Misfit Layer Compounds Research Articles

Determination of the modulated structure of the misfit layer compound (LaS) 1.196VS 2

Single crystal X-ray diffraction was used to determine the modulated structure of the misfit layer compound (LaS) 1.196VS 2. This compound crystallizes in the triclinic system with cell parameters: a s = 3.410 (1) Å, b s = 5.845(1) Å, c s = 11.191(2) Å, α ≈ 95.15(4), β ≈ 84.79(2)°, and γ ≈ 89.98(2)°, q = 0.5978(4) a s * − 0.002(1) b s * + 0.004(2) c s *, and V s = 221.2(1) Å 3. A (3 + 1)D superspace group, X 1 ¯ (α,β,γ), was used to analyze the complete structure ( X is referring to a pseudo C centering). The largest modulation amplitudes are observed for La–S (between La (subsystem 2) and S (subsystem 1)), as well as V–V distances. In connection with the large V–V modulation, we observed the formation of “linear vanadium clusters” that may impact on the transport properties.

Determination of the upper critical magnetic fields from fluctuation conductivity

The effect of fluctuations on the resistive transitions in magnetic field is analyzed in the frame work of the scaling procedure obtained theoretically by Ullah and Dorsey in two types of superconductors: in the cubic perovskite oxide Ba 1− x K x BiO 3 with a relatively high T c ∼ 23 K and in the low- T c but quasi two-dimensional misfit-layer compound (LaSe) 1.14(NbSe 2). In both cases the scaling is applicable in a significant range of magnetic fields around H c2. The temperature dependence of the upper critical magnetic fields H c2 determined from this analysis reveals a classical form for the type-II superconductor in contrast to a direct determination of H c2( T) from the resistive transitions but in a good agreement with independent determination by the non-dissipative methods.

Crystal structure and thermoelectric properties of the misfit-layered cobalt oxides

On the basis of the crystal chemistry and electronic structure of γ-Na x CoO 2, we explored many cobalt-based oxide systems and encountered the compound “Ca 3Co 4O 9”. Through the detailed structure analyses, the cobalt oxide has been revealed to be a misfit-layered compound, which consists of two interpenetrating monoclinic subsystems. The first subsystem [CoO 2] has a CdI 2-type triangular lattice while the second subsystem [Ca 2CoO 3] is built up from an ordered three-layered NaCl-type (RS) block. The structure formula can be expressed as [Ca 2CoO 3] p CoO 2, where p stands for the b-axis ratio, b CoO 2 / b RS∼0.62. A typical polycrystalline sample of the compound exhibits reasonable thermoelectric (TE) performance with thermopower S=130 μV K −1, resistivity=15 mΩ cm and thermal conductivity κ=1 W m −1 K −1 at 300 K. The resulting figure-of-merit Z ( S 2/ ρκ)=1.1×10 −4 K −1 is comparable to that of polycrystalline samples of the γ-phase, indicating that the present misfit cobalt oxide is a potential candidate for a thermoelectric material.

Preparation and characterization of intercalated misfit-layer compounds and natural superlattices

This paper presents the preparation and characterization of the new types of misfit-layer compounds and natural superlattices consisting of Ce x Nb 1− x− y □ y S (Q′) and Ni z NbS 2 (H) layers, where □ is an atomic deficit at a metal site. A Q′ layer is larger by about 1 Å in thickness than a CeS (Q) layer in (CeS) 1.16NbS 2. They are prepared by chemical vapor transport reaction in a closed silica tube under quasi-equilibrium conditions. The 1Q′/3H type of compound are grown as a single crystal while the 1Q′/4H type of compound is grown as composite crystals with the 1Q′/3H and 1Q/2H compounds. Natural superlattices which have a long period in a direction perpendicular to layers are found. Their chemical formulae are given by (Ce x Nb 1− x− y □ y S) m (Ni z ) n− m (NbS 2) n , where m and n are integers. It is found from an X-ray-photoelectron spectroscopy (XPS) study that Nb affects the valence and the bonding of Ce in the Q′ layers. It is in a higher oxidation state than Nb in NbS 2 layers. A scanning tunneling microscope (STM) study shows that some of the superlattices form a hexagonal 3 a 0× 3 a 0R30° supercell in the ( a, b)-plane and behave as a narrow-gap semiconductor so that no STM images are obtained at bias voltage less than 0.3 eV.

Nonadditivity of Properties and the Possibility of Formation of Ensembles of Incommensurate Structures in the GaSe–Ga2Se3System

The GaSe–Ga2Se3 system is studied by x-ray diffraction, electron microscopy, and differential scanning calorimetry. The results demonstrate that the nonadditivity of heat capacity observed near the eutectic composition after quenching is similar to the nonadditivity of properties in misfit layer compounds, which contain incommensurate structural components. The origin of this similarity is discussed in terms of the self-organization of interacting incommensurate elements in the melt and the formation of unstable suprastructural ensembles.

Synthesis of SnS2/SnS fullerene-like nanoparticles: a superlattice with polyhedral shape.

Tin disulfide pellets were laser ablated in an inert gas atmosphere, and closed cage fullerene-like (IF) nanoparticles were produced. The nanoparticles had various polyhedra and short tubular structures. Some of these forms contained a periodic pattern of fringes resulting in a superstructure. These patterns could be assigned to a superlattice created by periodic stacking of layered SnS(2) and SnS. Such superlattices are reminiscent of misfit layer compounds, which are known to form tubular morphologies. This mechanism adds up to the established mechanism for IF formation, namely, the annihilation of reactive dangling bonds at the periphery of the nanoparticles. Additionally, it suggests that one of the driving forces to form tubules in misfit compounds is the annihilation of dangling bonds at the rim of the layered structure.

Mixed‐Valence State of Europium in the Misfit Layer Compound (EuS)1.173NbS2.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Charge transfer in misfit layered compounds

In order to understand binding mechanisms and the resulting stability of misfit layered compounds, the valence bands of (PbS) 1.18TiS 2 and (PbS) 1.18(TiS 2) 2 were mapped along the ΓM(AL) direction in reciprocal space parallel to the surface. The results were compared to the experimental and the theoretical band structure of pure TiS 2. The changes in the electronic structure were analyzed with regard to the effects that are known to occur in intercalated transition metal dichalcogenides and that can directly be attributed to a charge transfer from the intercalated species to the host material. The theoretical determination of the densities of states for the pure TiS 2 and the misfit layered compound (PbS) 1.18TiS 2 confirms a charge transfer.

Mixed-Valence State of Europium in the Misfit Layer Compound (EuS)1.173NbS2

A new misfit layer compound (EuS)1.173NbS2 was synthesized, and its structure was solved in a (3+1)D superspace using the superspace group Xm2m(α00) with X referring to a pseudo F centering. The ce...

A 4D Composite Description for Ca0.82(Cu0.65Co0.35O2)

A misfit layer compound has been isolated in the Ca−Cu−Co−O system. It is constituted of two interpenetrating orthorhombic F sublattices exhibiting incommensurate periods along the [100] direction....

Crystal Structures and Magnetic Properties of the Two Misfit Layer Compounds: [SrGd 0.5S 1.5] 1.16 NbS 2 and [Sr(Fe,Nb) 0.5S 1.5] 1.13 NbS 2

Crystal structures of two new misfit compounds, [SrGd 0.5S 1.5] 1.16NbS 2 and [Sr(Fe,Nb) 0.5S 1.5] 1.13NbS 2, were determined through the composite approach, i.e., by refining each subpart ( Q, H-parts, and the common part) of these composite materials, separately. The Q-part is a three-atom-thick layer, with the NaCl-type structure, where external SrS planes enclose the inner GdS or (Fe,Nb)S plane; the structural difference between these two compounds lies in the central layer within the Q-part: Gd and S atoms are in special positions (octahedral coordination), while Fe and S atoms are statistically distributed on split (×4) positions (tetrahedral coordination) around a central unique site (=special position occupied by Nb). The H-part is a sandwich of sulfur planes enclosing the inner Nb plane as observed for the structure of the binary compound NbS 2 itself. The Sr–Gd derivative shows a paramagnetic behavior in the whole studied temperature range (2–300 K). On the other hand, antiferromagnetic interactions occur in the Sr–Fe derivative; the complex magnetic behavior of this compound is related to the statistical distribution of Fe atoms which leads to frustration of the magnetic interactions. At room temperature, experimental values obtained from Mössbauer spectrum correspond to Fe 3+ in tetrahedral sulfur environment: isomer shift δ=0.32 mm s −1, and quadrupole splitting Δ E=0.48 mm s −1.

Photoelectron spectra and surface valence fluctuation of Eu in the misfit-layer compound {(EuS) 1.15} 1.5NbS 2

The 1.5Q/1 H type of the misfit layer compound {(EuS) 1.15} 1.5NbS 2 has been studied by X-ray photoelectron spectroscopy. Eu 3d, 4d, 4f and 5p spectra are measured with Al and MgKα radiation. Their intensities are analyzed, taking the escape depth of photoelectrons emitted from the core levels into account. The results indicate that valence transfer of nearly-half Eu 2+ ions to Eu 3+ happens in the outermost surface layer and a reduced Eu–S distance brings about surface rearrangement. It is confirmed that Eu 2+ and Eu 3+ coexist in a bulk, in consistency with bond valence calculations and Mössbauer spectra. The XPS spectra are well interpreted in terms of the multiplet structures of Eu 2+ and Eu 3+.

On the Additivity of Properties and Structural Stability of Misfit Layer Compounds

Differential scanning calorimetry and temperature-dependent magnetic susceptibility measurements demonstrate that the misfit layer compounds (SnS)1.15(TaS2) and (PbS)1.14(TaS2) undergo a few phase transitions between 2 and 700 K. The transitions are interpreted as due to the incommensurability of the suprastructure components. The heat capacity data indicate that the properties of the misfit layer compounds do not obey the additivity rule.

Ullah-Dorsey Scaling of Fluctuation Conductivity Near the Superconducting Transition in (LaSe)1.14(NbSe2)

Magnetotransport measurements with the quasi two-dimensional misfit-layer compound (LaSe)1.14(NbSe2) with critical temperature Tc = 1.23 K are presented. The temperature dependence of the upper critical magnetic field Hc2 has been determined from an analysis of magnetoresistance data through the scaling procedure obtained theroretically by Ullah and Dorsey for the fluctuation conductivity. We show that in contrast to a direct determination of Hc2(T) with a positive curvature the temperature dependence of Hc2 obtained via the scaling procedure reveals a negative curvature in agreement with the Werthammer-Helfand-Hohenberg theory for conventional type-II superconductors.

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

Three misfit layer compounds with the same chemical formula Ca3Co4−xO9+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.

A new method to determine the resistivity anisotropy in lamellar superconductors

A six terminal method to determine the resistivity anisotropy of lamellar superconductors is presented. The main advantage of this method is to avoid the measurement of too small resistances while determining the resistivity anisotropy. A new resistance R d is introduced and the resistivity anisotropy can be determined by the ratio of R d and the longitudinal resistance for Γ⩽1, or R d and the transverse resistance for Γ⩾1 ( Γ is the so-called effective anisotropy). These two ratios have limited variations, which depend on the contact configuration. The room temperature resistivity anisotropy of the superconducting misfit layer compound (LaSe) 1.14(NbSe 2) 2 was determined using this new method.

Scanning Tunneling Microscopy, X-Ray Photoelectron Spectroscopy and Electron Energy-Loss Spectroscopy Studies of the Misfit Layer Compounds {(Pb1-xSbxS)1+y}mTS2(T=Ti and Nb;m=1 and 2)

The misfit-layer compounds {(Pb 1- x Sb x S) 1+ y } m TS 2 (T=Ti and Nb; m =1 and 2) are constructed of the regular stacking of single or double two-atom-thick Pb 1- x Sb x S (Q) layers and a three-atom-thick TS 2 (H) layer. They are studied by means of the scanning tunneling microscope (STM) and the x-ray photoelectron spectroscopy (XPS) and the electron-energy-loss spectroscopy (EELS) methods. For the 2Q/1H ( m =2) compounds, atomic-scale STM images are obtained from both the Q and H layers, whereas for the 1Q/1H compounds no STM images are obtained from a Q layer even if a Q layer consists of a (Pb,Sb)S layer. The electronic structures of the 2Q/1H compounds as well as the 1Q/1H compounds are discussed. It is found that as m increases, the x-ray photoemission threshold or the Fermi energy position shifts to lower binding energy by charge transfer from a Q layer.

Silver intercalation in PbS 1.18(TiS 2) n, n=1, 2, misfit layer compounds

The reaction between metallic Ag and PbS 1.18(TiS 2) n , n=1, 2, misfit layer compounds has been investigated by electrochemical technique, X-ray powder analysis and transmission electron microscopy. It was found that silver intercalation is possible only in the compound with n=2. The thermodynamic behavior and location of phase boundaries were studied in the temperature range 400–650 K. DC-conductivity and magnetic-susceptibility measurements were performed, and the data can be interpreted as an appearance of small polarons during silver insertion.

Ferromagnetism and Large Negative Magnetoresistance in Pb Doped Bi–Sr–Co–O Misfit-Layer Compound

Ferromagnetism and accompanying large negative magnetoresistance in Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail. Recent structural analysis of (Bi,Pb)${}_2$Sr${}_{3}$Co${}_2$O${}_9$, which has been believed to be a Co analogue of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta}$, revealed that it has a more complex structure including a CoO${}_2$ hexagonal layer [T. Yamamoto {\it et al.}, Jpn. J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only introduces holes into the conducting CoO${}_2$ layers but also creates a certain amount of localized spins. Ferromagnetic transition appears at $T$ = 3.2 K with small spontaneous magnetization along the $c$ axis, and around the transition temperature large and anisotropic negative magnetoresistance was observed. This compound is the first example which shows ferromagnetic long-range order in a two-dimensional metallic hexagnonal CoO${}_2$ layer.

New series CrLn8Te13Cl (Ln = Sm, Gd, Tb): pseudo-misfit-layer compounds.

The compounds CrLn8Te13Cl (Ln = Sm, Gd, Tb) have been prepared from the solid-state reaction of Ln, Cr, Te, and CrCl3 at 850 °C. These isostructural compounds crystallize in a new structure type that comprises a succession along the c axis of alternating distorted double rock-salt-type layers and a related CdI2-type layer. This structure bears some resemblance to the framework of certain misfit layer compounds.