Weyl Semimetal TaAs: Crystal Growth, Morphology, and

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Weyl Semimetal TaAs: Crystal Growth, Morphology, and Thermodynamics Zhilin Li,†,‡ Hongxiang Chen,†,‡ Shifeng Jin,† Di Gan,† Wenjun Wang,† Liwei Guo,*,† and Xiaolong Chen*,†,§ †

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China § Collaborative Innovation Center of Quantum Matter, Beijing 100190, China S Supporting Information *

ABSTRACT: Tantalum arsenide is experimentally verified as a Weyl semimetal recently. However, it is difficult to grow large TaAs single crystals due to the decomposition prior to reaching its melting point. Here we report an improved chemical vapor transport method using iodine as an agent to get large-size, high-quality TaAs single crystals up to 1 cm. Xray diffraction confirmed that they are tetragonal TaAs. Specific heat of TaAs was measured from 2 K to room temperature, and hence the entropy and enthalpy were obtained, which are helpful in designing the optimal growth conditions. The as-grown crystals exhibit polyhedral morphology consisting of {101}, {001}, {103}, and {112} facets. The key points in crystal growth include using tantalum in the form of foils instead of powder as the starting material, tilting ampule to enhance convections and controlling the concentration of agent iodine. These measures should be applicable to the growth of other transition metal arsenides and phosphides.

decompose at high temperatures. The chemical vapor transport (CVT) method is then expected to grow this crystal. In fact, single crystals of approximately 0.3 mm and 1.5 mm in size5,10 were recently obtained by researchers using iodine15 as the transport agent. However, the size limit prevents researchers from conducting other experiments that require larger crystals. Thus, an urgent need exists for further scientific research and potential applications for large-size, high-quality TaAs single crystals. In this Communication, we analyzed the thermodynamics for the reactions involved in the CVT process and obtained the optimized growth conditions. TaAs single crystals, with a size of up to 1 cm in one dimension and with a mass of nearly 1 g, were obtained by the CVT method using iodine as the agent. As far as we know, this is the largest TaAs single crystal ever reported up to now. The growth forms of crystals are determined by X-ray diffraction and morphology calculations. The key points involved in the crystal growth are to reduce the spontaneous nucleation in the initial growth stage and enhance the transport rate of the gaseous species. The results presented here should be applicable to the growth of other Weyl semimetal crystals such as TaP, NbAs, NbP, and even other transition metal pnictides. The experimental configurations used in our growth process are shown in Figure 1. In a typical run, tantalum foil (99.99%),

Weyl fermions are massless chiral fermions that were suggested in quantum field theory more than eight decades ago.1 Though having not been observed in high-energy physics, Weyl fermions are expected to exist in low-energy excitations of condensed matter systems.2,3 TaAs is a newly predicted4,5 Weyl semimetal and has been immediately confirmed by various experiments.6−8 Intense interest has been attracted since the discovery of this topological material as it shows a series of peculiar phenomena like Fermi arcs,5,9 ultrahigh mobility, negative magnetoresistance10 in parallel magnetic fields due to chiral anomaly,11 and very large magnetoresistance in vertical fields. TaAs may find applications in future spintronics and even quantum computation.5 TaAs crystallizes in a body-centered tetragonal unit cell with lattice constants a = 3.44 Å and c = 11.64 Å,12−16 and the space group is I41md (No. 109). Ta and As atoms are six coordinated to each other. It should be noted that this structure lacks a horizontal mirror plane and thus inversion symmetry, which is essential to realize Weyl semimetal. To grow single crystals, however, difficulties arise. Tantalum has a high melting point (∼3017 °C) and chemical inertness, while arsenic is easily sublimated at 613 °C before melting. Besides TaAs, other phases such as Ta3As, Ta2As, Ta5As4, and TaAs2 exist in the Ta−As binary system.15 TaAs itself does not melt up to 1400 °C but decomposes prior to reaching its melting point. These facts make it difficult to get TaAs single crystals from melts directly. It is known that halogens can react with Ta, As, and TaAs at relatively lower temperatures, and the halides tend to © XXXX American Chemical Society

Received: December 12, 2015 Revised: February 2, 2016

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DOI: 10.1021/acs.cgd.5b01758 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 1. Schemes of the chemical vapor transport experiments for crystallization of TaAs in a temperature gradient. TaAs is transported from the cooler end to the hotter part and crystallizes on the foil.

arsenic pieces (99.995%), and iodine (99.99%) were loaded in a 25 mL silica ampule, which was 10 cm in length and 1.8 cm in inner diameter. The ampule was originally filled with argon and then evacuated to a pressure below 1 Pa and sealed quickly to avoid the loss of iodine and arsenic. The mole ratio Ta:As:I2 = 1:1:0.05 (Caution: the total amount of arsenic and iodine should be limited so the pressure in the ampule at 1000 °C is less than 2 atm to avoid ampule explosion). Then the silica ampule was heated gradually from room temperature to 1000 °C over 72 h. During this period of time, tantalum reacted with arsenic almost completely to form TaAs polycrystalline. Afterward the ampule was put to a temperature gradient from 1020 to 980 °C where the CVT proceeded for 2 weeks and finally naturally cooled down to room temperature. Figure 2a shows the crystal structure of TaAs. Figure 2b shows the typical products of grown crystals. The X-ray

Figure 3. (a) X-ray diffraction pattern of TaAs powder, and of the (001), (101), and (112) facets of the TaAs single crystal. (b) X-ray rocking curves of the three different facets show good crystallinity with FWHM less than 0.033°. (c) Typical morphology of TaAs crystal with different facets labeled.

observed occasionally on the intersection line of (001) and (101) as elongated slim rectangular facets. Our calculations of morphology of TaAs crystal based on the Bravais-FriedelDonnay-Haker (BFDH) model17,18 show that the four most important facets should be {101}, {001}, {103}, and {112} facets, consistent with the observed ones. These faces have relatively larger lattice-plane-space and therefore a lower growth rate and thus are found more frequently than others in the asgrown crystals. The X-ray rocking curves (Figure 3b) show that the full widths at half maximum (FWHM) are 0.033°, 0.010°, and 0.033° for (004), (101), and (112) faces, respectively, suggesting the high quality of grown crystals in crystallinity. Under the optical microscope, regular spaced terraces and steps on the (101) facet can be seen (Figure 2c), while the (001) surface of the as-grown crystal is usually atomic flat as shown in the atomic force microscopy image in Figure 2d. To understand the growth process, we analyzed the thermodynamics of the transport reactions involved. As is well-known, transport reactions can be classified as exothermic and endothermic according to the thermodynamics of reaction between solid and transport agent. In the present case,

Figure 2. (a) Crystal structure of TaAs. (b) Photograph of TaAs single crystals. (c) Large and regular terraces on the (101) facet observed under an optical microscope. (d) Atomic force microscopy image of the (001) surface.

TaAs (s) + 4I 2 (g) ↔ TaI5 (g) + AsI3 (g)

(1)

Since the thermodynamic data for TaAs is not available, we first measured the specific heat as a function of temperature from 2 K to room temperature, obtained the Debye temperature θD = 352 K by data fitting, and then calculated its enthalpy and entropy (see Supporting Information, section S1). The thermodynamic data of other species were taken from ref 19 and 20, which are listed in Table 1. Then the enthalpy and entropy changes for the above reaction were calculated as ΔrHo = −319.975 kJ·mol−1, ΔrSo = −238.451 J·mol−1·K−1 at 298 K and ΔrHo = −294.060 kJ·mol−1, ΔrSo = −202.686 J· mol−1·K−1 at 1450 K. This is an exothermic reaction, suggesting that TaAs crystal growth will occur if gaseous species TaI5 and AsI3 migrate from the cooler end to the hotter end. The optimal growth temperature can be estimated according to van’t Hoff’s equation,

diffraction patterns (Figure 3a) of finely ground powder of the crystals are consistent with ICDD-PDF No. 33-1388, indicating that they are tetragonal TaAs.12,14,15 Note that, due to the symmetry of the TaAs lattice, some diffraction peaks like (001), (202) are systematically extinct. Well-developed TaAs crystal looks like a truncated dipyramid; see Figure 3a. On the basis of our X-ray diffraction measurements on the crystal facets, the facets can be divided into three groups: the two truncated surfaces are {001}, the trapezoid or isosceles triangular surfaces are {101}, and the rectangular ones {112}, as shown in Figure 3c. Since TaAs belongs to point group 4mm, the equivalent {101} and {112} planes should form a ditetragonal appearance. The observed morphology is one of the degenerated cases of the ideal form probably due to the anisotropic supply of gaseous species inside the ampule. Besides, facet (103) can be

ln K = −Δr H o/(RT ) + Δr S o/R B

(2) DOI: 10.1021/acs.cgd.5b01758 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Enthalpy and Entropy of Relevant Species19,20 species TaAs I2 TaI5 AsI3 a

a

Ho (298 K) (kJ·mol−1)

So (298 K) (J·mol−1·K−1)

Ho (1450K) (kJ·mol−1)

So (1450K) (J·mol−1·K−1)

−87.864 62.421 −197.066 38.911

59.868 260.685 472.341 391.816

−31.203 105.915 −35.778 134.175

137.143 320.149 692.828 522.225

Data were obtained from our heat capacity measurements. See Supporting Information, section S1.

Here K is the equilibrium constant, R is the ideal gas constant, ΔrHo and ΔrSo are the reaction enthalpy and entropy, respectively. At the optimal transport temperature, K should be equal to 1.21 This yields Topt = ΔrHo/ΔrSo. As ΔrHo and ΔrSo are temperature dependent, the solution is obtained graphically (see Supporting Information, section S2) as Topt = 1450 K, approximately 1180 °C. In experiments, however, we find that numerous tiny crystals are formed if the growth temperature is higher than 1100 °C, implying a speed-up in the spontaneous nucleation process. On the other hand, the impurity TaAs2 is often presented if the temperature is below 950 °C. Other impurity phases such as Ta3As, Ta2As, and Ta5As4 can be eliminated by a slight excess of As in the initial mixture of As and Ta. Taking these factors into consideration, the optimal growth temperature is chosen at around 1000 °C. In addition, a temperature gradient is needed to provide the necessary driving force for gaseous species in diffusion and convection between the cooler and hotter end. It is generally set as 5 °C/cm. Finally a temperature field of 1020 °C (crystallization zone) and 980 °C (source zone) is chosen in our optimized growth conditions. During the growth process, the gaseous species inside the silica ampule can be up to several atmospheric pressures, making the mean free path of gaseous molecules quite small. This is disadvantageous for diffusions. So the ampules can be tilted up to 30° from the horizon to enhance the convection and speed up the growth rate. The main obstacle encountered here in increasing crystal size is the spontaneous nucleation that can occur anywhere at the hotter half inside the ampule. This is, in particular, true if the Ta powder is used as the source material. It is found in our experiments that polished and annealed tantalum foil is a better choice than Ta powder to retard the spontaneous nucleation. This is probably because Ta powders usually have too many defects, which can act as nucleation centers. As the total mass of precursor is fixed, more nucleation centers undoubtedly result in numerous tiny single crystals. Instead, tantalum foils usually have much fewer defects. Polishing and annealing furtherly reduced the number of defects effectively. Another reason is that powders have very large specific surface area, and the reaction with iodine will be faster, leading to high concentration of gaseous iodides at the early growth stage. This, in turn, will result in enhanced spontaneous nucleation. Tantalum foil, in contrast, ensures a layer-by-layer and steady reaction, helpful not only in decreasing spontaneous nucleation at the early stage but also in supplying a steady flow of gaseous species to the seeds in the whole growth process. Figure 4a shows the grown crystals at an intermediate stage, which have advantages to grow larger in size at the expense of the numerous nearby tiny crystals on the polycrystalline foil (Figure 4b). The concentration of I2, the transport agent, is another critical parameter. Lower concentrations lead to slower reactions, while higher concentrations may have an increased risk of explosion. A concentration of 5 mg/mL is used in our

Figure 4. Scanning electron microscope image of the TaAs foil at the intermediate stage of the transport process. (a) Nucleation starts and small crystals appear. (b) Magnified image of the TaAs polycrystalline foil, the area among single crystals.

experiments. We advise that the concentration should range from 1 to 10 mg/mL for different purposes. In summary, single crystals of TaAs up to 1 cm in size can be grown by the CVT method using iodine as the agent. The involved reaction is exothermic when TaAs polycrystalline converts into gaseous iodides at the cooler end. Then TaAs should recrystallize at the hotter end around 1000 °C. The optimal growth temperature is estimated by thermodynamic data. The choice of Ta foil is helpful to retard spontaneous nucleation. Enhanced convection by tilting ampules to utilize gravity is realized to increase the growth rate. These measures should be applicable to the growth of other transition metal arsenides and phosphides.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01758. Heat capacity measurements and thermodynamic calculations, Estimation of optimal transport temperature (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(L.G.) Tel: 86-10-82649453. Fax: 86-10-82649646. E-mail: [email protected]. Web: http://www.blem.ac.cn/a02/ index/index-eng.html. *(X.C.) Tel: 86-10-82649039. Fax: 86-10-82649646. E-mail: [email protected]. Web: http://www.blem.ac.cn/a02/ index/index-eng.html. Author Contributions ‡

Z.L. and H.C. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.L.L. would like to thank T. P. Ying, H. Zhang, Z. P. Lin, K. K. Li, and M. Zhao of the Institute of Physics, CAS and Z. N. Guo C

DOI: 10.1021/acs.cgd.5b01758 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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of University of Science and Technology Beijing for fruitful discussions. This work was partly supported by the Ministry of Science and Technology of China (Grant No. 2011CB932700), the National Natural Science Foundation of China (Grants Nos. 51532010, 91422303, 51272279, 51472266, 51202286 and 51472265), and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB07020100).

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DOI: 10.1021/acs.cgd.5b01758 Cryst. Growth Des. XXXX, XXX, XXX−XXX