Local structure and defect chemistry of [(SnSe)1.15] (TaSe2) ferecrystals – A new type of layered intergrowth compound

Abstract The atomic structure of the family of ferecrystals [(SnSe)1.15]m(TaSe2) (m = 1, 3, and 6) was investigated by means of transmission electron microscopy. The tantalum in the TaSe2 layers was observed to have trigonal prismatic coordination similar to that found in the 2H polytype of bulk TaSe2. The structure of the SnSe constituent was found to be similar to that of orthorhombic α-SnSe. In the compounds with m = 1 and m = 3, regions with a local ordering of the layers along a commensurate axis, similar to the ordering in conventional misfit layer compounds, were observed. However, on a longer range the ferecrystals were found to exhibit a turbostratically disordered structure. Stacking defects were occasionally found in the samples in which a layer is interrupted and the surrounding layers are bent around these defects, while maintaining abrupt interfaces instead of interdiffusing. Volume defects were found in one sample of [(SnSe)1.15]1(TaSe2)1 in which a SnSe layer locally substitutes a part of a TaSe2 layer without interrupting the surrounding layers.