Quaternary Ln_xLa_(1-x)S-TaS₂ nanotubes (Ln=Pr, Sm, Ho, and Yb) as a vehicle for improving the yield of misfit nanotubes

The non-stoichiometric misfit layered compounds (MLC) of the general formula ((MX)_1+y)_m(TX₂)_n have been investigated quite extensively. Here MX is a monoatomic slab of a material with distorted rocksalt structure and TX₂ is a layered compound with hexagonal coordination. Recognizing the mismatch between the two (MX and TX₂) sublattices, nanotubes from the MLC of different compositions were described in the past. In particular, semimetallic nanotubes belonging to the family LnX-TaX₂ with Ln= rare-earth atom and X=S, Se, Te have been studied in the past. While some of them, like LaS-TaS₂ were obtained with moderately high yields, others like YbS-TaS₂ were scarce. In the present study, a new strategy for promoting the yield of such MLC nanotubes by alloying the LaS sublattice with another Ln atom is proposed. Detailed transmission electron microscopy investigation of the Ln_xLa_(1-x)S-TaS₂ (Ln= Pr, Sm, Ho, Yb) nanotubes show clearly that the substituting Ln atom resides in the rocksalt LaS sublattice of the nanotubes. Raman measurements show distinct differences between mixed tubes with open-shell (Pr, Sm, Ho) and closed-shell (La, Yb) rare-earth atoms. Density functional calculations show that the interplay between two important factors determine the enhanced stability of the mixed nanotubes- the size and electronic structure of the substituting rare-earth atom. The smaller is the substituting rare-earth atom (larger Z number), the more dissimilar it is to the original La atom. This dissimilarity enhances the incommensurability between the Ln_xLa_(1-x)S and the TS₂ subunits, promoting thereby the stability of the mixed MLC and nanotubes thereof. This strategy can be employed for enhancing the yield of these and other misfit nanotubes using different substituents of the right size and energy profile.