Polytypism, Polymorphism, and Superconductivity in Tase2−Xtex

Polytypism, polymorphism, and superconductivity PNAS PLUS in TaSe2−xTex

Huixia Luoa,1, Weiwei Xiea, Jing Taob, Hiroyuki Inouec, András Gyenisc, Jason W. Krizana, Ali Yazdanic, Yimei Zhub, and Robert Joseph Cavaa,1

aDepartment of Chemistry and cJoseph Henry Laboratories and Department of Physics, Princeton University, Princeton, NJ 08544; and bDepartment of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, NY 11973

Contributed by Robert Joseph Cava, February 9, 2015 (sent for review January 14, 2015; reviewed by J. Paul Attfield and Maw-Kuen Wu) Polymorphism in materials often leads to significantly different 3R form (20–22). The 3R form can be synthesized, but it is not the physical properties—the rutile and anatase polymorphs of TiO2 are stable variant (the 2H form is) and so has been the subject of little a prime example. Polytypism is a special type of polymorphism, study. In one of the other polymorphs, the 1T (T: trigonal) type, occurring in layered materials when the geometry of a repeating Ta is found in octahedral coordination in the Se-Ta-Se layers and structural layer is maintained but the layer-stacking sequence of the layer stacking along the c axis of the trigonal cell such that the the overall crystal structure can be varied; SiC is an example of structure repeats after only one layer (23) (Fig. 1A). Again, the 1T a material with many polytypes. Although polymorphs can have form has not been the subject of much study. Here we show that radically different physical properties, it is much rarer for polytyp- the 3R and 1T polymorphs are both quite stable in the TaSe2−xTex ism to impact physical properties in a dramatic fashion. Here we system and that they are both superconducting. For pure TaTe2, study the effects of polytypism and polymorphism on the super- the monoclinic structure is 1T based (Fig. 1A), but is distorted conductivity of TaSe2, one of the archetypal members of the large such that there are two nonequivalent Ta and three nonequivalent family of layered dichalcogenides. We show that it is possible to Te positions in the unit cell (24); we find TaSe2−xTex in this access two stable polytypes and two stable polymorphs in the polymorph to be nonsuperconducting down to 0.4 K. TaSe2−xTex solid solution and find that the 3R polytype shows We report the structures and superconducting properties of a superconducting transition temperature that is between 6 and SCIENCES TaSe − Te for 0 ≤ x ≤ 2. The 2H, 3R, 1T, and monoclinic dis- 17 times higher than that of the much more commonly found 2H 2 x x torted 1T-structure forms were successfully synthesized. Only APPLIED PHYSICAL polytype. The reason for this dramatic change is not apparent, but a small amount of Te doping (x = 0.02) changes 2H-TaSe into we propose that it arises either from a remarkable dependence of 2 the 3R polytype. Within the 3R polytype, TaSe2−xTex shows the Tc on subtle differences in the characteristics of the single layers present or from a surprising effect of the layer-stacking sequence coexistence of a CDW and superconductivity above 0.4 K for 0.1 ≤ x ≤ 0.35. The Te-rich limit of the 3R-TaSe1.65Te0.35 polytype on electronic properties that are typically expected to be domi- T nated by the properties of a single layer in materials of this kind. shows the highest c in the system, 2.4 K, which is 17 times higher than that of 2H-TaSe2. For 0.8 ≤ x ≤ 1.3, 1T-type TaSe2−xTex T – superconductivity | polytypism | polymorphism | dichalcogenide | emerges and shows a lower c, of 0.5 0.7 K. At higher Te sub- ≤ x ≤ charge-density wave stitutions (1.8 2), TaSe2−xTex changes again, into the monoclinic polymorph, and shows normal metallic behavior to 0.4 K. We argue that the isovalent Te/Se substitution acts to tune he MX2 layered transition-metal dichalcogenides (TMDCs, TM = Mo, W, V, Nb, Ta, Ti, Zr, Hf, or Re and X = Se, S, or the anisotropy of the layers, inducing the 3R to 1T transition, Te), have long been of interest due to the rich electronic prop- consistent with what has been proposed previously (25). The driving erties that emerge due to their low dimensionality (1–9). Struc- force for the 2H to 3R transition currently remains obscure. turally, these compounds can be regarded as having strongly bonded (2D) X–M–X layers, with M in either trigonal prismatic Significance or octahedral coordination with X, and weak interlayer X–X bonding of the van der Waals type. Many of these materials Although polymorphs of a substance can often have dramati- manifest charge-density waves and competition between charge- cally different physical properties, polytypes, which occur density waves (CDWs) and superconductivity, e.g., refs. 5–9. when the geometry of a structural layer is maintained but the Among the TMDCs, the 2H (H: hexagonal) polytype of tantalum number of layers in the layer-stacking sequence is changed, diselenide (2H-TaSe2) is considered one of the foundational rarely do. Here we find, using random substitution of Te for – materials (8 18), showing a transition from a metallic phase to some of the Se to induce structural changes in TaSe2, a classic an incommensurate charge-density wave (ICDW) phase at 123 layered dichalcogenide, so that the transition temperature to “ ” K, followed by a lock-in transition to a commensurate charge- superconductivity (Tc) is significantly different for different density wave (CCDW) phase at 90 K. It finally becomes a su- polytypes and polymorphs and especially differs when going perconductor with a rather low transition temperature (Tc)of from one polytype to another. This observation implies either 0.15 K. Although detailed studies have been performed on the a surprising sensitivity of Tc to the layer-stacking sequence or – physics of CDWs and superconductivity in 2H-TaSe2 (16 18), a similarly surprising sensitivity of Tc to the small changes in a comparative study of the superconductivity of the polytypes layer geometry that accompany the change in polytype. and polymorphs of TaSe2 from the chemical perspective has not been done. Author contributions: H.L., J.T., A.Y., Y.Z., and R.J.C. designed research; H.L., W.X., J.T., H.I., A.G., J.W.K., A.Y., and Y.Z. performed research; H.L., W.X., J.T., H.I., A.G., J.W.K., A.Y., TaSe2 is highly polymorphic, possibly the most polymorphic of Y.Z., and R.J.C. analyzed data; and H.L., W.X., J.T., H.I., A.G., J.W.K., A.Y., Y.Z., and R.J.C. the TMDCs (19). In some of its forms, notably the 2H and 3R wrote the paper. A (R: rhombohedral) polytypes (Fig. 1 ), Ta is found in trigonal Reviewers: J.P.A., University of Edinburgh; and M.-K.W., Academia Sinica. prismatic coordination in Se-Ta-Se layers that are stacked along The authors declare no conflict of interest. c the axis of the hexagonal (or rhombohedral) cell. The 2H and Freely available online through the PNAS open access option. — 3R polytypes differ only in their stacking periodicity the struc- 1To whom correspondence may be addressed. Email: [email protected] or huixial@ ture repeats after two layers in the 2H form and three layers in the princeton.edu.

www.pnas.org/cgi/doi/10.1073/pnas.1502460112 PNAS Early Edition | 1of7 Downloaded by guest on October 1, 2021 A Ta 3R-TaSe1.7Te 0.3 Monoclinic-TaTe2 Te Se

2H-TaSe 2 1T-TaSeTe

BC D 1.90 3R-TaSe Te 1.90 2-x x 3.64 TaSe Te

/ 2-x x 2H-TaSe2 1T-TaSe Te

) 2-x x / 1.88 a ) 1.88 axis 1T

c/n a 3.60 ( c/n a

( 1.86 1.86 3.56 1.84 1.84 (Å) 0.00 0.15 0.30 0.8 0.9 1.0 1.1 1.2 1.3 a 2H x in TaSe Te x in TaSe Te 3.52 2-x x 2-x x 3R 3R 3R-TaSe Te 1T-TaSe Te

2-x x 2-x x 3.48 2H 1T Mixed phase Mixed

Intersity (Arb. Unites) Intersity (Arb. Unites) 3.44 10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 0.0 0.2 0.4 0.6 0.81.0 1.2 1.4 2 (degree) 2 (degree) x in TaSe Te EF G2-x x 3.7 6.8 TaSe Te 3.8 TaSe Te TaSe Te 2-x x 2-x x 3.5 2-x x reduced c axis 1T 3.7 vdW Gap thickness 6.7 TaX 2 slab 1T

) thickness 3.3 2H ) 6.6 Å 3.6 3R (Å)

Å 3R ) ( 3.1 1T ( 6.5 2H z 3.5 Δ WG ( ⋅ c/n 2H 3R 2H 2.9 c 3.4 2H vd 6.4 3R 3R 2H 2.7

6.3 1T 3.3 1T 3R

Mixed phase Mixed Mixed phase Mixed Mixed phase Mixed 2.5 1T 6.2 3.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

x in TaSe2-xTe x x in TaSe2-xTe x x in TaSe2-xTe x

Fig. 1. Structural characterization and analysis of the polytypes and polymorphs of TaSe2−xTex.(A) The crystal structures of 2H-TaSe2, 3R-TaSe1.65Te0.35, 1T-TaSeTe, and monoclinic TaTe2.(B) Powder X-ray diffraction pattern for 3R-TaSe1.65Te0.35. Inset shows the reduced lattice parameter ratio, (c/n)/a, where n = number of layers per cell, for 2H-TaSe2 (38) and 3R-TaSe2−xTex.(C) Powder X-ray diffraction pattern for 1T-TaTeSe. Inset shows the reduced lattice parameter ratio, (c/n)/a, for 1T-TaSe2−xTex.(D) The variation of in-plane lattice parameter, a, with x for 2H-, 3R-, and 1T-TaSe2−xTex.(E) The variation of reduced stacking- direction lattice parameter, c/n, with x for 2H-, 3R-, and 1T-TaSe2−xTex.(F) The variation of the TaX2 slab thickness, ((c·(Δz)), with x for 2H-, 3R-, and 1T-TaSe2−xTex. (G) The variation of the van der Waals gap (vdWG) thickness with x for 2H-, 3R-, and 1T-TaSe2−xTex.

Results and Discussion 3R. The PXRD pattern for 3R-TaSe1.65Te0.35 is shown in the main B The polycrystalline samples of TaSe − Te were prepared as panel of Fig. 1 . For higher Te contents, a mixture of MX2 struc- 2 x x x = described in Methods. In the composition range of 0.02 ≤ x ≤ tures is encountered until 0.8, where TaSe2−xTex changes into 0.35, the samples have the noncentrosymmetric rhombohe- the 1T polymorph (P-3m1, space group 164), which exists until x = dral 3R structure (R3m, space group 160), evidenced by their 1.3. The powder diffraction pattern for one of the 1T compositions powder X-ray diffraction (PXRD) patterns. The 3R structure of the is shown in the main panel of Fig. 1C. Reitveld refinements of the materials in this composition range is also confirmed by single- PXRD data (Methods)wereusedtospecifythez coordinates of the crystal X-ray diffraction and electron diffraction. The detailed (Se,Te) atom positions across the 3R and 1T solid solutions, crystallographic data determined from the quantitative structure allowing the determination of the TaX2 and van der Waals layer refinements of a single crystal of the 3R phase are summarized in thicknesses separately. Tables 1 and 2. In the refined crystal structure of 3R-TaSe1.7Te0.3, To compare the structural stability regimes of the different c Ta atoms are located in trigonal prisms surrounded by a random forms of TaSe2−xTex, it is most instructive to divide the -axis mixture of Te and Se atoms. Refining the structure with the ideal lattice parameter by the number of layers in the stacking repeat, 3R atomic coordinates leaves a significant positive residual electron n, and then compare the thus-obtained reduced c/a ratios, (c/n)/a, density that is unaccounted for by the model. By investigating the to define the structural characteristics of single MX2 layers detailed electron density maps, layer-stacking faults, which are (“slabs” in the following) and their associated van der Waals a common occurrence in crystal structures of this type, were ob- gaps. In Fig. 1 B and C, Insets show the x variation of the reduced served through the presence of an “extra” atom site in the tantalum c/a ratio, (c/n)/a, for the 2H (n = 2, x = 0), 3R (n = 3, 0.02 ≤ x ≤ layer, occupied at the 5% level; thus about 5% of the layers in the 0.35), and 1T (n = 1, 0.8 ≤ x ≤ 1.3) phases, respectively. As c n a x 3R TaSe1.7Te0.3 crystal studied quantitatively are stacked in a way shown in the plots, the ( / )/ ratio increases with increasing in that violates the ideal A-B-C stacking of the 3R phase (e.g., in an the 3R form, with a discontinuity at the 2H to 3R transition, and A-B-A-B stacking); the remaining 95% of the structure is unfaulted is always less than the ideal ratio, which for the close-packed

2of7 | www.pnas.org/cgi/doi/10.1073/pnas.1502460112 Luo et al. Downloaded by guest on October 1, 2021 PNAS PLUS Table 1. Single-crystal crystallographic data for 330 K. For 3R TaSe1.9Te0.1, the superlattice peaks are first in 3R-TaSe1.70Te0.30 at 296(2) K incommensurate positions and weak but then sharpen and in- tensify significantly on reducing temperature until at 10 K they are Formula TaSe1.70Te0.30 sharp and strong, indicating that the CDW is well defined and or- F.W., g/mol 353.46 dered over a long range at low temperatures. The incommensurate Space group; ZR3m (no.160); 3 locations of the spots in reciprocal space at higher temperatures a,Å 3.4603(7) show that first there is an ICDW phase with the q vector larger than c,Å 19.523(4) 0.33. The ICDW diffraction spots then change position on cooling V,Å3 202.44(9) until they lock into nearly commensurate positions, q ∼ 0.32, at Absorption correction Multiscan around 100 K. The illuminated areas from which the electron dif- Extinction coefficient 0.006(3) fraction patterns were obtained are relatively large, with beam −1 μ,mm 66.441 diameters greater than 300 nm. The small value of incommensu- θ range, ° 3.130–29.555 ration observed in the low-temperature locked-in CDW phase, hkl ranges −4 ≤ h,k ≤ 4 which is less than 0.01 from the commensurate value of 0.33, may −27≤ l ≤ 27 come from defects and domain walls in the low-temperature CDW No. reflections; Rint 714; 0.0451 phase; as the STM topographic images show (see below), the CDW No. independent reflections 189 is locked into a commensurate relationship with the underlying No. parameters 14 atomic lattice over the vast majority of the material. We thus R1; wR2 (all I) 0.0530; 0.1347 consider this to be a commensurate CDW with a wave vector of Goodness of fit 1.284 q = 0.33. The 10-K electron diffraction pattern shown in Fig. 2A − 3 – Diffraction peak and hole, e /Å 7.885; 3.371 is an indication of the quality of this low-temperature CCDW The numbers in parentheses are the standard error of the refined parameters. phase. At the lock-in transition, the intensity of the diffracted spots from the CDW increases dramatically, an indication of its strengthening in the CCDW state. The temperature-dependent trigonal-prismatic arrangement is considered to be 2.0 (26). characteristics of the CDW in 3R TaSe1.9Te0.1 obtained from the The (c/n)/a ratio collapses for the 1T polymorph and changes electron diffraction study are summarized in Fig. 2C.Thus,in

x c n a SCIENCES relatively little with increasing .Inthiscase,the( / )/ ratios analogy to the 2H form of TaSe2, 3R-TaSe1.9Te0.1 first has an c n a are slightly larger than the ideal value of 1.633 (26). The ( / )/ ICDW and then locks in to a CCDW phase on further cooling. APPLIED PHYSICAL ratio where the 3R polytype becomes unstable and the 1T poly- The q vectors of the CCDW phase, q1 = q2 = 0.33, indicate a tri- morph becomes stable is consistent with expectations for MX2 pling of the in-plane unit cell along both a1 and a2 by the CDW. phases, as has been described by others (26). STM measurements on 3R-TaSe1.9Te0.1 andon3R-TaSe1.7Te0.3 Fig. 1 D and E shows the variations of the room-temperature provide additional characterization of the CCDW phase. Topo- in-plane lattice parameters (a) and the reduced lattice parame- graphic images on the atomic scale (Fig. 3) display the in-plane unit ters (c/n) in the stacking direction for the 2H, 3R, and 1T forms cell tripling in real space on the surface of both samples below the of TaSe2−xTex over the full range of composition stability. An CDW transition temperature. Whereas the CDW superlattice in overall trend toward larger a and c values with increasing x is TaSe1.9Te0.1 is clearly visible (Fig. 3 A and B), that in TaSe1.7Te0.3 observed across the whole family as larger Te replaces Se, but is moderately masked by the disorder (Fig. 3 D and E). However, the composition dependence is also generally different in the the Fourier transform of the topographic images for both samples different structural forms and there are discontinuities at the (Fig. 3 C and F) unambiguously reveals the primary peaks of the q = q = transitions between forms. A mixture of MX2 phases is en- CCDW at 1 2 0.33. Hence, the CDW is present for both countered at higher x until the distorted 1T structure of TaTe2 is high and low Te contents in the 3R phase. Remarkably, even in found for 1.8 ≤ x ≤ 2.0. the presence of the strong disorder due to the Se/Te solid so- It is instructive to look at the changes in structure within the lution, we observe that the phase of the CDW is unperturbed and ∼ × TaSe2−xTex series in more detail by considering separately only a single-domain CDW appears in the field of view ( 40 nm the composition dependencies of the TaX2 slab thickness and the 40 nm). Furthermore, the STM data indicate that the apparent van der Waals gap thickness. This is shown in Fig. 1 F and G, tripling of the cell by the CDW in both in-plane directions de- where the variation of the TaX2 slab thickness, [(c·(Δz), where c duced from the electron diffraction patterns is not due to the is the c-axis length and Δz is the difference in refined z coor- overlap of single-q domains in different orientations; i.e., it is dinates for the X positions above and below the Ta layer] and a 2D CDW. Further, the diffracted spots from the CDW phase that of the similarly determined van der Waals gap (vdWG) are visible in single-crystal diffraction experiments at 100 K; the thickness are presented. As shown in Fig. 1F, the thickness of the data show that the q vector is in plane only; there is no c-axis TaX2 slab increases essentially continuously with increasing x component. The data therefore show that for the CCDW in the across all forms, with a change in slope ongoing from the 3R to 1T 3R polymorph, q1 = 0.33, q2 = 0.33, and q3 = 0. In the CCDW polymorphs. In contrast, the van der Waals gap thickness changes state the 2H-TaSe2 polytype is also reported have the wave vec- very little with x in either the 3R or the 1T polytypes, with a sig- tors q1 = q2 = 0.33 and q3 = 0 (27, 28). In other words, in both 2H nificant collapse of thickness on passing from the 3R to 1T polymorphs. The factors determining the behaviors observed in Fig. 1 F and G are not presently known. Table 2. Atomic coordinates and equivalent isotropic To determine whether CDWs are present in the 3R and 1T displacement parameters of TaSe1.70Te0.30

forms of TaSe2−xTex, the materials were studied at low temper- Atom Wyckoff Occupancy xy z Ueq atures by electron diffraction and scanning tunneling microscopy (STM). The temperature-dependent electron diffraction patterns Ta1 3a 0.95(1) 0 0 0.2931(1) 0.009(1) obtained from single-crystal domains (Fig. 2 A and B) reveal Ta2 3a 0.05(1) 2/3 1/3 0.293(3) 0.009(1) critical information about the CDWs in both materials. In the 3R Se/Te1 3a 0.85/0.15 1/3 2/3 0.2059(3) 0.012(1) Se/Te2 3a 0.85/0.15 1/3 2/3 0.3804(3) 0.011(1) form, a strong CDW appears on cooling TaSe2−xTex below am- 2 bient temperature. The CDW gives rise to extra peaks in the Ueq is defined as one-third of the trace of the orthogonalized Uij tensor (Å ). electron diffraction patterns, which are already weakly visible at The numbers in parentheses are the standard error of the refined parameters.

Luo et al. PNAS Early Edition | 3of7 Downloaded by guest on October 1, 2021 A C 0.025 0.35 T = 10 K T = 83 K T = 155 K 0.34

0.020 a / 0.33

(a.u.)

CDW CDW q 0.015 0.32

CDW 0.31 0 100 200 300 T = 223 K T = 295 K T = 330 K 0.010 Temperature (K)

0.005 TCDW

Intensity of q 0.000 0 50 100 150 200 250 300 B Temperature (K) T = 10 k T = 78 K T = 125 K D 0.025 0.32

0.31

0.020 a /

(a.u.) 0.30

CDW CDW q CDW 0.015 0.29 0.28 T = 180 K T = 240 K T = 330 K 0 100 200 300 (110) 0.010 Temperature (K) - (000) (210) 0.005

Intensity of q 0.000 050100 150 200 250 300 350 Temperature (K)

Fig. 2. Characterization of the charge-density waves in the 3R and 1T polymorphs. Temperature dependence of the incommensurate CDW state in the a–b

plane is shown. (A) temperature dependence of electron diffraction patterns of polycrystalline 3R-TaSe1.7Te0.3.(B) Temperature dependence of electron diffraction patterns of polycrystalline 1T-TaTeSe. (C and D) CDW vector qCDW as a function of temperature for (C) 3R-TaSe1.6Te0.3 and (D) 1T-TaSeTe.

and 3R polytypes of TaSe2, the electronic instabilities that lead to atures and increases to a maximum of 2.4 K for 3R-TaSe1.65Te0.35, the CCDWs are 2D in character. substantially higher than that observed in the 2H form. The The situation is somewhat different for the 1T polymorph (Fig. 2 superconducting transition is clearly observed through the presence B and D). In this case, weak, diffuse superlattice spots whose in- of a full shielding signal in the temperature-dependent magneti- tensities and positions are relatively independent of temperature zation measurements (Fig. 4D). between 10 K and 330 K are observed in the electron diffraction The main panel of Fig. 4B shows the temperature dependence experiments. Here the in-plane q vector is farther from the com- of the normalized electrical resistivity (ρ/ρ300K) for the poly- mensurate value, near q = 0.30, but the weakness and diffuseness crystalline samples of 1T-TaSe2−xTex (0.8 ≤ x ≤ 1.3). In this case, of the spots make them invisible in a single-crystal diffraction ex- the residual-resistivity ratio is very small, RRR = ρ300K/ρn < 1.3, periment and so we have no information about their c-axis com- which we interpret as a reflection of the substantial Se-Te dis- ponent. These spots likely represent an ICDW phase with q1 = q2 = order present. No signature of a CDW lock-in transition is seen in 0.30 and q3 = 0 that is ordered over short spatial distances and ρ(T), consistent with the electron diffraction data. At low tem- stable over the whole temperature range studied. Alternatively they peratures, a clear, sharp (ΔTc < 0.1 K) drop of ρ(T) is observed, may have a chemical origin, such as might occur due to short-range signifying the onset of superconductivity (Fig. 4B, Inset). The Se/Te ordering. Further work will be required to determine which sample with x = 1showsthehighestTc, 0.73 K. Finally, Fig. 3C is the case. presents the temperature dependence of the normalized resistiv- We next consider the electronic properties of the phases. The ity for polycrystalline samples of the monoclinic polymorph of A main panel of Fig. 4 shows the temperature dependence of the TaSe2−xTex (1.8 ≤ x ≤ 2). This variant shows metallic behavior, normalized electrical resistivity, (ρ/ρ300K), for the polycrystalline similar to what has been previously reported (24), with no super- 3R-TaSe2−xTex (0.02 ≤ x ≤ 0.35) samples. All of the 3R samples conducting transition down to 0.4 K. have resistivities below 10 mohm/cm at 300 K with a metallic Further information on the electronic properties and super- temperature dependence, and all show the signature of the lock- conductivity in the 3R and 1T variants of TaSe2−xTex was ob- in to the CCDW phase at around 100 K through a change in tained from specific heat measurements. The main panels of slope of ρ(T). A similar change in slope of ρ(T) is observed in Fig. 4 E and F show the temperature dependence of the zero- 2 many TMDC systems at the onset of a CDW that localizes some field specific heat, Cp./T vs. T ,for3R-TaSe1.65Te0.35 and 1T- but not all of the electrons at the Fermi surface (29). A look at the TaSe1.2Te0.8. The normal state-specific heat at low temperatures 3 derivative of the ρ(T)curves(Fig.4D, Inset) indicates that the (but above Tc) obeys the relation of Cp = γT + βT ,whereγ and β impact of the CDW lock-in transition, which appears to have describe the electronic and phonon contributions to the heat ca- a temperature that is independent of Te content, weakens with pacity, respectively, the latter of which is a measure of the Debye increasing Te content in the 3R phase. Correspondingly, a super- temperature (θD). By fitting the data in the temperature range of conducting transition is found (Fig. 4A, Inset). With higher Te 2–10 K, we obtain the electronic specific heat coefficients γ = 7.25 −1 −2 −1 −2 doping in the 3R phase, the superconducting transition temperature mJ·mol ·K for 3R-TaSe1.65Te0.35 and γ = 2.91 mJ·mol ·K (Tc) increases; the superconductivity is first found at low temper- for 1T-TaSe1.2Te0.8 and the phonon specific heat coefficients

4of7 | www.pnas.org/cgi/doi/10.1073/pnas.1502460112 Luo et al. Downloaded by guest on October 1, 2021 A D PNAS PLUS

B C E F

Fig. 3. (A–F) Visualization of the charge-density wave on the surface of 3R-TaSe1.9Te0.1 (A–C) and 3R-TaSe1.7Te0.3 (D–F)bySTM.(A and B) Real space to- × × – = − = = pographic images of (A) 300-Å 300-Å and (B) 60-Å 60-Å areas on the cleaved a b surface of TaSe1.9Te0.1 [VBias 800 mV, (A) I 100 pA, and (B) I 60 pA] SCIENCES at T = 48 K, which show the tripling of the in-plane unit cell. The CDW remains unchanged around the bright spots on the surface, which are associated with the substituted Te atoms. (C) The Fourier transform of a 440-Å × 440-Å large topographic image reveals wave vectors corresponding to the atomic modu- APPLIED PHYSICAL

lation (black circles) and q vectors of the commensurate charge-density wave phase (q1 = q2 = 0.33, red circles). Higher harmonics of q1 and q2 are marked by gray circles. (D–F) Similar topographic images (D and E) of the surface of TaSe1.7Te0.3 at VBias = 300 mV, I = 200 pA, and T = 27 K and (F) Fourier transform of a 490-Å × 490-Å large area. The CDW is clearly observed despite the disorder induced by the higher Te substitution.

−1 −4 β = 0.93 mJ·mol ·K for 3R-TaSe1.65Te0.35 and β = where n is the number of atoms per formula unit (n = 3) and R −1 −4 1.82 mJ·mol ·K for 1T-TaSe1.2Te0.8.Usingthesevaluesofβ,we is the gas constant; the θD values are found to be 184 K for 4 1/3 estimate the Debye temperatures by the relation θD = (12π nR/5β) , 3R-TaSe1.65Te0.35 and 147 K for 1T-TaSe1.2Te0.8. As shown in

ABC1.0 x=0.35 1.4 1.0 x=0.3 Monoclinic TaSe2-xTe x x=0.25 1T-TaSe2-xTe x x=0.2 1.2 x=2 0.8 x=0.15 0.8 x=1.9 x=0.1 1.0 x=1.8 0.6 0.0004 0.8 0.6 TCDW 0.0008 0.0003 0.6 0.0002 0.4 x=0.8 0.0004 0.4 0.0002 x=0.9 0.0001 0.0001 0.4 x=1 0.2 0.0000 0.2 0.0000 0.0000 x=1.2 0.4 0.6 0.8 0.0 1.0 2.0 3.0 0.2 x=1.3 0.0 1.0 2.0 3.0 T (K) T (K) T (K) 0.0 0.0 0.0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 0 50 100 150 200 250 300 T (K) T (K) T (K)

D E 120 15 F 15 0.0000 6 10 T = 0.6 K TaSe Te TaSe Te 100 4 c 1.2 0.8 -0.0005 FC 1.65 0.35 5 12 H = 10Oe 0 2 80 -5 -0.0010 0.000006 0 -10 9 -2 60 0 1 2 3 4 -0.0015 0.000004 T 0.0 0.3 0.6 0.9 1.2 1.5 LO T (K) 5 T (K) M’ (emu) M’ -0.0020 0.000002 40 0.00000 -0.0025 50 100 150 20 3 T (K) ZFC -0.0030 0 0 1.8 2.0 2.2 2.4 2.6 2.8 3.0 0 20 40 60 80 100 1 2 3 4 T (K)

Fig. 4. Characterization of the electronic properties of TaSe2−xTex.(A) The temperature dependence of the ratio (ρ/ρ300K) for 3R-TaSe2−xTex.(Inset) Enlarged view of low-temperature region (0.4–3 K), showing the superconducting transition. (B) The 1T-TaSe2−xTex.(Inset) Enlarged view of low-temperature region (0.4–0.8 K) showing the superconducting transition. (C) Monoclinic TaSe2−xTex (1.8 ≤ x ≤ 2). (Inset) Enlarged view of the low-temperature region showing the absence of superconductivity. (D) The temperature dependence of dc magnetic susceptibility for 3R-TaSe1.65Te0.35.(Inset)Enlargedviewofdρ/dT of 3R-TaSe2−xTex showing 2 CDW lock in temperature (TLO). (E) The temperature dependence of specific heat Cp of 3R-TaSe1.65Te0.35, presented in the form of Cp/T vs. T (main panel) and Cel/T 2 vs. T (Inset). (F) The temperature dependence of specific heat Cp of 1T-TaSe1.2Te0.8, presented in the form of Cp/T vs. T (main panel) and Cel/T vs. T (Inset).

Luo et al. PNAS Early Edition | 5of7 Downloaded by guest on October 1, 2021 Fig. 4 E and F, Insets, both materials display a large specific heat saturating, but varies very little with this important characteristic jump at Tc. The superconducting transition temperatures are of a single layer in the 1T form. A dramatic distinction is seen in excellent agreement with the Tcs determined in the ρ(T) between the 2H and 3R polytypes on this plot. Finally, the −1 −2 measurements. We estimate ΔC/Tc = 8.7 mJ·mol ·K for 3R- overall behavior of the TaTexSe2-x system is summarized in the −1 −2 TaSe1.65Te0.35 and ΔC/Tc = 3.93 mJ·mol ·K for 1T-TaSe1.2Te0.8. structural and electronic phase diagram shown in Fig. 5C.OnTe The normalized specific heat jump values ΔC/γTc arefoundtobe substitution for Se in 2H-TaSe2, the 3R polytype is immediately 1.20 for 3R-TaSe1.65Te0.35 and 1.35 1T-TaSe1.2Te0.8, which are near stabilized and in TaSe2−xTex exists in the range of 0.02 ≤ x ≤ that of the Bardeen–Cooper–Schrieffer (BCS) weak-coupling limit 0.35. The 3R-TaSe2−xTex shows the coexistence of a CDW value (1.43), confirming bulk superconductivity. Using the Debye and superconductivity in this composition range. The super- temperature (θD) and the critical temperature Tc and assuming that conducting transition temperature is found to maximize at the the electron–phonon coupling constant (λep) can be calculated from limit of the structural stability of the 3R phase, x = 0.35. The the inverted McMillan equation (30), maximum Tc, 2.4 K, is substantially higher than that for 2H- TaSe2. We conclude that 3R-TaSe2−xTex can be considered as 1:04 + μ p lnðθD=1:45TcÞ a good candidate for characterizing the balance between CDW λep = : ð1 − 0:62μ p ÞlnðθD=1:45TcÞ − 1:04 formation and superconductivity in the 3R polytype of the layered TMDCs. With further Te doping, the 3R polytype becomes c n a The values of λep obtained are 0.64 for 3R-TaSe1.65Te0.35 and unstable as its ( / )/ ratio approaches the structural stability 0.51 for 1T-TaSe1.2Te0.8 and suggest weak coupling supercon- limit expected for TMSCs, and the 1T polymorph, with octahe- ductivity. With the Sommerfeld parameter (γ) and the electron– dral rather than trigonal prismatic coordination for the Ta, phonon coupling (λep), the density of states at the Fermi level can emerges at x = 0.8. The 1T polytype structure exists from x = 0.8 2 2 be calculated from NðEFÞ = ð3=π kBð1 + λep ÞÞγ.ThisyieldsN(EF) = to 1.3. The 1T-TaSe2−xTex-x displays superconducting transitions T x 1.88 states/eV formula unit for optimal 3R-TaSe1.65Te0.35 and below 1 K and the c does not change significantly with . The N(EF) = 0.82 states/eV formula unit for 1T-TaSe1.2Te0.8.Thesecom- 1T-TaSe2−xTex appears to display a weak, short-range ordered pare with λep = 0.4 and N(EF) = 1.55 for the 2H-TaSe2 (31). The incommensurate CDW at temperatures as high as 330 K, but somewhat larger N(EF)andλep values for the 3R polytype may further work will be necessary to support that conclusion. The be why it has dramatically higher Tc than the 2H polytype, but monoclinic polymorph exists over a limited composition range in why these parameters are different in the 2H and 3R polytypes TaSe2−xTex, from x = 1.8 to 2.0; it is metallic but not super- is not currently known. conducting above 0.4 K. Fig. 5A shows the variation of the superconducting Tc with the In conclusion we have shown that the TaSe2−xTex system, reduced c/a ratio (c/n)/a, for 2H-TaSe2, 3R-TaSe2−xTex (0.1 ≤ x ≤ based on the isoelectronic substitution of Te for Se in TaSe2,is 0.35), and 1T-TaSe2−xTex (0.8 ≤ x ≤ 1.3), where n = number of an excellent venue for investigating the influence of polytypism layers in the stacking repeat, and c and a are the unit cell and polymorphism on superconductivity in the layered transition parameters. Tc increases smoothly with (c/n)/a in the 3R form, metal dichalcogenides. It may be that the major impact of the but is constant in the 1T form. The 2H, 3R, and 1T forms are change in polytype from 2H to 3R in this system, which increases clearly distinguished in this plot, showing that the super- Tc by a factor of 6–17, is in the final analysis actually still a 2D conducting Tc is not a simple function of the overall structural effect. The 3R polytype is stable for larger (c/n)/a and slab characteristics across the whole family. The distinction between thickness ratios for a single layer than it is possible to obtain in the polymorphs and polytypes is seen even more dramatically in the 2H polytype. If one of these is the primary reason for the Fig. 5B, where Tc is plotted as a function of the aspect ratio of difference in Tc, then it indicates a remarkable sensitivity of Tc the TaX2 slab across the whole family. Tc increases nearly im- to the aspect ratio of the TaX6 triangular prisms that make possibly steeply with the slab aspect ratio in the 3R form before up the single layers, evidenced especially in Fig. 5B. Alternatively,

AC 3.0 5.0 TaSe Te 2.5 2-x x 4.5 Superconductivity in 2.0 2H (K) 3R TaSe Te dichalogenides c 1.5 3R 4.0 2-x x T 1T 1.0 1T 3.5 0.5 2H 3.0 R3m P-3m1 C2/m 0.0

(K)

1.84 1.85 1.86 1.87 1.88 1.89 c B (c/n)/a T 2.5 2.8 2.0 2.4 TaSe 2-xTe x 2.0 3R 1.5 1.6

(K)

c 2H 1.0

1.2 2H T 3R 0.8 1T 1T 0.5

0.4 2H

Monoclinic metal Monoclinic Mixed phase Mixed 3R-SC

Mixed phase Mixed 1T-SC 0.0 0.0 0.96 0.98 1.00 1.02 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 c⋅ Δz a ( ( ))/ x in TaSe2-xTe x

Fig. 5. Structural and superconducting phase diagram for TaSe2−xTex.(A) The variation of the superconducting Tc with the reduced c/a ratio (c/n)/a, for 2H- TaSe2, 3R-TaSe2−xTex (0.1 ≤ x ≤ 0.35), and 1T-TaSe2−xTex (0.8 ≤ x ≤ 1.3), where n = number of layers in the stacking repeat, and c and a are the unit cell parameters. The different superconducting forms are very clearly distinguished in this plot. (B) The variation of the superconducting Tc with the TaX2 slab thickness, (c·(Δz)), for 2H-TaSe2, 3R-TaSe2−xTex (0.1 ≤ x ≤ 0.35), and 1T-TaSe2−xTex (0.8 ≤ x ≤ 1). Again, the different superconducting forms are very clearly distinguished. (C) The composition stability ranges of the 2H, 3R, 1T, and monoclinic MX2 forms in TaSe2−xTex and the dependence of Tc on x. The TaX2 coordination polyhedra are highlighted. Single-phase regions are shown in pink, and multiple-phase regions are shown in blue.

6of7 | www.pnas.org/cgi/doi/10.1073/pnas.1502460112 Luo et al. Downloaded by guest on October 1, 2021 the difference in superconducting transition temperature of the a Quantum Design Physical Property Measurement (PPMS). For the super- PNAS PLUS polytypes, similarly suggested to be quite different in Fig. 5B,may conducting samples, Tc was taken as the intersection of the extrapolation of depend on differences in the electronic structure that arise as the steepest slope of the resistivity ρ(T) in the superconducting transition re- ρ a result of the differences between a two-layer stacking repeat and gion and the extrapolation of the normal state resistivity ( n) (33). Magneti- zation measurements as a function of temperature and applied field were a three-layer stacking repeat, in other words how the nominally 2D carried out in a Quantum Design Magnetic Property Measurement (MPMS). Fermi surface is modulated along c, the perpendicular direction, Selected resistivities for TaSe2−xTex (0.1 ≤ x ≤ 0.25; 0.7 ≤ x ≤ 1.3; and 1.8, 1.9, by the layer stacking. Further investigation by both experiment and 2) and heat capacities for TaSe1.65Te0.35 and TaSe1.2Te0.8 were measured in and electronic structure calculations will be required to resolve the PPMS equipped with a 3He cryostat. which of these is the case or whether a different strong influence Single crystals from the samples were mounted on the tips of glass fibers. on Tc is present. The Tc of the 1T polymorph is intermediate Room temperature intensity data were collected on a Bruker Apex II dif- fractometer with Mo radiation K (λ = 0.71073 Å). Data were collected over between that of the 2H and the 3R, with Tc a factor of 5 larger a1 than that in the 2H variant, but as the TaX coordination scheme a full sphere of reciprocal space with 0.5° scans in ω with an exposure time of 6 θ is octahedral rather than trigonal prismatic in this polymorph, 10 s per frame. The 2 range extended from 4° to 60°. The SMART software different aspects of its electronic structure may determine its was used for data acquisition. Intensities were extracted and corrected for Lorentz and polarization effects with the SAINT program. Empirical ab- superconducting transition temperature. Further study of this sorption corrections were accomplished with SADABS, which is based on system may prove to be of significant interest. modeling a transmission surface by spherical harmonics, using equivalent > σ – Methods reflections with I 2 (I)(3437). With the SHELXTL package, the crystal structures were solved using direct methods and refined by full-matrix least 2 Polycrystalline samples of TaSe2−xTex (0 ≤ x ≤ 2) were synthesized in two squares on F (36). All crystal structure drawings were produced using the steps by solid-state reaction. First, mixtures of high-purity fine powders of Ta program VESTA (37). [99.8 atomic (atm) %], Te (99.999 atm%), and Se (99.999 atm%) in the ap- Before the STM measurements, the samples were cleaved and transported propriate stoichiometric ratios were thoroughly ground, pelletized, and to the microscope in ultrahigh vacuum. The experiments were performed on

heated in sealed evacuated silica tubes at a rate of 1 °C/min to 700 °C and TaSe1.9Te0.1 at 48 K and on TaSe1.7Te0.3 at 27 K with a home-built variable held there for 48 h. Subsequently, the as-prepared powders were reground, temperature STM. Temperature-dependent electron diffraction measure- repelletized, and sintered again, heated at a rate of 3 °C/min to 1,000 °C and ments were performed on a JEOL 2100F microscope at Brookhaven National held there for 48 h. Single crystals were grown by the chemical vapor trans- Laboratory equipped with a liquid-helium cooling sample holder. port (CVT) method with iodine as a transport agent. One gram as-prepared

powders of TaSe2−xTex mixed with 50 mg iodine was sealed in evacuated silica ACKNOWLEDGMENTS. This research was primarily supported by the Army SCIENCES tubes and heated for 1 wk in a two-zone furnace, where the temperatures of Research Office (ARO) Multidisciplinary University Research Initiative (MURI) source and growth zones were fixed at 1,050 °C and 1,000 °C, respectively. The on superconductivity, Grant FA-9550-09-1-0953. ARO MURI Grant FA-9550- APPLIED PHYSICAL identity and phase purity of the samples were determined by powder X-ray 10-1-0553 supported the single-crystal diffraction work, and the Depart- diffraction (PXRD) (Rigaku; Cu Kα radiation, graphite diffracted beam mono- ment of Energy (DOE) Basic Energy Sciences (BES) supported the powder chromator). Unit cell parameters and the z coordinate of the Se+Te atom diffraction refinements through Grant DE FG02-08ER46544. The electron diffraction study at Brookhaven National Laboratory was supported by positions (the only structural variable) were refined from the powder dif- the DOE BES, by the Materials Sciences and Engineering Division under fraction data through use of the FULLPROF diffraction suite (32). For the 3R Contract DE-AC02-98CH10886, and through the use of the Center for Func- ± polymorph, the Se/Te positions were constrained to be symmetric at z around tional Nanomaterials. The STM work was supported under the ARO-MURI the Ta positions, yielding regular TaX6 prisms. Measurements of the temper- Program W911NF-12-1-0461 and the National Science Foundation Grant ature dependence of the electrical resistivity were heat capacity performed in DMR-1104612.

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