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

Abstract

n 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.