscopy, and first-principles calculations of Cs2O

Oxides of cesium play a key role in ameliorating the photoelectron emission of various opto-electronic devices. However, due to their extreme reactivity, their electronic and optical properties have hardly been touched upon. With the objective of better understanding the electronic and optical properties of Cs₂O in relationship to its structure, an experimental and theoretical study of this compound was undertaken. First-principles density functional theory calculations were performed. The preferred structural motif for this compound was found to be anti-CdCl₂. Here three Cs-O-Cs molecular layers are stacked together through relatively weak van-der-Waals forces. The energy bands were also calculated. The lowest transition at 1.45 eV, was found to be between the K point in the valence band to the Γ point in the conduction band. A direct transition at 2 eV was found in the center (Γ) of the Brillouin zone. X-ray powder diffraction, transmission electron microscopy and selected area electron diffraction were used to analyze the synthesized material. These measurements showed good agreement with the calculated structure of this compound. Absorption measurements at 4.2 K indicated two optical transitions with somewhat higher energy (indirect one at 1.65 and a direct transition at 2.2 eV, respectively). Photoluminescence measurements also showed similar transitions, suggesting that the lower indirect transition is enhanced by three nearby minima at 1.5 eV in the Brillouin zone.     

Three Forms of the Misfit Layered Cobaltite [Ca2CoO3] [CoO2]1.62·A 4D Structural Investigation

Three misfit layer compounds with the same chemical formula Ca₃Co_4−xO_9+δ were isolated in the Ca–Co–O system. They exhibit either a monoclinic or an orthorhombic symmetry. These crystals are constituted of two interpenetrating C sublattices showing incommensurate periods along the b axis. The structure of the three crystals can be described as an alternate stacking along [001] of distorted rock-salt-type slabs [Ca₂CoO₃] and of [CoO₂] layers displaying a distorted CdI₂-type structure. Relating to the symmetry and the different observed c periods, different sequences are found for the [CoO₂] layers running along [001] within the three crystals. Two of them were determined by single X-ray diffraction using the 4D superspace formalism. A significant displacive modulation is implied, acting mainly on Ca and O atoms involved in the intersystem bonding scheme. This modulation leads to a noticeable distortion of the CoO₆ octahedra of the [CoO₂] layers. Strong interactions, via Ca–O bonds, are evidenced between the two sublattices. A systematic positional disorder is observed inside the [CoO] layer.     

Crystal structure and magnetic properties of Co2TeO3Cl2 and Co2TeO3Br2

The crystal structures of the two new synthetic compounds Co₂TeO₃Cl₂ and Co₂TeO₃Br₂ are described together with their magnetic properties. Co₂TeO₃Cl₂ crystallize in the monoclinic space group P2₁/m with unit cell parameters a=5.0472(6) Å, b=6.6325(9) Å, c=8.3452(10) Å, β=105.43(1)°, Z=2. Co₂TeO₃Br₂ crystallize in the orthorhombic space group Pccn with unit cell parameters a=10.5180(7) Å, b=15.8629(9) Å, c=7.7732(5) Å, Z=8. The crystal structures were solved from single crystal data, R=0.0328 and 0.0412, respectively. Both compounds are layered with only weak interactions in between the layers. The compound Co₂TeO₃Cl₂ has [CoO₄Cl₂] and [CoO₃Cl₃] octahedra while Co₂TeO₃Br₂ has [CoO₂Br₂] tetrahedra and [CoO₄Br₂] octahedra. The Te(IV) atoms are tetrahedrally [TeO₃E] coordinated in both compounds taking the 5s² lone electron pair E into account. The magnetic properties of the compounds are characterized predominantly by long-range antiferromagnetic ordering below 30 K.     

Structural changes of NaxCoO2 (x=0.74) at high pressures

Structural changes in the layered compound γ-Na_xCoO₂ (x=0.74) are studied by in situ Raman scattering and energy-dispersive X-ray diffraction methods at pressures up to 41 GPa. The pressure dependence of the lattice parameters indicate that γ-Na_xCoO₂ has a strong anisotropic compressibility before 15 GPa and the unit cell is easily compressed between layers. The discontinuity of the lattice parameters and Raman observations reveal that a phase transition occurred at pressures between 10 and 12 GPa. The high-pressure phase has the same hexagonal symmetry and the phase transition may be due to the pressure-induced rearrangement of one of the Na cations in the unit cell.     

Dielectric relaxation in a new double perovskite oxide Ho2MgZrO6

A new double perovskite oxide holmium magnesium zirconate Ho₂MgZrO₆ (HMZ) was prepared by solid state reaction technique. The crystal structure has been determined by powder X-ray diffraction which shows monoclinic phase at room temperature with cell parameters a = 9.3028 ± 0.0030 Å, b = 5.2293 ± 0.0008 Å, c = 4.4009 ± 0.0009 Å, β = 103.3746 ± 0.0166°. An analysis of complex permittivity with frequency was carried out assuming a distribution of relaxation times. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. At the high temperature range, conductivity data satisfy the variable range hopping (VRH) model. In this regime, the conductivity of sample obeys Mott’s T^1/4 law, characteristic of VRH. High temperature data indicates the formation of thermally activated small polarons. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures.     

Low temperature synthesis of bilayer hydrated cesium cobalt oxide

Bilayer hydrated Na_0.35CoO₂·1.3H₂O structure has re-directed superconductivity research in recent years. Here, we develop a low temperature synthesis method to prepare a novel hydrous Cs_0.2CoO₂·0.63H₂O compound in one step. The bilayer-hydrate of Cs_0.2CoO₂·0.63H₂O with a greatest interlayer spacing d=10.0(2) Å among alkali cobalt oxides has been grown in crystal form. Magnetic susceptibility measurement of Cs_0.2CoO₂·0.63H₂O displays a paramagnetic behavior down to 1.9 K. With the assistance of low temperature molten CsOH solvent, crystals of Rb_0.30CoO₂·0.36H₂O and K_0.35CoO₂·0.4H₂O can be grown. The results provide the capability for preparing a novel hydrous structure and the systematic investigation of interlayer coupling effect of alkali ion insertion compounds.     

Synthesis and structural characterization of two cobalt phosphites: 1-D (H3NC6H4NH3)Co(HPO3)2 and 2-D (NH4)2Co2(HPo3)3

Two new cobalt phosphites, (H₃NC₆H₄NH₃)Co(HPO₃)₂ (1) and (NH₄)₂Co₂(HPO₃)₃ (2), have been synthesized and characterized by single-crystal X-ray diffraction. All the cobalt atoms of 1 are in tetrahedral CoO₄ coordination. The structure of 1 comprises twisted square chains of four-rings, which contain alternating vertex-shared CoO₄ tetrahedra and HPO₃ groups. These chains are interlinked with trans-1,4-diaminocyclohexane cations by hydrogen bonds. The 2-D structure of 2 comprises anionic complex sheets with ammonium cations present between them. An anionic complex sheet contains three-deck phosphite units, which are interconnected by Co₂O₉ to form complex layers. Magnetic susceptibility measurements of 1 and 2 showed that they have a weak antiferromagnetic interaction.     

A novel layered bimetallic phosphite intercalating with organic amines: Synthesis and characterization of Co(H2O)4Zn4(HPO3)6·C2N2H10

A new layered cobalt-zinc phosphite, Co(H₂O)₄Zn₄(HPO₃)₆·C₂N₂H₁₀ has been synthesized in the presence of ethylenediamine as the structure-directing agent. The compound crystallizes in the monoclinic system, space group Cc (No. 9), a=18.2090(8), b=9.9264(7), c=15.4080(7) Å, β=114.098(4)°, V=2542.3(2) Å³, Z=4, R=0.0323, wR=0.0846. The structure consists of ZnO₄ tetrahedra, CoO₆ octahedra and HPO₃ pseudopyramids through their vertices forming bimetallic phosphite layers parallel to the ab plane. Organic cations, which reside between the inorganic layers, are mobile and can be exchanged by NH₄⁺ cations without the collapse of the framework.     

Structural order and disorder in Co-based layered cuprates CoSr2(Y,Ce)sCu2O5+2s (s=1-3)

Crystal structures of a homologous series of Co-based layered cuprates, CoSr₂(Y,Ce)_sCu₂O_5+2s (s=1-3), have been investigated by high-resolution electron microscopy (HREM) and electron diffraction (ED) techniques. For all the three phases ED patterns showed double periodicity along a direction parallel to the CoO layers, indicating a regular alternation of two types of CoO₄-tetrahedra chains within the layers. Also seen was ordering of the chains along the layer-stacking direction for the s=1 phase (Co-1212); ED patterns simulated based on the proposed superstructure model well reproduced the observed patterns. For the s=2 (Co-1222) and s=3 (Co-1232) phases in which an additional fluorite-type layer-block is inserted between two CuO₂ planes, HREM and ED analysis revealed complete disorder of the CoO₄ chains along the layer-stacking direction. This implies that the interlayer ordering is mainly controlled by the distance between the neighboring CoO layers.     

Crystal Structure of a Lithium Ion-Conducting Perovskite La2/3−xLi3xTiO3 (x=0.05)

The crystal structure of a lithium ion-conducting perovskite La_0.62(2)Li_0.16(1)TiO_3.01(3) obtained by furnace-cooling was refined by the Rietveld method using neutron-diffraction data at room temperature and 77 K. The adopted space group was Cmmm (No. 65). The anti-phased tilts of TiO₆ octahedra along the b-axis in addition to alternative arrangement of La ions along the c-axis was confirmed. The position of Li was refined to be off-centered and in two equivalent positions in an A-site. The obtained structural information implies that the lithium ion conduction among A-sites occurs in the vicinity of La-poor layers two-dimensionally rather than three-dimensionally in this compound.     

Magnetic susceptibility of the one-dimensional conductor (H3O)1.6[Pt(C2O4)2]nH2O

The magnetic susceptibility of the dioxalatoplatinate acid (H₃O)_1.6[Pt(C₂O₄)₂]nH₂O, a mixed valency planar (MVP) compound with columnar structure and one-dimensional metallic conduction properties, has been measured in the temperature range 1.5–300°K. The observed paramagnetism, like in other MVP compounds, may be described by a linear $\text{1}\text{T}$ dependence with a break in the slope at a characteristic temperature. The Curie constants as well as the point of discontinuity have been found to depend on the crystal water content. The results are discussed in terms of different theoretical concepts.     

Synthesis and crystal structure of the palladium oxides NaPd3O4, Na2PdO3 and K3Pd2O4

NaPd₃O₄, Na₂PdO₃ and K₃Pd₂O₄ have been prepared by solid-state reaction of Na₂O₂ or KO₂ and PdO in sealed silica tubes. Crystal structures of the synthesized phases were refined by the Rietveld method from X-ray powder diffraction data. NaPd₃O₄ (space group Pm3¯n, a=5.64979(6) Å, Z=2) is isostructural to NaPt₃O₄. It consists of NaO₈ cubes and PdO₄ squares, corner linked into a three-dimensional framework where the planes of neighboring PdO₄ squares are perpendicular to each other. Na₂PdO₃ (space group C2/c, a=5.3857(1) Å, b=9.3297(1) Å, c=10.8136(2) Å, β=99.437(2)°, Z=8) belongs to the Li₂RuO₃-structure type, being the layered variant of the NaCl structure, where the layers of octahedral interstices filled with Na⁺ and Pd⁴⁺ cations alternate with Na₃ layers along the c-axis. Na₂PdO₃ exhibits a stacking disorder, detected by electron diffraction and Rietveld refinement. K₃Pd₂O₄, prepared for the first time, crystallizes in the orthorhombic space group Cmcm (a=6.1751(6) Å, b=9.1772(12) Å, c=11.3402(12) Å, Z=4). Its structure is composed of planar PdO₄ units connected via common edges to form parallel staggered PdO₂ strips, where potassium atoms are located between them. Magnetic susceptibility measurements of K₃Pd₂O₄ reveal a Curie-Weiss behavior in the temperature range above 80 K.