Nanotubes from the Misfit Compound Alloy LaS-Nb_xTa_(1–x)S₂

Misfit layered compounds (MLC) with the composition (LaS)_1.15TaS₂ (for simplicity denoted as LaS-TaS₂) and LaS-NbS₂ were prepared and studied in the past. Nanotubes of LaS-TaS₂ could be easily synthesized, while tubular structure of the LaS-NbS₂ were found to be rather rare in the product. To understand this riddle, quaternary alloys of LaS-Nb_xTa_(1–x)S₂ with ascending Nb concentration were prepared herein in the form of nanotubes (and platelets). Not surprisingly, the concentration of these quaternary nanotubes shrank (and the relative density of platelets increased) with increasing Nb content in the precursor. The structure and chemical composition of such nanotubes was elucidated by electron microscopy. Conceivably, the TaS₂ in the MLC compounds LnS-TaS₂ (Ln = lanthanide atom) crystallizes in the 2H polytype. High resolution transmission electron microscopy showed however that, invariably, MLC nanotubes prepared from 80 at% Nb content in the precursor belonged to the 1T polytype. Raman spectroscopy of individual tubes revealed that up to 60 at% Nb, they obey the standard model of MLC, while higher Nb lead to large deviations, which are discussed in brief. The analysis indicated also that such nanotubes do not exhibit the pattern assigned to charge density wave transition so typical for binary 1T-TaS₂. The prospect for revealing interesting quasi-1D behavior of such quaternary nanotubes is also briefly discussed.