AN ELECTRON-DIFFRACTION STUDY OF THE MISFIT LAYER COMPOUND (NDS)(N)NBS2

Misfit layer compounds of the type (NdS)(n)NbS2 have been made by iodi ne vapour transport. They have been characterised by energy dispersive X-ray analysis, transmission electron microscopy and electron diffrac tion. An orthorhombic form with Am2m symmetry has been observed. The u nit cell contains two layers of NdS and two layers of NbS2. The layers have rectangular symmetry and are stacked alternately with their plan es 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 MI and c=2.266 nm. The symmetry of the material observed in the present work is Am2m which di ffers from the Fm2m reported by others. It is argued that the lower sy mmetry occurs because of a displacement of Nb atoms in one of the laye rs in the unit cell. A comparison between calculated and observed elec tron diffraction patterns and energy arguments are used to support the proposed explanation. Copyright (C) 1996 Elsevier Science Ltd.