Monoclinic structures of niobium trisulfide.

Two new polymorphs of niobium trisulfide are established by single crystal x-ray diffraction. NbS₃-iv crystallizes in the monoclinic space group <italic>P</italic>2₁/<italic>c</italic> with lattice parameters <italic>a</italic> = 6.7515(5) Å, <italic>b</italic> = 4.9736(4) Å, <italic>c</italic> = 18.1315(13) Å, and <italic>β</italic> = 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 <italic>P</italic>2₁/<italic>m</italic> with lattice parameters <italic>a</italic> = 4.950(5) Å, <italic>b</italic> = 3.358(4) Å, <italic>c</italic> = 9.079(10) Å, <italic>β</italic> = 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 <italic>b</italic>-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. [ABSTRACT FROM AUTHOR]