Article pubs.acs.org/IC
Structural Channels and Atomic-Cluster Insertion in CsxBi4Te6 (1 ≤ x ≤ 1.25) As Observed by Aberration-Corrected Scanning Transmission Electron Microscopy Ruixin Zhang,†,‡ Huaixin Yang,*,†,‡ Cong Guo,† Huanfang Tian,† Honglong Shi,§ Genfu Chen,†,∥ and Jianqi Li*,†,‡,∥ †
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China § School of Science, Minzu University, Beijing 100081, China ∥ Collaborative Innovation Center of Quantum Matter, Beijing 100190, China S Supporting Information *
ABSTRACT: Microstructural analyses based on aberration-corrected scanning transmission electron microscopy (STEM) observations demonstrate that lowdimensional CsxBi4Te6 materials, known to be a novel thermoelectric and superconducting system, contain notable structural channels that go directly along the b axis, which can be partially filled by atom clusters depending on the thermal treatment process. We successfully prepared two series of CsxBi4Te6 single-crystalline samples using two different sintering processes. The CsxBi4Te6 samples prepared using an air-quenching method show superconductivity at approximately 4 K, while the CsxBi4Te6 with the same nominal compositions prepared by slowly cooling are nonsuperconductors. Moreover, atomic structural investigations of typical samples reveal that the structural channels are often empty in superconducting materials; thus, we can represent the superconducting phase as Cs1−yBi4Te6 with considering the point defects in the Cs layers. In addition, the channels in the nonsuperconducting crystals are commonly partially occupied by triplet Bi clusters. Moreover, the average structures for these two phases are also different in their monoclinic angles (β), which are estimated to be 102.3° for superconductors and 100.5° for nonsuperconductors.
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INTRODUCTION
The microstructures of both thermoelectric materials and superconductors have considerable influences on their physical properties.13−23 Typical thermoelectric examples are embedded nanoscale precipitates in bulk materials that scatter heatcarrying phonons, e.g., PbTe-SrTe18,19 and PbS-PbTe,20 which reduce κ to enhance ZT. High power factors have been realized in the CoSb3 skutterudites by adjusting the total filling fraction of fillers in the nanoscale cages with different charge states; in addition, lattice thermal conductivity can also be significantly reduced.21 Phase separation was revealed in the superconducting AyFe2−xSe2 material system and shown to be important to the superconductivity of this system.22,23 At the same time, we note that the crystal structure of CsBi4Te6 is unique, containing anisotropic [Bi4Te6] rods linked with Bi−Bi bonds to form slabs and the Cs ions intercalated among the two-dimensional slabs, as shown in Figure 1a, showing an essentially one-dimensional structure, which is consistent with the characteristic needle-like morphology of the crystal.2,9 Here, we report our observations of structural channels in CsxBi4Te6
The narrow-gap semiconducting material CsBi4Te6 as the first member (n = 6) of the possible homology of the type Cs4[Bi2n+4Te3n+6]1 has received considerable attention in recent decades as a promising thermoelectric material for lowtemperature applications, possessing a thermoelectric figure of merit of 0.82 at 225 K when doped appropriately.2,3 All of the experimental and theoretical studies of the electronic structure of CsBi4Te6 have revealed the existence of strong anisotropy in this material.4−7 The electrical conductivity σ, thermopower S, and thermal conductivity κ all show strong anisotropies in different crystallographic directions.8 Recently, superconductivity was discovered in the p-type (Birich or Cs-poor) doped thermoelectric material CsBi4Te6.9 The Tc of the p-type CsBi4Te6 is 4.4 K, and the critical field achieves 10 T, despite the relatively low superconducting fractions, e.g., a typical superconducting volume fraction of ∼3% for Cs0.96Bi4Te6 and ∼11% for CsBi4.1Te5.9 with the field normal to the samples.9 While superconductivity has been discovered upon doping in other semiconductors, such as PbTe,10 GeTe,11 and Bi2Se3,12 the mechanism for the emergence of superconductivity is still under debate. © XXXX American Chemical Society
Received: August 26, 2016
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DOI: 10.1021/acs.inorgchem.6b02077 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
samples along the [010] zone-axis direction were prepared through the preparation method for a cross-section transmission electron microscope sample, including embedding samples, cutting with a wire saw, and thinning. The TEM samples along the other zone-axis directions were prepared by crushing the single crystals into fine fragments. Then the resulting suspensions were dispersed on holey copper grids coated with thin carbon films. Resistivity measurements were performed down to 1.8 K in a Quantum Design Physical Property Measurement System (PPMS) using the standard four-point probe method, and the current direction is parallel to the direction of crystal growth (b axis). Magnetization measurements were performed using a Quantum Design SQUID magnetometer at a magnetic field of 10 Oe with the field normal to the orientation of the needle-like morphology (b axis).
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Figure 1. (a) Structural model of the monoclinic phase CsBi4Te6, illustrating the layered structural features and the Bi−Bi bonds. (b) SEM image shows the needle-like morphology of the air-quenched CsBi4Te6 crystals. (c) X-ray diffraction data for the air-quenched and slowly cooled CsBi4Te6. All peaks can be characterized with the monoclinic space group C2/m of CsBi4Te6. (Insert) Difference in the peak positions between the two samples.
that can be filled partially by atom clusters (presumed to be Bi ions). Two stable structural states are identified, i.e., the structural channels are either empty or occupied by triplet Bi clusters. Further research shows that the structural channels are often empty in superconducting phase Cs1−yBi4Te6 and partially occupied by triplet Bi clusters in nonsuperconducting CsBi4.07Te6. Moreover, the average structures for these two phases and the common defects and structural alterations associated with the insertion of the atomic clusters in the channels are also examined.
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RESULTS AND DISCUSSION
First, two series of the CsxBi4Te6 (1 ≤ x ≤ 1.25) single crystals were examined by SEM, demonstrating that these crystals often adopt very similar needle-like morphologies as reported in refs 2 and 3 and can be easily cleaved along the a−b plane where the Cs layers are accommodated. Figure 1b shows an SEM image of an air-quenched CsBi4Te6 sample, illustrating the morphological features of the well-aligned CsBi4Te6 crystals. The direction of the crystal growth is along the b-axis direction. Powder X-ray diffraction was employed to measure the average structures of these CsxBi4Te6 samples as shown in Figure S1 (Supporting Information). Careful analysis of the experimental data demonstrates that all reflection peaks can be well indexed by the monoclinic lattices, and no impurity phase is observed. The structural refinement for all samples shows that the lattice parameters vary slowly with increasing Cs content in both sintering processes, and the monoclinic angles (β) in the airquenched samples are slightly larger than those in the slowly cooled samples. For instance, Figure 1c shows the X-ray diffraction data of air-quenched CsBi4Te6 and slowly cooled CsBi4Te6 with the lattice parameters of a = 51.921(1) Å, b = 4.403(4) Å, c = 14.512(5) Å, and β = 101.8° and a = 51.919(4) Å, b = 4.410(8) Å, c = 14.510(8) Å, and β = 100.8°, respectively, clearly exhibiting the difference in the peak positions resulting from the monoclinic angle (β) between the two samples; the unit cell volume for the slowly cooled sample (3264.2 Å3) is slightly larger than that of the airquenched sample (3247.87 Å3). Though we made numerous attempts to characterize the detailed atomic structures base on the X-ray diffraction data, it is noted the powder X-ray data are strongly affected by the preferred orientation in the singlecrystalline samples due to the essential anisotropic structural features, i.e., the needle-like morphology. Careful analysis on (00L) reflection peaks also revealed clear alterations of lattice strains (or local distortions) in this layered system. The maximum strains of the air-quenched and slowly cooled CsBi4Te6 samples are estimated to be 0.47% and 0.79%, respectively. The lattice distortion of air-quenched CsBi4Te6 is smaller than that of slowly cooled ones, suggesting the atomic insertion in structural channels of the nonsuperconducting (slowly cooled) samples could locally yield visible structural changes, modulations, and defects as discussed in the following context. The chemical compositions of the synthesized single crystals are further analyzed by EDS equipped on the scanning electron microscope and generally consistent with the nominal composition. We list the chemical compositions of the two series of samples with nominal compositions of CsxBi4Te6 in Table S1 (Supporting Information). The measurements of magnetic susceptibility and resistivity for all CsxBi4Te6 crystals have been performed and analyzed at
EXPERIMENTAL SECTION
Synthesis of CsxBi4Te6. To extensively investigate the microstructural features and physical properties of CsxBi4Te6, we first performed our studies on the influence of the Cs doping content and the existence of n-type CsxBi4Te6 superconductor (1< x ≤ 1.25), and single crystals of CsxBi4Te6 (x ranges from 1 to 1.25) were prepared: small excesses (∼1%) of Cs and Bi2Te3 with designed compositions were placed in a small alumina crucible in a silica tube that was evacuated to
