Abstract
Two new polymorphs of niobium trisulfide are established by single crystal x-ray diffraction. NbS3-iv crystallizes in the monoclinic space group P21/c with lattice parameters a = 6.7515(5) Å, b = 4.9736(4) Å, c = 18.1315(13) Å, and β = 90.116(2)°. Its structure is based on chains of [NbS6] trigonal prisms containing Nb–Nb pairs with a bond length of 3.0448(8) Å; this pairing causes the chains to corrugate slightly along their axis, a feature also present in triclinic NbS3-i that leads to semiconductor properties. The stacking arrangement of chains is different in these polymorphs, however, with NbS3-i having an ABCDE repeating sequence of chain bilayers and NbS3-iv having an AB repeating sequence. HRTEM studies show the presence of topotactically-oriented intergrown zones and numerous dislocations, which result in mosaic structuring. A second new polymorph, NbS3-v, crystallizes in the monoclinic space group P21/m with lattice parameters a = 4.950(5) Å, b = 3.358(4) Å, c = 9.079(10) Å, β = 97.35(2)°. In contrast to NbS3-iv, NbS3-v maintains fixed a Nb–Nb bond distance of 3.358(4) Å along the chains, and it has an ABCDE repeating sequence of chain bilayers similar to NbS3-i. High resolution scanning transmission electron microscopy (HR-STEM) imaging of an exfoliated NbS3-v nanoribbon shows the continuous [NbS6] chains oriented along the b-axis. These results provide the first firmly established structural data for monoclinic NbS3. In addition, SEM images show the formation of NbS3 rings and cylinders, and a combination of powder x-ray diffraction and Raman spectroscopy provides a way to distinguish between NbS3 polymorphs.
Highlights
Many classes of materials have been proposed as replacements for semiconductor-based electronics technology as it progresses in complexity and miniaturization.[1]
A high-pressure modification of NbS3 prepared at 700 ◦C with 2 GPa pressure was reported by Kikkawa and co-workers in 1982.33 The monoclinic unit cell deduced from powder x-ray diffraction data is similar to that of monoclinic NbSe3 (Table I), but atomic positions for NbS3HP could not be determined.[33]
It is curious that Jellinek and co-workers published two papers prior to their report of NbS3-i’s triclinic structure in which they reported monoclinic unit cells extremely similar to what we have found for NbS3-iv.[16,17]
Summary
Many classes of materials have been proposed as replacements for semiconductor-based electronics technology as it progresses in complexity and miniaturization.[1]. Jellinek and co-workers first proposed a monoclinic ZrSe3-type NbS3 structure in 1960.16 They reported a definitive structure in 1978, using single crystal x-ray diffraction data to refine the NbS3 structure with a triclinic unit cell (notably having α = γ = 90◦).[22] This polymorph is known as NbS3-i in the literature.
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