Impact of the chemical vapor transport agent on polymorphism in the quasi-1D NbS₃ system

Recent advancements in the synthesis, exfoliation, and characterization of low-dimensional materials have stimulated interest in their fundamental properties and applications. The transition metal di- and trichalcogenides, MX₂ and MX₃ (where M = transition metal and X = chalcogen), are two classes of low-dimensional materials at the heart of these developments. Although the 2D MX₂ materials have been more widely studied, quasi-one-dimensional (quasi-1D) MX₃ compositions possess many attractive properties, including high anisotropy, superconductivity, semiconducting to metallic conductivity, and charge density waves (CDWs) that originate in the structure and bonding of these materials.^1–7 

Crystalline MX₃ structures consist of asymmetric trigonal prismatic MX₆ units (M = selected transition metal; X = S, Se, Te) assembled into infinite parallel chains of face-sharing polyhedra. These chains are arranged into bilayers that associate through van der Waals interactions, leading to the quasi-1D (rather than true 1D) properties of these materials. Structural diversity arises through differences in metal–metal bonding along the chains, variations in chalcogen–chalcogen bonding within the coordination sphere of the metal, the strength of interchain interactions, the relative positions of neighboring chains, and the stacking arrangements of bilayers.⁴ Specifically, for transition metal trisulfides, trigonal bipyramidal asymmetry emerges from differences in the shortest S⋯S distance in each trigonal bipyramid, i.e., within the (S₂)²⁻ moiety of the M⁴⁺(S)²⁻(S₂)²⁻ fragment. The number of different (X₂)²⁻ interactions within a unit cell is one differentiating characteristic of the three prototype MX₃ structures, i.e., the ZrSe₃-, TaSe₃-, and NbSe₃-types, which exhibit one, two, or three distinct (X₂)²⁻ interactions, respectively. These structures also vary in the relative arrangement of MX₃ chains within the bilayers.

Niobium and tantalum trisulfides display interesting and productive polymorphism, and the resulting structural variations are distinctive of 1D materials. NbS₃ is significantly more diverse than any other MX₃ system.^5–8 NbS₃-I and NbS₃-IV both contain distorted NbS₃ chains featuring Nb pairs or dimers, which results in alternating short and long distances between Nb centers (where short distances correspond to Nb–Nb bonding).^4,8,9 This scenario is believed to effectively confine the lone conducting electron of each Nb center, resulting in semiconducting rather than metallic properties.⁷ This metal pairing appears to be unique to NbS₃-I and -IV among all MX₃ compositions.

On the other hand, NbS₃-II and NbS₃-V contain chains of Nb⁴⁺ at uniform distances of ∼3.35 Å.^8,10,11 The literature describing NbS₃-II shows that this polymorph is metallic with unusually high CDW transitions at 450–475, 360, and 150 K, which can be discerned from the superstructures revealed by selected area electron diffraction and from nonlinear transport behavior.^12–14 Zybtsev et al. have reported evidence for the existence of NbS₃-II subphases or polytypes designated as “low-Ohmic” and “high-Ohmic” with different electrical properties.^14,15

Polymorphs are valuable for several reasons. With sufficient control over their formation, they can be harnessed to clarify key structure–property relationships. Because structural differences can impact properties to a significant degree, polymorphism also presents an opportunity to engineer materials for specific applications. Well-known examples include the carbon allotropes (diamond, graphite, C₆₀, and nanotubes)¹⁶ and the silicon carbide polytypes (200+ known forms).¹⁷ In this latter system, 4H-SiC is an important ceramic material and also a commercialized wide bandgap semiconductor.

The primary challenge associated with polymorphs is phase pure synthesis and isolation. In the case of NbS₃, all polymorphs can be prepared by chemical vapor transport (CVT), which yields needle- or ribbon-like MX₃ crystals up to several centimeters in length. The recently reported NbS₃-IV and NbS₃-V polymorphs were prepared using elemental transport agents (I₂ or Br₂ for NbS₃-IV, excess S for NbS₃-V).^8,18 Reaction temperatures and gradients influenced which major phase was isolated (670→570 °C for NbS₃-IV vs 700→670 °C for NbS₃-V). Here, we find that switching the transport agent to ammonium chloride, NH₄Cl, can provide phase control such that solely polymorph NbS₃-VI is isolated. This result is surprising because polymorph control in CVT and chemical vapor deposition (CVD) is usually accomplished via temperature, pressure, the use of crystalline seeds/substrates (epitaxy), or the application of cooling/quenching protocols at the conclusion of crystal growth.¹⁹ We are not aware of other instances where transport agent selection determines a product's atomic structure.

NH₄Cl has been used as a CVT transport agent and chemical vapor deposition (CVD) “growth promoter” or “synergistic additive” for a variety of metal chalcogenides (e.g., FeS₂, TiS₂, VTe₂, and ZnSe), metal nitrides (e.g., ZrNCl), metal oxides (e.g., ZnO, WO₂, InWO₃, and Cu₂OSeO₃), and other materials.^20–23 At temperatures beyond ∼340 °C, NH₄Cl(s) sublimes and decomposes into two highly effective transport species, NH₃(g) and HCl(g). Ammonia may further decompose into H₂(g) and N₂(g) at sufficiently elevated temperatures, especially in the presence of a catalyst. Thus, in addition to serving as an in situ source of HCl(g), NH₄Cl can generate an advantageous reducing atmosphere.

To produce NbS₃-VI, we conducted CVT from the elements and NH₄Cl at a final temperature gradient of 670→570 °C. After two weeks of growth, we isolated silver-gray, wirelike crystals of NbS₃-VI. CVT at the same temperature with I₂ or Br₂ instead yields NbS₃-IV. Compositional analyses with energy dispersive x-ray spectroscopy (EDS) showed that the experimental S/Nb atomic ratio matches the anticipated theoretical value within error (Table S1). Elemental maps of Nb and S show even distributions of both elements throughout the crystals (Fig. S1).

Bulk structural characterization by powder x-ray diffraction (PXRD) provided experimental patterns that match well with droplines from the single crystal x-ray structure of NbS₃-VI (Fig. S2) once orientation effects are accounted for. The diffraction patterns of NbS₃-IV, NbS₃-V, and NbS₃-VI have many common features, making the polymorphs challenging to differentiate by PXRD alone. For this reason, polymorph identification in the NbS₃ system must be assessed by unit cell determination.

To establish the NbS₃ polymorph discovered in this work, we solved the structure of NbS₃-VI from single crystal x-ray diffraction data. Key crystallographic parameters and atomic coordinates are summarized in Tables S2 and S3, and Table I makes comparisons among known NbS₃ structures. We find that the combination of β angle and unit cell volume (Table S4) serves to distinguish among NbS₃-IV, -V, and -VI.

The overall unit cell dimensions of NbS₃-VI are most similar to those of NbS₃-I. However, NbS₃-VI crystallizes in the monoclinic space group Pm rather than the triclinic space group P$1¯$ of NbS₃-I. Like other NbS₃ polymorphs, the structure of NbS₃-VI is based on infinite chains of face-sharing, niobium sulfide trigonal bipyramids arranged into layers [Fig. 1(a)] separated by a ∼3.0 Å van der Waals gap. The outstanding feature of NbS₃-VI is its combination of two chain types; that is, NbS₃-VI is constructed of alternating “corrugated” chains (where Nb–Nb pairing leads to alternating distances between Nb⁴⁺ along the chains) and “uniform” chains (where the absence of pairing leads to regular distances between Nb⁴⁺). These chains are illustrated in Fig. 1, and Table I includes key metrics that define the chains in each structure. In particular, the distances between Nb⁴⁺ in NbS₃-VI vary from as short as 3.007(6) Å within Nb₂ pairs that make up corrugated chains, to as long as 3.736(6) Å between neighboring Nb₂ pairs. These values suggest that Nb pairs in NbS₃-VI are more strongly bonded than in NbS₃-I and NbS₃-IV. In comparison, the distance between Nb ions in uniform chains of NbS₃-VI is 3.3724(10) Å, just slightly longer than those in NbS₃-V [3.358(4) Å].

The presence of both uniform and corrugated chains in NbS₃-VI results in three different S–S pairing distances: one in the uniform chains, and two in the corrugated chains. The shortest S–S distance is slightly longer in the uniform chains (2.12 Å) than those in the corrugated chains (1.987 and 2.002 Å). The longer S–S distances (long side of the S3 triangle) in the corrugated chains of NbS₃-IV and VI also differ. In NbS₃-VI, these S–S distances fall in the range of 3.361–3.372 or 3.822–3.897 Å, consistent with a distorted triangular base. Thus, it appears that the presence of both uniform and corrugated chains in each bilayer causes the uniform chains to be more symmetric than those in NbS₃-V, whereas the corrugated chains are more distorted than those in NbS₃-IV.

An interesting aspect of NbS₃-VI is that its structural components—semiconducting, corrugated chains of NbS₃ and metallic, uniform chains of NbS₃—have distinctly different electronic character. This feature can be related conceptually to the alternating octahedral and trigonal prismatic layers incorporated within 4H- and 6R-TaX₂.^24,25 We also note that the mixed chains in NbS₃-VI lead to “Janus” motifs where one face of the bilayer displays corrugated chains and the other face displays uniform chains. In comparison, previously described Janus metal chalcogenide nanosheets are characterized by different chalcogen substitution on each 2D face.^26–28 

We expanded this study to include Nb_xTi_x−1S₃, which is known to form solid solutions across its full compositional range with isovalent substitution of the slightly smaller Ti⁴⁺ (60.5 pm ionic radius) for Nb⁴⁺ (68.0 pm ionic radius).^29,30 Crystals of Nb_0.56Ti_0.44S₃ were grown by CVT using excess sulfur as the transport agent. To minimize inhomogeneous crystal nucleation and favor crystal growth at the cooler end of the ampule rather than throughout the ampule, we first applied a reverse temperature gradient (hot growth zone → cold source zone) prior to forward transport (hot source → cold growth). This process removes crystallization seeds from the walls of the ampule in the growth zone region and previously was reported to improve CVT crystal growth.^31,32 For these experiments, we initially positioned the sealed ampule over the edge of the tube furnace so that the source zone was suspended outside of the furnace at a temperature of ∼25 °C, while the empty growth zone was positioned within the furnace at 600 °C. After one week, the ampule was completely inserted into the furnace and a forward gradient of 600 → 500 °C (source zone–growth zone) was established for crystal growth. This technique significantly improved the CVT growth of (Nb,Ti)S₃, leading to larger and more uniform crystals.

Scanning electron microscopy (SEM) imaging revealed flat, ribbon-like crystal morphologies (Fig. S3). EDS provided an experimental composition of Nb_0.56Ti_0.44S₃ (Table S5) and elemental mapping verified homogeneity (Fig. S3). Single crystal x-ray diffraction showed that this material crystallizes in the monoclinic space group P2₁/m with unit cell parameters (Table I) similar to those of both NbS₃-V and TiS₃.^8,33,34 As illustrated in Fig. 2, each Nb_0.56Ti_0.44S₃ unit cell contains two uniform-type chains with metal–metal spacings of 3.3692(6) Å, a value that is intermediate between the d(Nb–Nb) of 3.358(4) Å in NbS₃-V and 3.3724(10) Å in NbS₃-VI.⁸ The isosceles triangle-based prisms are consistent with the so-called A-variant structure of TiS₃.^33,34 The base of each triangle contains an S₂ pair at 2.038 Å separation, similar to a distance of 2.039 Å in TiS₃. Bridging M^…S linkages help to assemble neighboring chains into bilayers; the 2.638 Å interchain distance in Nb_0.56Ti_0.44S₃ exceeds that of both NbS₃-V (2.619 Å, 2.635 Å) and TiS₃ (2.416 Å), reflecting the impact of Nb⁴⁺/Ti⁴⁺ substitution on interchain interactions.

Further characterization of NbS₃-VI entailed Raman spectroscopy. A representative spectrum is shown in Fig. 3, along with comparative spectra from NbS₃-IV and NbS₃-V. At first glance, the spectra of NbS₃-VI and -IV have numerous similarities, whereas the spectrum of NbS₃-V is quite different.^30–32 Several strong resonances exhibited by both NbS₃-IV and -VI include ∼145, ∼155, ∼189, and ∼335 cm⁻¹, which also have been reported for NbS₃-I and -II.^30,32,33 By analogy, we can tentatively assign the resonance at ∼145 cm⁻¹ as a lattice compression mode, whereas the one at ∼155 cm⁻¹ is most likely the intrachain ν(Nb–Nb) stretching mode. The ∼189 cm⁻¹ peak can be associated with intrachain ν(Nb–S) stretches. The presence of the lower intensity shoulder at ∼195 cm⁻¹ as well as the absence of the two distinct ν(Nb–S₂) stretches differentiate NbS₃-VI and are consistent with the reduced influence of weaker ν(Nb–S) interactions with corrugated chains. The peaks appearing between 250 and 350 cm⁻¹ are found in both NbS₃-IV and -VI spectra with similar intensities. However, the peaks at 198, 375, and 565 cm⁻¹ are significantly reduced in intensity in NbS₃-VI compared to NbS₃-IV, with the 550 cm⁻¹ peak of NbS₃-IV notably absent from the spectrum of NbS₃-VI. This resonance is likely the ν(S–S) stretch associated with S–S pairing.

Morphological characterization of NbS₃-VI by scanning electron microscopy (SEM) is shown in Fig. 3. Previous work revealed consistent variations among NbS₃ polymorphs, specifically wider, more ribbon-like crystals of NbS₃-I and -IV vs thinner, more wire-like crystals of NbS₃-II and V.^8,9,35 Figure 3(a) shows that NbS₃-VI possesses a fibrous morphology, with individual crystals ranging from ∼100 nm to ∼100 μm in width, and from several micrometers to centimeters in length. Most of the NbS₃-VI fibers exhibit a ribbon-like morphology similar to those of NbS₃-I and IV. However, wire-like crystals resembling NbS₃-II and V are present in the sample, which may be due to mixed phases or mechanical cleavage during sample preparation. A closer examination of the NbS₃-VI surface reveals a striated surface texture of parallel ridges [Fig. 3(b)]. These features continue along the length of the NbS₃-VI ribbons (the b-axis) with varied width and spacing. This texture, as well as the sample cleavage, indicates the strong growth anisotropy along the quasi-1D NbS₃ chains and the weaker interchain interactions along the a- and c-directions. Similar features are apparent in TiS₃, TaSe₃, and NbSe₃.^36–38 

High resolution (scanning) transmission electron microscopy [HR-(S)TEM] was used to visualize NbS₃ crystals at the nanometer and atomic scales. Figure 4 shows NbS₃-VI crystals under three transmission imaging modes: low magnification TEM, high magnification TEM, and HR-STEM. At low magnification (panel a), the exfoliated NbS₃-VI appears as multilayered nanoribbons. Selected area electron diffraction (SAED) (panel b, inset) confirms the single crystalline nature of the narrower ribbon at lower right. Indexing the observed diffraction spots shows the [001] zone axis to be parallel to the imaging direction. Notably, the SAED pattern lacks the extra rows of spots corresponding to superstructures in NbS₃-II and -V caused by CDW phases.^11,39 Their absence suggests that this NbS₃ polymorph does not exhibit CDWs at room temperature.

High resolution TEM of the NbS₃-VI nanoribbons reveals the superstructure visible in Fig. 4(b): evenly spaced and perpendicular linear features crossing the nanoribbon length and width. The orientation of the NbS₃ chains along the length of the nanoribbon, confirmed by SAED, can be attributed to the features parallel to the nanoribbon edge. The perpendicular lines, which have been observed for NbS₃-I and -IV nanoribbons as well (but not NbS₃-II and -V), are most likely a result of increased electron density between neighboring chains caused by the close chain proximity and corrugated chains within NbS₃ bilayers.

The structure of NbS₃-VI was further clarified by atomic resolution STEM imaging. In Fig. 4(c), the orientation of the NbS₃ chains runs horizontally across the image. The linear features observed by HR-TEM are still present at higher magnification, reappearing every fifth column of atoms, yet they do not appear to directly link neighboring chains along the [100] axis. At this magnification and with the aid of the enlarged view in Fig. 4(d), it is clear that the linear features seen at lower magnification are generated by overlapping chains: uniform chains in a “top row” and corrugated chains in a “bottom row,” which can be identified by distinctive spacings between the Nb⁴⁺ centers [Fig. 4(e)].

In summary, the preparation and characterization of NbS₃-VI further expand the possibilities for transition metal trichalcogenide structure and bonding. Single crystal x-ray diffraction and atomic-resolution electron microscopy establish the unique features of NbS₃-VI that distinguish it from other known polymorphs of NbS₃, namely, NbS₃-VI contains both uniform and corrugated NbS₃ chains with equidistant and paired Nb⁴⁺, respectively. Access to NbS₃-VI is enabled by ammonium chloride as the crystal growth transport agent, which represents an alternative approach to polymorph engineering. Although the selection of the transport agent has always been a key aspect of CVT reactions, the results described in this contribution demonstrate an additional role that it can play in the development of low-dimensional materials, especially 1D van der Waals materials.

See the supplementary material for synthetic details, crystallographic information, atomic coordinates, and additional characterization data.

This work was supported in part by a U.S. National Science Foundation Emerging Frontiers of Research Initiative 2-DARE grant (No. NSF EFRI-1433395). Transmission electron microscopy studies were carried out at EAG Materials, Netherlands, with the help of M. A. Verheijen.