Synthesis and Electron Microscopy Study of the Quaternary Misfit Layer Chalcogenides {(Bi,Nd)S}1+δCrS2 and {(Pb,Nd)Se}1+δ(NbSe2)2.

In the present work, two compounds, in the Bi-Nd-Cr-S and Pb-Nd-Nb-Se systems, not reported to date were synthesized. The chemical composition and the structural determination of these complex compounds, at atomic resolution, was performed through conventional and aberration-corrected electron microscopy including selected area electron diffraction, high resolution (HR) transmission electron microscopy (TEM), HR scanning TEM, and the analytical associated techniques X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy. The average compositions are [(Bi0.4,Nd0.6)S]1.25CrS2 and [(Pb0.5,Nd0.5)Se]1.15(Nb1.0Se2)2, respectively. By using these electron microscopy techniques, we confirmed that both compounds can be described in term of two interpenetrated sublattices that fit along a but do not fit along b, giving rise to an incommensurate modulation. A closer inspection along the stacking direction of the subcell has provided an ideal structural model for [(Bi0.4,Nd0.6)S]1.25CrS2 based on the intergrowth of one layer of CrS2, three atoms thick, (111) B1 type, and one layer of (Bi, Nd)Se, two atoms thick, (100) B1 type. In [(Pb0.5,Nd0.5)Se]1.15(Nb1.0Se2)2 we found that two layers of NbSe2, which adopt the 2H-NbSe2 polytype, alternate with one layer of (Pb, Nd)Se B1 type. In addition, crystals showing extended defects, associated with the weak interaction between the layers, were frequently found.