An electron diffraction study of the misfit layer compound (NdS)nNbS2

Misfit layer compounds of the type (NdS)_nNbS₂ have been made by iodine vapour transport. They have been characterised by energy dispersive X-ray analysis, transmission electron microscopy and electron diffraction. An orthorhombic form with Am2m symmetry has been observed. The unit cell contains two layers of NdS and two layers of NbS₂. The layers have rectangular symmetry and are stacked alternately with their planes perpendicular to the c-axis. A curious feature of the structure is that the layers have a common b unit cell parameter but the a values, which depend on the metal element in a given layer, are incommensurate. The lattice parameters for the material are a = 0.5625 nm for the NdS layer, a = 0.331 nm for the NbS₂ layer, b = 0.5733 nm and c = 2.266 nm. The symmetry of the material observed in the present work is Am2m which differs from the Fm2m reported by others. It is argued that the lower symmetry occurs because of a displacement of Nb atoms in one of the layers in the unit cell. A comparison between calculated and observed electron diffraction patterns and energy arguments are used to support the proposed explanation.