Electron microscopy study of tubular crystals (BiS)1+δ(NbS2)n

Abstract

Big crystals (mm-sized) with anomalous tubular morphology were synthesised in the Bi–Nb–S system. Their structural study by electron microscopy and related techniques revealed that they can be described as misfit layer structures, a type of composite modulated structures. The misfit monolayer ≈BiNbS3 and bilayer ≈BiNb2S5 phases appear as well in the preparation but with lamellar morphology as it is typical for these kind of compounds. They consist of periodical intergrowth of pseudotetragonal layers BiS (Q) with pseudohexagonal layers NbS2 (H). ≈BiNbS3 has a stacking sequence …Q,H,Q,H,… and ≈BiNb2S5 has a stacking sequence …Q,H,H,Q,H,H,…Backscattered electron imaging, wavelength dispersive X-ray spectroscopy and transmission electron microscopy of transversal cross-sections of single tubular crystals showed that these crystals present a strong compositional and inter-laminar stacking disorder along the tube radius. This disorder is suggested to be the cause for the original wrapping of the layers that gives rise to the tubes. Besides the disordered areas, some ordered slabs have been found with stacking sequences corresponding to binary 3R–NbS2 (≈6Å), ≈BiNb2S5 (≈17.4Å) (which dominate in the crystal) and to a new related phase ≈BiNb4S9 with a stacking sequence …Q,H,H,H,H,Q,H,H,H,H,… and a periodicity of ≈29.2Å.