Article pubs.acs.org/crystal
Hexagonal Phase Intergrown with the Tetragonal Weyl Semimetal TaAs C. Guo,† H. F. Tian,† H. X. Yang,† K Sun,†,‡ L. L. Wei,†,‡ G. F. Chen,†,§ and J. Q. 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 § Collaborative Innovation Center of Quantum Matter, Beijing 100190, China ABSTRACT: TaAs as a well-known topological Weyl semimetal has attracted intense interests in recent study. Our structural investigations on the microstructure properties of TaAs samples using Cs-corrected transmission electron microscopy and scanning transmission electron microscopy evidently show the presence of a hexagonal phase intergrown with the known tetragonal TaAs (NbAs-type). The structure of the hexagonal phase is identified to have the WC-type structure (space group P6̅m2) with hexagonal c-axis along the [100]tetra or [010]tetra direction. The crystallographic correlation between the hexagonal and tetragonal phase, as well as the relevant local atomic motions, has been carefully analyzed.
I. INTRODUCTION
In this article, the local structural features of TaAs have been investigated by using Cs-corrected transmission electron microscopy. Based on the experimental images for atomic structures, a hexagonal phase and three types of transition mechanisms from main phase (NbAs-type) to the hexagonal phase were recognized in the Weyl semimetal. Then the structure of the hexagonal phase was confirmed as the WC-type (space group P6̅m2) with 90° orientation domains. We modified the model proposed by Boller and Parthe for NbP and TaP to explain our experimental data in TaAs, and the additional reflection spots can been well indexed. We also discussed the relationship between the hexagonal phase and the high pressure phase reported by Zhou.12
Weyl fermions, massless fermions predicted by Hermann Weyl in 1929 as the solution of the Dirac equation,1 have not yet been observed in high energy physics. In 2011, however, it was predicted that Weyl Fermions can be realized in condensed matter physics, as the electronic quasi-particles in the family of pyrochlore iridates and the ferromagnetic spinel compound HgCr2Se4.2,3 Following recent theoretical investigations of Weyl Fermions in the simple semimetal TaAs and relevant isostructural compounds TaP, NbAs, and NbP, 4 Weyl Fermions have been discovered experimentally shortly after in TaAs,5,6 and then it was quickly identified in following study.7 Very recently, the experimental measurements and theoretical analyses reveal a rich variety of physical properties in TaAs,8−11 such as remarkable structural features of the Fermi surface topology,8 the large magnetoresistance, the high carrier mobility,9 and remarkable transition under the pressure of around 14 GPa.12 It is well-known that the physical properties of a material depend notably on the local structural properties.13,14 The chiral Weyl fermions can appear in TaAs because the spindoublet degeneracy of the bands is removed by breaking special inversion symmetry.4 In 1963, the arsenide TaAs crystallizes in the noncentrosymmetric space group I41md.15 Then the phase relations were explored by Murray.16 In 1984, Willerstrom have discussed the structure of the stacking disorders in NbP, TaP, NbAs, and TaAs.17 These results support the disordered model proposed by Boller and Parthe for NbP and TaP with regions of the NbAs-type alternating with regions of the WC-type within a coherent lattice.15 However, the additional reflections observed in samples of TaAs cannot be accounted by this model very well.17 © XXXX American Chemical Society
II. EXPERIMENTAL METHODS Crystals of TaAs were grown by chemical vapor transport.7 A previously reacted polycrystalline TaAs sample was filled in a quartz ampule using 2 mg/cm3 of iodine as transporting agent. After evacuating and sealing, the ampule was kept at the growth temperature for 3 weeks. Large polyhedral crystals with dimensions up to 1.5 mm were obtained in the temperature range of ΔT = 1150−1000 °C. The average stoichiometry was determined by energy-dispersive X-ray (EDX) spectroscopy. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) observations were performed in the JEOL ARM200F equipped with double aberration correctors and operated at 200 kV. The TEM samples were prepared by crushing the single crystals into fine fragments, then the resulting suspensions were dispersed on a holey copper grid coated with a thin carbon film. Received: November 29, 2016 Revised: March 2, 2017 Published: March 17, 2017 A
DOI: 10.1021/acs.cgd.6b01743 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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III. RESULTS AND DISCUSSION First, our TEM observations along the relevant directions have demonstrated that the main phase in the TaAs material adopts a tetragonal structure (NbAs-type) in a good agreement with previously reported data. Moreover, a careful structural analysis of TEM images reveals that the TaAs crystals often show up visible contrast inhomogeneity arising from the presence of intergrowth layers of a hexagonal phase with the tetragonal TaAs. In order to better characterize microstructural properties of the TaAs material, we have chosen several thin areas and obtained a series of TEM images along the main zone axis directions. Figure 1a presents a bright-field (BF) TEM image of
from our structural measurements that the minor hexagonal phase in the TaAs samples often has a volume fraction between 10% and 20%. It is noted that the type 2 structure is also observed by Besara, but which was only discussed as a defective structure.19 The schematic illustrations for all three layered structures are shown in inserted images of Figure 1e. It is wellknown that the tetragonal phase TaAs is predicted to be a Weyl semimetal and has been extensively studied by angle resolved photoemission spectroscopy (ARPES).5,6 Weyl semimetals can be considered as 3D analogues to graphene in terms of the electronic structures, and a variety of remarkable properties originating from the Weyl nodes have been observed, such as topological surface states with Fermi arcs and a negative longitudinal magnetoresistance (MR) due to the chiral anomaly. Interestingly, the ab initio calculations indicate that a hexagonal phase TaAs, having the same structure as discussed in above context, is also a Weyl semimetal and could be a dominant structure at the pressure around 14 GPa.12 Moreover, the hexagonal phase has only a single set of Weyl nodes, which appear exactly on the same energy level, this fact makes the observations of Fermi arcs more clear and provides a proper platform for characterizing the essential properties of topological Weyl semimetals. In this articles, we discovered a new approach to research the hexagonal phase TaAs at room temperature and atmospheric pressure. We now go on to discuss the observed atomic stacking nature and relevant atomic motion between the tetragonal and hexagonal structure. As shown in Figure 2a, the Weyl semimetal
Figure 1. (a) BF TEM image of a typical TaAs single crystal. (b) SAED pattern from the single crystal taken along the [100] zone axis direction. (c) BF TEM image of a TaAs crystal reveals the presence of intergrowth phase. (d) SAED pattern of the TaAs sample also taken along the [100] zone axis direction. (e) STEM HAADF image of a TaAs crystal along the [100] direction, illustrating the presence of intergrowth layers of hexagonal and tetragonal phase. The schematic illustrations for local structural features are shown in the left panel. Figure 2. (a) Structural models illustrating the crystallographic relationships for crystal structures of TaAs: the body-centered tetragonal NbAs-type with the noncentrosymmetric space group I41md. (b) Type 1 and (c) type 2 structures for TaAs. Actually they are the hexagonal WC-type phase (space group P6̅m2) projected along the directions with a 90° rotation with respect to one another.
a typical TaAs single crystal. The selected area electron diffraction (SAED) pattern in Figure 1b reveals that the crystal is a tetragonal structure (NbAs-type) in a good agreement with previously reports. Figure 1c shows a BF TEM image of a TaAs crystal clearly exhibiting the presence of high density of intergrowth lamella stacking along the c-axis direction. Figure 1d shows the SAED pattern from this region along the [100] direction in which the diffraction streaks arising from the hexagonal (minor) phase appear at the position of k ≠ 2n along the c*-axis direction. The STEM high angle annular dark field (HAADF) image, Figure 1e, shows the atomic images illustrating the notable layered structural features for the intergrowth phases. It is known the dot brightness in the HAADF images for a specific atomic column is proportional to Zn (1 < n < 2, Z is the atomic number);18 therefore, it is easily identifiable in the present image that the Ta atoms are the bigger and brighter dots, and the As atoms are the less bright ones. Our analysis also shows that there are three kinds of atomic stacking configurations in this area, i.e., the main tetragonal phase (space group I41md) with atomic stacking layers of BAAB, type 1 for the minor hexagonal phase with atomic stacking layers (ABAB), and type 2 for the minor hexagonal phase with stacking layers (AAAA). It is estimated
TaAs crystallizes in a body-centered tetragonal NbAs-type structure with the noncentrosymmetric space group I41md, a = 3.437 Å, c = 11.656 Å. This structure consists of four Ta−As layers per unit cell, with the sequence of B(0)-A(0)-A(1/2)B(1/2) along the c axis direction. The numbers in parentheses indicate the site position of Ta−As layers in the a direction. Our careful analysis revealed that, in general, there is one (only one) identical parameter between two neighboring Ta−As layers. Based on this rule, we can directly achieve layered patterns for type 1 and type 2 structures, as shown in Figure 2b,c, respectively. In the structural point of view, it is recognizable that hexagonal phase can be obtained by an intracrystalline slip operation of L = (1/2 1/2 0) for the upper half of tetragonal unit cell. Interestingly, type 1 and type 2 structure with a 90° rotation with respect to one another are the identical hexagonal structures viewed along different directions, this WC-type hexagonal structure has a space B
DOI: 10.1021/acs.cgd.6b01743 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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group P6̅m2, a = c = 3.437 Å, and the unit cell is indicated by green solid lines in Figure 2b,c. Hence, it is expected that the coexistence of Type 1 and Type 2 structures in the NbAs samples could yield visible domains, and the hexagonal 90° orientation domains are stacked along the c direction as discussed in following context. Figure 3 shows a low magnification STEM HAADF image taken along the [001] direction, clearly exhibiting the presence
Figure 4 shows a schematic diagram about the intergrowth structure in TaAs crystals. The gray matrix presents the
Figure 4. Schematic diagram for the intergrowth structure in TaAs crystals.
tetragonal NbAs-type TaAs, and the red and yellow slices indicate type 1 and type 2 structure (the hexagonal phase), respectively. The slices of the hexagonal phase in general have a thickness of about 10 Å in the [001] direction of the tetragonal phase, and the hexagonal c-axis is going along the [100]tetra or [010]tetra direction. Based on the atomic resolution STEM images, we can further analyze the local structural features and atomic configurations in correlation of the transition from tetragonal NbAs-type structure to the hexagonal WC-type structure in TaAs. Figure 5a reveals a Ta−As layer B(1/2) gradually changes to the A(0). This kind of antisites disorder is highlighted by red arrows. In another case as shown in Figure 5b, the Ta−As layer shows up a clear shift of L = [1/2, 1/2, 0] due to the presence of a vacancy at the Ta site as indicated by a red box. Interestingly, very recent theoretical study on Weyl semimetals shows that Ta vacancy is a predominant defect in TaAs crystals.20 Moreover, it is also noted in our experimental investigations that the presence of low-angle boundary could result in clear changes of atomic structures, as typically illustrated in Figure 5c. An extra half-plane of Ta atoms is found at the boundary, which yields a 4.0° angle difference between crystals crossing this boundary and which could allow the stable hexagonal phase as shown in the right region of the image. Based on the structural analysis and previous reported data,17 we can conclude that the hexagonal TaAs phase can be a metastable phase and intergrown with the tetragonal TaAs phase due to the defects and internal stress at room temperature. It is possibly formed during the early stages of the crystal growth during thermal processing. As the temperature increases, the metastable WC-type hexagonal structure partially transforms into a stable NbAs-type structure, resulting in a disordered structure comprising variable amounts of the WC- and NbAs-type layers within a common coherent lattice. It is known that the NbAs-type structure TaAs has two types of Weyl nodes located on different energy level. Importantly, the hexagonal phase of TaAs is a topological semimetal with only a single set of Weyl nodes exactly on the same energy level, and the iso-energetic feature might be critical for the technological application of Weyl semimetals in microelectronics.
Figure 3. (a) Low-magnification STEM HAADF image of TaAs observed along the [001] zone axis direction. (b) SAED pattern obtained from the type 1 dominant area. (c) Corresponding kinematic simulated pattern for the type 1 structure. (d) Typical experimental SAED pattern for an area with high density of domains. (e) Simulated diffraction pattern, which can be indexed by the coexistence of type 1 and type 2. The symbols “×” denote the double diffraction spots.
of the high density of domains in Figure 3a. Although the thickness of the intergrowth layers for the hexagonal phase often has order of 10 Å along the c-axis direction, the extension of the hexagonal regions perpendicular to c direction is much larger, it could be as large as 10 nm. The SAED pattern in Figure 3b is obtained from the type 1 dominant area in which the systemic forbidden diffraction spots (h = 2n + 1; k = 2m) appear evidently. The corresponding kinematic simulated pattern for type 1 structure, shown in Figure 3c, is in good agreement with the experimental pattern. Figure 3d shows a typical experimental SAED pattern for area, which has high density of domains. The theoretical simulated pattern (Figure 3e) is consistent with the experimental pattern, which can be indexed by the coexistence of type 1 and type 2 structures. The symbols “×” in Figure 3e denote the double diffraction spots. Thus, all additional extra reflections observed in samples of TaAs can be well accounted for by the structural model.
IV. CONCLUSIONS In conclusion, the microstructure properties of TaAs have been investigated by using Cs-corrected TEM and STEM. Based on the STEM images and diffraction data, a hexagonal phase, C
DOI: 10.1021/acs.cgd.6b01743 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS
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REFERENCES
Article
This work was supported by National Basic Research Program of China 973 Program (No. 2015CB921300 and No. 2012CB821404), the National Key Research and Development Program of China (No. 2016YFA0300300), the Natural Science Foundation of China (Grants No. 11604372, No. 11274368, No. 51272277, No. 91221102, No. 11190022, No. 11474323, and No. 91422303), and “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (No. XDB07020000).
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Figure 5. Typical local atomic structures as observed in our experiments, which could result in structural transition from NbAstype tetragonal structure to the hexagonal WC-type structure in TaAs: (a) antisites type; (b) vacancy type; (c) low-angle boundary type.
intergrown with the tetragonal TaAs phase, has been observed. The transition mechanisms from main phase (NbAs-type) to the hexagonal phase have been analyzed in correlation with local atomic motions and defective structure as well. The hexagonal phase is found to have the WC-type structure with a space group P6̅m2. We have improved the structural models as proposed by Boller and Parthe for NbP and TaP and successfully interpret the additional spots in the diffraction patterns and structural transition associated with alterations of atomic configurations as revealed in the intergrowth structures.
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[email protected]. ORCID
C. Guo: 0000-0002-9916-1984 Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.cgd.6b01743 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.cgd.6b01743 Cryst. Growth Des. XXXX, XXX, XXX−XXX
