Monoclinic structures of niobium trisulfide

Monoclinic structures of niobium trisulfide Matthew A. Bloodgood,1 Pingrong Wei,1 Ece Aytan,2 Krassimir N. Bozhilov,3 Alexander A. Balandin,2 and Tina T. Salguero1,a 1Department of Chemistry, University of Georgia, Athens, Georgia 30602, USA 2Nano-Device Laboratory (NDL) and Phonon Optimized Engineered Materials (POEM) Center, Department of Electrical and Computer Engineering, University of California–Riverside, Riverside, California 92521, USA 3Central Facility for Advanced Microscopy and Microanalysis, University of California–Riverside, Riverside, California 92521, USA

composition, structure, and morphology of any particular MX 3 sample, either one-or two-dimensional character may dominate.Much like MX 2 materials, MX 3 materials are amenable to mechanical or chemical exfoliation along the van der Waals gap, 6,14 and recent studies show that their properties can be maintained or even enhanced by nanostructuring. 12,15ere we consider NbS 3 , arguably the least-well-understand metal trichalcogenide.7][18][19] Offsetting this complexity and piquing our interest are intriguing reports about the electrical and charge density wave (CDW) properties of certain NbS 3 samples, including multiple CDW transitions and room temperature charge transport. 3,20,21However, further progress has been impeded by the absence of well-established structural information, spectroscopic characterization, or even reproducible synthetic details.Therefore, in this contribution, we aim to clarify the polymorphism of NbS 3 as part of the foundation required to understand its electronic properties.
To date, NbS 3 has been reported to crystallize in four forms: types I, II, III, and a high-pressure phase (Table I).Jellinek and co-workers first proposed a monoclinic ZrSe 3 -type NbS 3 structure in 1960. 16They reported a definitive structure in 1978, using single crystal x-ray diffraction data to refine the NbS 3 structure with a triclinic unit cell (notably having α = γ = 90 • ). 22This polymorph is known as NbS 3 -i in the literature.Compared to other MX 3 structures, NbS 3 -i has several unique features.The most remarkable feature is Nb-Nb pairing along the NbS 3 chain axis that manifests as alternating Nb-Nb bonds (∼3.0 Å) and gaps (∼3.7 Å).4][25][26][27] Another consequence is the doubling of the b-axis (6.730 Å) compared to other MX 3 structures (typically ∼3.3 Å), which is needed to accommodate Nb-Nb pairs in the unit cell (Fig. 1).A recent report by Fedorov and co-workers demonstrated the solution-based exfoliation of NbS 3 -i into nanoribbons.A second polymorph, NbS 3 -ii, was identified in 1978 based on electron diffraction studies, which showed inter-row pairs of satellite diffraction lines that resolved into rows of spots with varying temperature. 28These rows occurred at irrational positions with similar separations as NbS 3i, leading to the proposal that NbS 3 -ii was a superstructure of NbS 3 -i with different chain stacking.Several years later, Prodan and co-workers introduced a model monoclinic NbS 3 -ii unit cell with lattice parameters 2a , b /2, 2c , β based on the NbS 3 -i unit cell (lattice parameters a, b, c, β) (Table I). 29,30In parallel, Roucau and co-workers proposed a closely related model monoclinic NbS 3ii unit cell with lattice parameters 2a , 2c , b /2, β based on a different multiple of the NbS 3 -i unit cell (Table I). 26,31,32In the almost forty years since NbS 3 -ii was identified, however, its atomic structure has remained unsolved.During this time, continued interest in this polymorph has been motivated by its three CDWs: a low temperature transition at 150 K, a second transition at 330-370 K, and a high temperature transition at 620-650 K. 21,26 The transition at 330-370 K, just above room temperature, is an unusual one for CDW transport, and according to Zybtsev and co-workers, this CDW exhibits high transport velocities and sliding coherence. 21 high-pressure modification of NbS 3 prepared at 700 • C with 2 GPa pressure was reported by Kikkawa and co-workers in 1982. 33The monoclinic unit cell deduced from powder x-ray diffraction data is similar to that of monoclinic NbSe 3 (Table I), but atomic positions for NbS 3 -HP could not be determined. 33Yet another polymorph, designated NbS 3 -iii, was reported by Zettl and co-workers in 1982.In this case, powder x-ray diffraction data provided only partial lattice parameters.However, NbS 3 -iii can be distinguished by a characteristic CDW transition at 155 K. 20 Subsequent electrical measurements by Zybtsev and co-workers on "low-ohmic" and "high-ohmic" NbS 3 samples showed that low-ohmic NbS 3 exhibits CDW transitions at both 150 and 360 K, whereas high-ohmic NbS 3 exhibits a single CDW transition at 150 K. 3 On this basis, it has been postulated that low-ohmic and high-ohmic NbS 3 correspond to NbS 3 -ii and NbS 3 -iii, respectively; an alternate interpretation is that low-and high-ohmic NbS 3 as well as NbS 3 -iii are sub-phases of NbS 3 -ii. 21his letter reports the growth and characterization of two new monoclinic polymorphs of NbS 3 , designated NbS 3 -iv and NbS 3 -v following the nomenclature currently in place.Interestingly, we isolated NbS 3 -iv under synthetic conditions previously reported to produce NbS 3 -i and NbS 3 -v under synthetic conditions previously reported to produce NbS 3 -ii. 3,34Single crystal x-ray diffraction studies of both NbS 3 -iv and NbS 3 -v provide unambiguous structural information about these polymorphs.Such definitive characterization is necessary for accurate identification, especially because NbS 3 polymorphs form under similar conditions and often more than one can be found in the same crystal growth batch. 28Furthermore, electron microscopy analysis provides information about structural defects in NbS 3 -iv and atomic order in NbS 3 -v, and Raman spectroscopy of NbS 3 -iv and NbS 3 -v provides data that can be used to distinguish between polymorphs.
Niobium trisulfide samples were synthesized using niobium powder (99.99%,STREM) and sublimed sulfur powder (Baker, >99%) using a chemical vapor transport method.Iodine (Baker, 98%) at 4-6 mg/cm 3 was used for the growth of the NbS 3 -iv phase, whereas excess sulfur (∼10%) was utilized as the transport agent for the growth of NbS 3 -v crystals.For all syntheses, niobium and sulfur were lightly ground by mortar and pestle before being placed in a quartz ampule (∼13 cm 3 ).The ampules were then evacuated and backfilled with argon three times before flame sealing under vacuum (∼10 2 Torr).Reactions were placed in a two-zone furnace with a fixed gradient for 10-14 days; upon completion, the ampule/furnace was allowed to cool to room temperature naturally.
NbS 3 -iv crystals used for x-ray diffraction analysis were prepared at 670 • C with a 100 • C gradient (670 • C source zone → 570 • C growth zone).This synthesis temperature is comparable to temperatures used to prepare NbS 3 -i, 11,27,34 and the only difference here is the use of I 2 as a transport agent.This NbS 3 polymorph crystallizes in a monoclinic unit cell, and the structure was solved using space group P2 1 /c.Additional details are provided in Tables S1 and S2 of the supplementary material, and crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (structure number 1574381).
The unit cell of NbS 3 -iv is larger than that of NbS 3 -i because the c-axis is doubled, resulting in twice as many chains per unit cell (Fig. 1).The asymmetric unit of NbS 3 -iv is Nb 2 S 6 , making Z formally 4, which becomes 8 when normalized to the formula NbS 3 .Also, in contrast to other MX 3 structures, the chain axis in NbS 3 -iv is the a-axis rather than the b-axis (Fig. 1).Looking down the aaxis, the structure of NbS 3 -iv in cross section is similar to the model ZrSe 3 -type structure, specifically the "A-variant." 35However, a difference between these structures is their layer-stacking order.This is clearly seen when examining multiple unit cells of each polymorph (Fig. 1).The chains in NbS 3 -i realign every five bilayers, where a bilayer is defined as a pair of chains bounded by van der Waals gaps (Fig. 1 where pairs of blue or green chains designate the bilayers).Thus, the repeat sequence is ABCDE.However, NbS 3 -iv experiences a realignment of the chain after only two bilayers, making the repeat sequence AB.
In the chain cross section of NbS 3 -iv, three sulfur atoms surround a niobium atom to form acute, isosceles triangles.The bases of the triangles are made of a 2.015 Å S-S bond; the presence of this bond is consistent with the formulation of NbS 3 as Nb(S 2 )S. 22The sides of the triangles are separated by longer S• • • S distances of 3.404-3.418Å or 3.787-3.828Å.Two interchain Nb-S bonds of 2.623 and 2.654 Å exist between each chain, which hold the bilayer together.Each bilayer is separated from the next by a distance of ∼2.8 Å, the van der Waals gap.
Looking down the b-axis (Fig. 1), we see that NbS 3 -iv contains Nb-Nb pairing with a bond distance of 3.0448 Å, much like NbS 3 -i.Thus, NbS 3 -iv demonstrates that it is possible to have Nb-Nb pairing within a space group with P2 1 /c symmetry, i.e., it is not essential to reduce the symmetry to triclinic as in NbS 3 -i.It is curious that Jellinek and co-workers published two papers prior to their report of NbS 3 -i's triclinic structure in which they reported monoclinic unit cells extremely similar to what we have found for NbS 3 -iv. 16,17It is possible that they actually made NbS 3 -iv in earlier work, which was based on the synthesis of NbS 3 from the elements, whereas their structural study was conducted on NbS 3 crystals generated through the thermal decomposition of NbS 2 Cl 2 . 22he differences in preparative conditions may have led to the formation of NbS 3 -i versus NbS 3 -iv.In addition, van Smaalen has re-interpreted the triclinic space group P 1 of NbS 3 -i as a monoclinic superspace group, proposing that this is an uncommon example of a modulated structure. 36he relatively high wR value (11.38%) associated with the refinement of NbS 3 -iv reflects the presence of structural disorder, possibly including sulfur vacancies, faulting or slippage along the van der Waals planes, and twinning, all of which have been observed in various MX 3 materials. 28,35,37o identify the specific disorder in NbS 3 -iv, the TEM analysis was performed on a Thermo Fisher Scientific Themis 300 instrument, fitted with a X-FEG electron gun and CETA 4K × 4K CMOS camera, at an accelerating voltage of 300 kV.Specimens were prepared by dispersing a small amount of as-synthesized NbS 3 -iv powder in distilled water without grinding and depositing a droplet of the resulting suspension onto lacey carbon grids.The TEM study revealed that the sample consists of three types of particles: (1) long thin whiskers with aspect ratios >100 and thicknesses <100 nm that are elongated parallel to the a-axis [Fig.2(a)]; (2) prismatic crystals with aspect ratios <20, with a long side parallel to the a-axis, a short side parallel to the b-axis, and thicknesses <100 nm along the c-axis [Fig.2(b)]; (3) small aggregates consisting of isometric particles <50 nm in diameter.
The fibers of NbS 3 -iv are twisted around their long axis, resulting in the development of numerous dislocations that can be identified through disruptions of the lattice fringes parallel to {001} [Fig.2(a)].The prismatic particles consist of topotactically intergrown zones 5-50 nm wide parallel to the b-axis and several microns long parallel to the a-axis [Fig.2(b)].They are coherently intergrown with interfaces parallel to the {010} planes.The individual crystals exhibit a mosaic structure with fine domains about 10 nm in diameter [Fig.2(c)] that are coherently intergrown with small angular mis-orientation around the c-axis, which cause changes in the contrast and appearance of the lattice fringes resolved by HRTEM imaging.It is possible that the observed variations in the lattice images along the c-axis are caused by stacking defects; it is possible to resolve such features by imaging down the a-axis, but this has not been done here due to the unfavorable particle shapes.The inset shows a magnified view of the area marked by a white box.The complex lattice contrast and the mosaic structure of low-angle domains is revealed here; the slight tilt of the domains causes considerable change in the lattice fringe contrast, which prevents the reliable interpretation of any stacking disorder that might be present parallel to the c-axis.
Further exploration of the NbS 3 system provided yet another monoclinic NbS 3 polymorph, NbS 3 -v.It was prepared at 700 • C with a 30 • C gradient (700 • C source zone → 670 • C growth zone) using 10% excess sulfur for transport.This polymorph also crystallizes in the P2 1 /m space group.Additional details are provided in Tables S1 and S2 of the supplementary material, and crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (structure number 1574382).As illustrated in Fig. 1, the unit cell of NbS 3 -v is similar to that of NbS 3 -i but with a halved b-axis; the lattice parameters can be expressed as a , b /2, c , β based on the NbS 3 -i unit cell (Table I).These dimensions result in two chains per unit cell, albeit the unit cell is only one [MX 6 ] trigonal prism wide (Z = 2).This unit cell is smaller than those proposed for NbS 3 -ii.
In the chain cross section of NbS 3 -v, three sulfur atoms surround a niobium atom to form acute isosceles triangles with bases composed of 2.020 Å S-S bonds and sides formed by S• • • S distances of 3.576-3.612Å.Interestingly, the interchain Nb-S interactions alternate in pairs of 2.619 Å and 2.635 Å Nb-S bonds, which results in a small but measurable chain pairing within the bilayer sheet.Although NbS 3 -v features the same ABCDE stacking that occurs in NbS 3 -i (Fig. 1), it contains a single, uniform Nb-Nb distance of 2.358 Å, leading to ideal (non-corrugated) [MX 6 ] chains.Theoretically, NbS 3 with such an undistorted structure should display metallic conductivity due to a single electron remaining on each Nb 4+ after creation of the [NbS 6 ] chains. 23,38bS 3 -iv and NbS 3 -v can be exfoliated in a similar fashion as other MX 3 materials.5,6,13  There are no defects visible at this magnification in this particular area of the sample or others imaged along this nanoribbon, although the high wR value (21.99%) associated with the refinement of NbS 3 -v points to the presence of significant structural disorder in the larger crystal used for x-ray diffraction analysis.
In experimental material research, powder x-ray diffraction (PXRD) is usually the initial characterization method used to identify crystalline phases and evaluate phase purity.The patterns for NbS 3 -i, iv, and v are compared in Fig. 4(a).Although the theoretical patterns (calculated from singlecrystal XRD structures) of NbS 3 -i and NbS 3 -iv are similar, each has distinct peaks in the range 10 • -40 • 2θ that should make polymorph identification unambiguous.In practice, however, the experimental peak intensities of NbS 3 -iv (powder mount) are altered by orientation effects, such that the resulting pattern can be matched to either NbS 3 -i or NbS 3 -iv.In the case of NbS 3 -v, its theoretical pattern is essentially identical to that of NbS 3 -i, but experimentally NbS 3 -v powder generates a distinct pair of peaks at ∼22 • 2θ.Thus, we caution that PXRD alone is not sufficient to differentiate between all NbS 3 polymorphs.
Raman spectroscopy provides additional data to differentiate NbS 3 -iv and NbS 3 -v.As shown in Fig. 4(b), the spectra of these polymorphs are clearly different.However, the spectrum of NbS 3 -v is not as clearly resolved as that of NbS 3 -iv, which may be due to the structural disorder present.0][41]   show that the morphology of NbS 3 -iv and NbS 3 -v is fibrous.NbS 3iv crystals generally appear as wider, straight fibers, whereas NbS 3 -v crystals look more like flexible wires.These morphological observations are similar to descriptions of NbS 3 -i (larger, straighter needles) and NbS 3 -ii (finer, bent whiskers) in the literature. 22,30,34In addition to the fibers, both products contain numerous cylinders and rings in various stages of winding/unwinding [Figs.4(d) and 4(e)].Moreover, energy dispersive spectroscopy (EDS) analysis indicates that these microstructures have a NbS 3 stoichiometry identical to the fibers.3][44][45][46] Indeed, Enyashin and Ivanovskiȋ have provided a theoretical basis indicating that NbS 3 ring structures are energetically favorable. 47Continuing work will be required to understand the formation mechanism of such morphologies in the NbS 3 system and characterize the distortions that accompany crystal bending.
In summary, the crystal structures of two new niobium trisulfide polymorphs, NbS 3 -iv and NbS 3 -v, greatly enrich our understanding of this system.Both polymorphs crystallize in monoclinic unit cells, which fills a long-standing gap of knowledge about monoclinic NbS 3 .We find that the structure of NbS 3 -iv is related to triclinic NbS 3 -i with respect to Nb-Nb pairing along the chain axis.In comparison, the structure of NbS 3 -v is undistorted along the chain axis but contains subtle chain pairing.This structural information will be extremely helpful for the interpretation of spectroscopic data and modeling the electronic properties of NbS 3 -iv and NbS 3 -v.
See supplementary material for additional information about the x-ray diffraction studies of NbS 3 -iv and NbS 3 -v, including atomic coordinates.

FIG. 1 .
FIG.1.Views of NbS 3 -i, iv, and v: from left to right including unit cell, chain cross section, layer, and perspective views.The NbS 3 bilayers are differentiated by color to illustrate the van der Waals gaps, which is further illustrated with a red plane in each perspective view.The structures of these three polymorphs have been established by single crystal x-ray diffraction.

FIG. 2 .
FIG. 2. (a) Bright field TEM image of a NbS 3 -iv fiber.Insets show (i) HRTEM image (lower left) of the crystal along the b-axis and (ii) the corresponding indexed FFT pattern (upper right).The location of the HRTEM image is indicated by a black arrow.In the HRTEM inset, multiple disruptions of the lattice fringes parallel to {001} are visible; some dislocations are indicated by white arrows.(b) Bright field high magnification TEM image of a prismatic NbS 3 -iv particle.Insets shown are (i) low magnification BF TEM image of the whole particle (upper) and (ii) the corresponding selected area electron diffraction pattern along the c-axis, which confirms the single crystal nature of the whole particle.The topotactically oriented individual intergrown zones are clearly visualized, with some of the coherent interfaces between zones indicated by black arrows.(c) HRTEM image of the particle in (b).The inset shows a magnified view of the area marked by a white box.The complex lattice contrast and the mosaic structure of low-angle domains is revealed here; the slight tilt of the domains causes considerable change in the lattice fringe contrast, which prevents the reliable interpretation of any stacking disorder that might be present parallel to the c-axis.
Figure 3 shows HAADF-STEM images of a ∼40 nm wide NbS 3 -v nanoribbon exfoliated by sonication in ethanol.The high-resolution image in panel (b) reveals the [NbS 6 ] chains along the b-axis.Both the Nb-Nb bonds with 3.37 Å spacing and the van der Waals gap between chains are clearly visible.
FIG. 3. HAADF-STEM images of an exfoliated NbS 3 -v nanoribbon along the b-axis at low (a) and high magnifications (b), together with a structural model for the observed atomic spacings (c).

FIG. 4 .
FIG. 4. (a) Powder x-ray diffraction patterns of NbS 3 -i, NbS 3 -iv, and NbS 3 -v; drop lines for NbS 3 -i are from JCPDS card no.04-007-1130.Powder x-ray diffraction data for NbS 3 -iv, and NbS 3 -v were collected on a Bruker Advance D8 diffractometer with Co-Kα radiation at 35 kV and 40 mA from 5 • to 80 • 2θ with a 0.01 step size at 0.4 s/step.(b) Raman spectroscopy of NbS 3 -iv and NbS 3 -v.Raman data were collected on a Renishaw InVIA microscope at 785 nm with a 50× objective lens.SEM images of NbS 3 -iv [(c) and (d)] and NbS 3 -v [(e) and (f)].SEM was conducted on a FEI Teneo FE-SEM instrument equipped with an Oxford Instruments X-MAX N detector at 5-20 keV. 11

TABLE I .
Unit cell and synthesis parameters for previously reported NbS 3 polymorphs as well as new polymorphs NbS 3 -iv and NbS 3 -v.