Long period superstructures in Cu intercalated VS₂: Cu_0.75VS₂ ¹

In Cu_0.75VS₂, a phase transition exhibiting a large temperature "hysteresis" is found below room temperature. Electron diffraction and microscopy show that, below the transition temperature, a one-dimensional superstructure is formed, the crystal being fragmented into domains belonging to three orientation variants. A model for the superstructure within one variant is proposed in which the Cu atoms are assumed to occupy strips of the two families of tetrahedral interstices within the same Van der Waals gap. Within the strips, the room temperature ordered structure is formed. Adjacent strips are separated by a type of APB planes which, if regularly spaced give rise to the superstructure. This model seems to be confirmed by computer generated diffraction patterns. Although it is generally believed that the presence of incommensurate reflections in these compounds is associated with the formation of a deformation modulated structure, this work shows that the long period antiphase boundary model (LPAPB) provides an alternative explanation. Physical arguments in favor of the LPAP model are also given, although some questions still remain to be answered.