Determination by Means of X-ray Absorption Spectroscopy and Transmission Electron Microscopy of the Local Structure of the 2D-Misfit Compound [Sr(Fe0.77Nb0.23)0.5S1.5]1.13NbS2

Abstract

The [Sr(Fe0.77Nb0.23)0.5S1.5]1.13NbS2 phase belongs to the 2D-misfit compounds family which are formed of two kinds of slabs labeled Q and H. The Q-part is a three-layer-thick slab, with the NaCl type structure, where external SrS planes enclose the inner (Fe,Nb)S plane. In a previous paper it was shown that there is a statistical distribution of the iron and sulfur atoms in the central layer of the Q-part. On the other hand, niobium atoms adopt an octahedral environment. To get a better description of this central layer, X-ray absorption and HREM studies were undertaken. EXAFS measurements clearly show that the environment of Fe atoms is less complex than that suggested from the structural determination. The existence of only one value for the Fe−S distance confirms the presence of [FeS4] tetrahedra as previously shown by the Mossbauer study. Both [NbS6] and [FeS4] polyhedra can arrange themselves in clusters. Electron diffraction and HREM studies reveal two new additional periodicities which can origina...