New Order In (Bis)(1.19)(Bi1/3cr2s4) Misfit Layer Compound.

In a sample with nominal composition BiCr2S5 a new misfit layer compound has been synthesized and studied via transmission electron microscopy (TEM). It presents a composite modulated structure with a composition that can be formulated as (BiS)(1.19)[(CrS2)-Bi-1/3-(CrS2)]. It consists of the periodic stacking of a Q layer (BiS) with two H layers (CrS2), horizontal ellipsis QHHQHH horizontal ellipsis , with additional bismuth atoms inserted between the H layers. Selected area electron diffraction (SAED) patterns show an almost commensurate fit between the pseudo-tetragonal Q (BiS) and the pseudo-orthohexagonal H (Bi1/3Cr2S4) subcells along the misfit direction a, being b the same for both sublattices and c the layers stacking direction. In addition to the characteristic misfit modulation between the two sub-structures, a second modulation is observed in the H sub-structure produced by the ordering of bismuth atoms within the Van der Waals gap that occurs between consecutive H layers. High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) images show ordered stacking sequences between the (BiS) and (Bi1/3Cr2S4) layers. Besides, disordered intergrowths have been occasionally found as well as different kinds of twinning defects.