Misfit Layer Compounds Research Articles

Structural and electrical properties of a new ([SnSe]1.16)1(NbSe2)1 polytype

A new polytype of the misfit layer compound ([SnSe]1.16)1(NbSe2)1 with extensive rotational disorder was prepared from designed modulated elemental reactants. This polytype, previously referred to as a ferecrystal due to the extensive rotational disorder, formed over a range of compositions and precursor thicknesses and the resulting c-axis lattice parameters ranged from 1.2210(4) to 1.2360(4)nm. These values bracket the value published for the crystalline misfit layer compound prepared at high temperature. The a- and b-axis in-plane lattice parameters of both the SnSe and NbSe2 constituents were incommensurate, which differs from the misfit layer compound formed via high temperature reaction that has a common b-axis lattice parameter for the two constituents. The in-plane area per unit cell of the ferecrystal is 1–2% larger than the compound formed at high temperature. The ferecrystalline ([SnSe]1.16)1(NbSe2)1 compound is 1.6 times more conductive than the misfit layer compound. Hall effect measurements indicate that the ferecrystal is a p-type metal and that the higher conductivity is a consequence of higher mobility of carriers in the ferecrystalline compound.

ChemInform Abstract: Telluride Misfit Layer Compounds: [(PbTe)1.17]m(TiTe2)n.

AbstractTelluride misfit layer compounds are reported for the first time.

Telluride Misfit Layer Compounds: [(PbTe)1.17]m(TiTe2)n

Telluride misfit layer compounds are reported for the first time. These compounds were synthesized using a novel approach of structurally designing a precursor that would form the desired product upon low-temperature annealing, which allows the synthesis of kinetically stable products that do not appear on the equilibrium phase diagram. Four new compounds of the [(PbTe)(1.17)]m(TiTe2)n family are reported, and their structures were examined by a variety of X-ray diffraction techniques.

Telluride Misfit Layer Compounds: [(PbTe)1.17]m(TiTe2)n

AbstractTelluride misfit layer compounds are reported for the first time. These compounds were synthesized using a novel approach of structurally designing a precursor that would form the desired product upon low‐temperature annealing, which allows the synthesis of kinetically stable products that do not appear on the equilibrium phase diagram. Four new compounds of the [(PbTe)1.17]m(TiTe2)n family are reported, and their structures were examined by a variety of X‐ray diffraction techniques.

Effects of Transition Metal Substitution on the Thermoelectric Properties of Metallic (BiS)1.2(TiS2)2 Misfit Layer Sulfide

The misfit layer compounds (BiS)1.2(TiS2)2, with a natural superlattice structures, are of substantial interest as thermoelectric materials. In this work we doped the Ti sites of (BiS)1.2(TiS2)2 with a series of transition metal (TM) elements (V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), to optimize its transport properties and thermoelectric performance. X-ray diffraction confirmed all the resulting compositions were single-phase. X-ray photoelectron spectroscopy revealed the valence states of the doping elements, indicating they behave as acceptors and reduce the carrier concentration; this was also apparent from Hall measurements. However, because of the non-parabolic band structure of (BiS)1.2(TiS2)2, reduction of carrier concentration by doping with most of the TM elements did not improve the Seebeck coefficient. Exceptions were V and Cr. For these elements, the effective mass of electrons was maintained, or even enhanced, resulting in improvement of the Seebeck coefficient. Furthermore, the stacking disorder present in undoped (BiS)1.2(TiS2)2 was not observed for the TM element-doped samples, resulting in increased lattice thermal conductivity. Although the power factor of these materials was not optimized, because of the large reduction in electronic thermal conductivity upon doping, the Cr-doped sample had a higher figure of merit than the undoped material.

Misfit-layered compound PbTiS3 with incommensurate modulation: Transmission electron microscopy analysis and transport properties

The microstructural characteristic of the misfit-layered compound PbTiS3 has been studied with transmission electron microscopy. All the incommensurate modulation-induced satellite spots and main diffraction spots of basic sublattices can be indexed systematically with a superspace group method. Finally, the relationship between the electronic transport properties and the crystal structure is discussed.

Local structure and defect chemistry of [(SnSe)1.15] (TaSe2) ferecrystals – A new type of layered intergrowth compound

Abstract The atomic structure of the family of ferecrystals [(SnSe)1.15]m(TaSe2) (m = 1, 3, and 6) was investigated by means of transmission electron microscopy. The tantalum in the TaSe2 layers was observed to have trigonal prismatic coordination similar to that found in the 2H polytype of bulk TaSe2. The structure of the SnSe constituent was found to be similar to that of orthorhombic α-SnSe. In the compounds with m = 1 and m = 3, regions with a local ordering of the layers along a commensurate axis, similar to the ordering in conventional misfit layer compounds, were observed. However, on a longer range the ferecrystals were found to exhibit a turbostratically disordered structure. Stacking defects were occasionally found in the samples in which a layer is interrupted and the surrounding layers are bent around these defects, while maintaining abrupt interfaces instead of interdiffusing. Volume defects were found in one sample of [(SnSe)1.15]1(TaSe2)1 in which a SnSe layer locally substitutes a part of a TaSe2 layer without interrupting the surrounding layers.

Ceramics, squared: J. Carlos Diez and colleagues discuss thermoelectric ceramics and the challenge of optimizing their properties

Thermoelectric materials can directly convert a temperature gradient into an electric voltage thanks to the Seebeck effect. Since its discovery in 1823, this characteristic has allowed the use of metallic junctions to control temperatures. These thermocouples possess a low electrical resistivity and Seebeck coefficient with high thermal conductivity. The thermoelectric performances of these materials are evaluated by the dimensionless figure-of-merit (ZT = SsT/k, where S is the Seebeck coefficient, T is the absolute temperature, s is the electrical conductivity, and k is the thermal conductivity) which is very small, indicating that metallic materials possess poor thermoelectric behavior. The introduction of intermetallic semiconductor materials, with higher thermoelectric performances than their metallic counterparts, has resulted in their practical application in thermoelectric modules. These modules can be found as Peltier devices, when used for refrigeration purposes, or Seebeck devices when used for electrical energy generation. Current applications of this kind of material include the environmentally friendly recoveryof industrial and automobilewasteheat, and inthe production of electrical energy in radioisotope thermoelectric generators installed on spacecrafts, or in lighthouses located in isolated regions. Nevertheless, the application of semiconductor materials for energy generation has been limited due to their relatively low thermal stability under air, which can result in the release of heavy and/or toxic elements, and degradation and/or oxidation processes at high temperatures which can diminish their performances. Nowadays, research effort is being focused on ceramic materials with relatively high thermoelectric performances which can work at higher temperatures than the intermetallic ones. Some of these ceramics are based on Co-oxides, with a high Seebeck coefficient and low electrical resistivity and thermal conductivity. Moreover, they are mostly composed of abundant, less expensive and more environmental friendly elements than the intermetallic compounds. Their main drawback can be found in their relatively low (compared with the intermetallic materials) ZT values, meaning that raising the figure-or merit is the most pressing task to be performed, so that they may be used in practical power generation applications. One of these Co-oxides is [Bi0.87SrO2]2[CoO2]1.82, described as a misfit layer compound with monoclinic symmetry, composed – in turn – of two subsystems exhibiting incommensurate periodicities. The structure of the misfit layer crystal can be described as an alternation along the c-axis of distorted rock-salt-type slabs, formed from [BiO] and [SrO] layers (the first subsystem), and of [CoO2] layers (the second subsystem and the electrical conducting one) displaying a distorted CdI2-type structure [1]. In order to improve the performance of these layered materials texturing techniques can be applied.

Synthesis, structure and electrical properties of a new tin vanadium selenide

Abstract The turbostratically disordered misfit layer compound (SnSe) 1.15 VSe 2 was synthesized and structurally characterized. Electrical transport measurements suggest this compound undergoes a charge or spin density wave (CDW or SDW) transition, which has not been observed in previous misfit layer compounds. The (SnSe) 1.15 VSe 2 compound, created through the modulated elemental reactants technique, contains highly oriented intergrowths of SnSe bilayers and VSe 2 structured Se–V–Se trilayers with abrupt interfaces between them perpendicular to the c -axis. X-ray diffraction data and transmission electron microscope images show that each constituent has in-plane crystallinity but that there is a random rotational disorder between the constituent layers. Temperature-dependent electrical resistivity data and Hall measurements are consistent with (SnSe) 1.15 VSe 2 being a metal, however an abrupt increase in the resistivity occurs between 30 and 100 K. The carrier concentration decreases by approximately 1 carrier per vanadium atom during this temperature interval.

Structure of Turbostratically Disordered Misfit Layer Compounds [(PbSe)0.99]1[WSe2]1, [(PbSe)1.00]1[MoSe2]1, and [(SnSe)1.03]1[MoSe2]1

AbstractThe structures of three new misfit layer compounds, [(PbSe)0.99]1[WSe2]1, [(PbSe)1.00]1[MoSe2]1, and [(SnSe)1.03]1[MoSe2]1, prepared as thin films from designed elementally modulated reactant precursors, have been investigated by synchrotron X‐ray diffraction and scanning transmission electron microscopy. This is the second report of a misfit layer compound based on tin selenide and the first structure determinations of tungsten‐ and molybdenum‐containing misfit layer compounds. The compounds have mismatches in both the a‐ and b‐axis (in‐plane) lattice parameters of the component structural units, which result in misfit parameters, δ, both greater and smaller than zero. Pole figures of hk0‐type reflections indicate that c‐axis‐oriented (basal plane) dichalcogenide‐structured units are randomly oriented within the a‐b plane. The widths of diffraction peaks of type hk0, 00l, and hkl (h, k ≠ 0; l ≠ 0) indicate coherence lengths of approximately (6 to 7) nm within the a‐b plane, the full thickness of the film along the c‐axis, and (1 to 2) nm in mixed‐index directions, respectively, consistent with random rotational disorder between the constituents, commonly termed turbostratic disorder in the clay literature. The intensities of 00l‐ and hk0‐type diffraction peaks were modeled to determine the structure of atomic planes along the c axis and the projection of the constituent structures onto the a‐b plane, respectively.

Intercalation: Building a Natural Superlattice for Better Thermoelectric Performance in Layered Chalcogenides

A natural superlattice with composition (SnS)1.2(TiS2)2, built by intercalating a SnS layer into the van der Waals gap of layered TiS2, has been directly observed by high-resolution transmission electron microscopy (HRTEM). The thermoelectric performance is improved in the direction parallel to the layers because the electron mobility is maintained while simultaneously suppressing phonon transport, which is attributed to softening of the transverse sound velocities due to weakened interlayer bonding. In the direction perpendicular to the layers, the lattice thermal conductivity of (SnS)1.2(TiS2)2 is even lower than the predicted minimum thermal conductivity, which may be caused by phonon localization due to the translational disorder of the SnS layers parallel to the layers. Moreover, we propose a large family of misfit-layer compounds (MX)1+x(TX2)n (M = Pb, Bi, Sn, Sb, rare-earth elements; T = Ti, V, Cr, Nb, Ta; X = S, Se; n = 1, 2, 3) with a natural superlattice structure as possible candidate high-performance thermoelectric materials.

不整合積層構造を持つ層状熱電化合物(MS)1+x(TiS2)2 (M = Pb, Sn, Bi)の熱伝導機構

不整合積層構造を持つ層状熱電化合物(MS)1+x(TiS2)2 (M = Pb, Sn, Bi)の熱伝導機構

Ab initiostudy of stabilization of the misfit layer compound(PbS)1.14TaS2

In the present work we perform ab initio electronic-structure calculations of the ${(\text{PbS})}_{1.14}{\text{TaS}}_{2}$ misfit layer compound in order to understand the basic mechanism of its stabilization. Density-functional based calculations were carried out in commensurate unit cells containing 74 or 296 atoms using the WIEN2K code. The two experimentally predicted mechanisms of stabilization, namely, metal cross substitution (interchange of Pb and Ta atoms in the PbS and ${\text{TaS}}_{2}$ layers) and nonstoichiometry (substitution of Pb by Ta), were investigated. The results show clearly that the nonstoichiometry mechanism plays the significant role. When Pb inside the PbS layer is substituted by Ta with concentrations around 0.13--0.19, a stabilization is found with respect to the parent PbS and ${\text{TaS}}_{2}$ compounds. These results are explained by an analysis of partial densities of states. The calculated x-ray photoemission spectroscopy core-level shifts are in reasonable agreement with experiments.

ChemInform Abstract: Structure, Electrical Transport and Magnetic Properties of the Misfit Layer Compound (SmS)1.19TaS2 “SmTaS3”.

Abstract Single-crystal X-ray diffraction has shown that (SmS)1.19TaS2 is a misfit layer compound built alternately of double layers of SmS, an approximately {100} slice of NaCl-type SmS and sandwiches of TaS2 with tantalum in slightly distorted trigonal prisms of sulfur. The two structural units are both described in the orthorhombic space group Fm2m, a 1 = 5.552(3) A , b 1 = 5.679(4) A , c 1 = 22.50(4) A , Z = 8 for the SmS part and a 2 = 3.293(1) A , b 2 = 5.679(4) A , c 2 = 22.50(4) A , Z = 4 for the TaS2 part. Corresponding axes are parallel and the layer stacking direction is c. The composition is determined by the ratio a 1 a 2 and the number of formula units in each cell. Sm-S distances in the SmS double layer are close to those in SmS in the metallic high-pressure form with the scheme Sm3+ (e−)S2−. The electrical transport properties are in agreement with an almost filled 5dz2 band of the TaS2 part of the structure, indicating electron donation from the SmS to the TaS2 part. The magnetic susceptibility χ does not follow a Curie-Weiss law; χ−1 goes to zero with decreasing temperature after a small discontinuous drop at about 50 K.

ChemInform Abstract: Crystal Structure Determination of the Misfit Layer Compound (HoS)1. 23NbS2.

Abstract Single-crystal X-ray diffraction has shown that (HoS)1.23NbS2 is a misfit layer compound built of alternate double layers of HoS, approximately a {001} slice of NaCl-type HoS, and sandwiches of NbS2 with niobium in trigonal prisms of sulphur like niobium in 2HNbS2. The unit cell dimensions of the HoS subsystem are a1 = 5.396(1) A, b1 = 5.661(1) A, c1 = 22.195(8) A, β1 = 89.81(2)°, space group F2, Z = 8. The NbS2 lattice with unit cell dimensions a2 = 3.312(1) A, b 2 = 5.661(1) A , c2 = 11.125(7) A, β2 = 86.99(3)°, space group C2, Z = 2 shows twinning, the twin plane being (001). Corresponding a and b axes in the layers of the two subsystems are parallel, while the c axes diverge slightly. The two subsystems have a common ( b∗, c∗ ) reciprocal lattice plane which corresponds in real space to a common projection along [100]. Refinements were performed of the subsystem reflections (Okl excluded) and the common Okl reflections to fix the origin of one subsystem with respect to the other. The HoS distances in the HoS double layer are 2.743(4), 2.754(4), 2.765(4), 2.83(1) and 2.95(1) A; holmium is also coordinated to one or two sulphur atoms of NbS2 (the coordination varies because of the incommensurateness along [100]). The shortest distance is 2.72 A for coordination with one sulphur atom (two other sulphur atoms at distances of 3.18 A) and 2.86 A for coordination with two sulphur atoms of NbS2, holmium atoms in rows parallel to [100] lying in between rows of sulphur atoms of NbS2 sandwiches. The Ho-S distances point to Ho(III). The Nb-S distances are 2.453(8) A (4X) and 2.472(8) A (2X).

ChemInform Abstract: Structure Determination of a New Misfit Layer Compound (PbS)1.18(TiS2) 2.

Abstract (PbS)1.18(TiS2)2 is a misfit layer compound in which two types of slabs, PbS and 2(TiS2), alternate along the c direction. The |TiS2| blocks (about 11.39 A thick) are interleaved by |PbS| layers, thus leading to a value for c of 17.46 A. The misfit between the two slab types occurs along the a direction; the parameter values being a( PbS )=5.761 A and a( TiS 2 )=3.390 A respectively. This yields a ratio of approximately 1.699, which is irrational but close to 5/3. The common in-plane b parameter is equal to 5.873 A. The PbS unit consists of a {001} slice (half an edge thick) of an NaCl-type f.c.c. cell.

Structural study by X-ray diffraction and transmission electron microscopy of the misfit compound (SbS 1− xSe x) 1.16(Nb 1.036S 2) 2

In the Sb–Nb–S–Se system, a new misfit layer compound (MSL) has been synthesized and its structure was determined by combining single crystal X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. It presents a composite crystal structure formed by (SbS 1− x Se x ) slabs stacking alternately with double NbS 2 layers and both can be treated as separate monoclinic subsystems. The (SbS 1− x Se x ) slabs comprise a distorted, two-atom-thick layer with NaCl-type structure formed by an array of {SbX 5} square pyramids joined by edges (X: S, Se); the NbS 2 layers consist of {NbS 6} trigonal prisms linked through edge-sharing to form sheets, just as in the 2H–NbS 2 structure type. Both sublattices have the same lattice parameters a = 5.7672(19) Å, c = 17.618(6) Å and β = 96.18(3)°, with incommensurability occurring along the b direction: b 1 = 3.3442(13) Å for the NbS 2 subsystem and b 2 = 2.8755(13) Å for the (SbS 1− x Se x ) subsystem. The occurrence of diffuse scattering intensity streaked along c * indicates that the (SbS 1− x Se x ) subsystem is subjected to extended defects along the stacking direction.

Low-Thermal-Conductivity (MS)1+x(TiS2)2 (M = Pb, Bi, Sn) Misfit Layer Compounds for Bulk Thermoelectric Materials

A series of (MS)1+x(TiS2)2 (M = Pb, Bi, Sn) misfit layer compounds are proposed as bulk thermoelectric materials. They are composed of alternating rock-salt-type MS layers and paired trigonal anti-prismatic TiS2 layers with a van der Waals gap. This naturally modulated structure shows low lattice thermal conductivity close to or even lower than the predicted minimum thermal conductivity. Measurement of sound velocities shows that the ultra-low thermal conductivity partially originates from the softening of the transverse modes of lattice wave due to weak interlayer bonding. Combined with a high power factor, the misfit layer compounds show a relatively high ZT value of 0.28~0.37 at 700 K.

Muon spin relaxation study of misfit-layered cobalt dioxide

Abstract In order to clarify the nature of a possible magnetic phase transition at Tm=13 K for the misfit layered cobalt dioxide with a double rocksalt (RS-) type block, [Ca0.85OH]1.16RS[CoO2], we have measured the muon spin rotation and relaxation ( μ + SR ) spectra in the temperature ( T ) range between 1.8 and 20 K. All μ + SR parameters, such as the dipolar interaction frequency ( f μ OH ) due to the formation of μ + –OH, its asymmetry ( A μ OH ) and relaxation rate ( σ μ OH ), are found to be independent of T both above and below T m . This is a clear indication that the transition at T m is not due to magnetic order but has a structural and/or electronic origin. In addition, we have been able to determine the muon stopping site for this misfit layered compound. It is found to be located within the CaOH blocking layers, where the muon and OH-group form a water-like H μ + O unit as indicated by a very slow oscillation in the zero-field relaxation spectrum.

Rational Synthesis and Characterization of a New Family of Low Thermal Conductivity Misfit Layer Compounds [(PbSe)0.99]m(WSe2)n

We describe here a general synthesis approach for the preparation of new families of misfit layer compounds and demonstrate its effectiveness through the preparation of the first 64 members of the [(PbSe)0.99]m(WSe2)n family of compounds, where m and n are integers that were systematically varied from 1 to 8. The new compounds [(PbSe)1+y]m(WSe2)n were synthesized by annealing reactant precursors containing m layers of alternating elemental Pb and Se followed by n layers of alternating elemental W and Se, in which the thickness of each pair of elemental layers was calibrated to yield a structural bilayer of rock salt structured PbSe and a trilayer of hexagonal WSe2. The compounds are kinetically trapped by the similarity of the composition profiles and modulation lengths in the precursor and the targeted compounds. The structural evolution from initial reactant of layer elements to crystalline misfit layer compounds was tracked using X-ray diffraction. The crystal structures of new compounds were probed us...