A new type of deformation modulated superstructure in 1T‐VSe₂ and its relation with other superstructures in transition metal dichalcogenides

A number of different commensurate deformation modulated superstructures related to charge density waves is known to occur in octahedrally coordinated transition metal dichalcogenides, in particular the a \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {13} $\end{document} type (in TaS₂ and TaSe₂) and the a \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {19} $\end{document} type (in NbTe₂ and TaTe₂). Now a new superstructure of the same family in VSe₂, with a cell edge of a \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {7} $\end{document}, is discovered. In the same material a further superstructure with unit cell edges 3a and an incommensurate modulated structure are observed. A pattern of weak spots due to a structure with a unit cell edge 2a \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {7} $\end{document} is attributed to amplitude modulation of the CDW associated with the a \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {7} $\end{document} structure. A unified geometrical model explaining the formation of these various superstructures in the transition metal dichalcogenides is proposed.