Polytypism, polymorphism, and superconductivity 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 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 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,of0.50.7 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. E1174–E1180 | PNAS | Published online March 3, 2015 www.pnas.org/cgi/doi/10.1073/pnas.1502460112 Downloaded by guest on September 26, 2021 PNAS PLUS 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 3.8 TaSe Te TaSe Te TaSe Te SCIENCES 2-x x 2-x x 3.5 2-x x 6.7 reduced c axis 1T 3.7 TaX slab vdW Gap thickness 2 1T APPLIED PHYSICAL ) 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.