Crystallographic features related to a van der Waals coupling in the layered chalcogenide FePS3

We investigated the crystallographic structure of FePS3 with a layered structure using transmission electron microscopy and powder X-ray diffraction. We found that FePS3 forms a rotational twin structure with the common axis along the c*-axis. The high-resolution transmission electron microscopy images revealed that the twin boundaries were positioned at the van der Waals gaps between the layers. The narrow bands of dark contrast were observed in the bright-field transmission electron microscopy images below the antiferromagnetic transition temperature, T N ≈ 120 K. Low-temperature X-ray diffraction showed a lattice distortion; the a- and b-axes shortened and lengthened, respectively, as the temperature decreased below T N. We propose that the narrow bands of dark contrast observed in the bright-field transmission electron microscopy images are caused by the directional lattice distortion with respect to each micro-twin variant in the antiferromagnetic phase.