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 {article}{empty}{document}√13{document} type (in TaS2 and TaSe2) and the a {article}{empty}{document}√19{document} type (in NbTe2 and TaTe2). Now a new superstructure of the same family in VSe2, with a cell edge of a {article}{empty}{document}√7{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 {article}{empty}{document}√7{document} is attributed to amplitude modulation of the CDW associated with the a {article}{empty}{document}√7{document} structure. A unified geometrical model explaining the formation of these various superstructures in the transition metal dichalcogenides is proposed.