2021-01-222021-01-222017-05-252041-17232041-1723http://dx.doi.org/10.34944/dspace/487628541270 (pubmed)http://hdl.handle.net/20.500.12613/4894© The Author(s) 2017. Materials with a negative Poisson's ratio, also known as auxetic materials, exhibit unusual and counterintuitive mechanical behaviour - becoming fatter in cross-section when stretched. Such behaviour is mostly attributed to some special re-entrant or hinged geometric structures regardless of the chemical composition and electronic structure of a material. Here, using first-principles calculations, we report a class of auxetic single-layer two-dimensional materials, namely, the 1T-type monolayer crystals of groups 6-7 transition-metal dichalcogenides, MX 2 (M=Mo, W, Tc, Re; X=S, Se, Te). These materials have a crystal structure distinct from all other known auxetic materials. They exhibit an intrinsic in-plane negative Poisson's ratio, which is dominated by electronic effects. We attribute the occurrence of such auxetic behaviour to the strong coupling between the chalcogen p orbitals and the intermetal t 2g -bonding orbitals within the basic triangular pyramid structure unit. The unusual auxetic behaviour in combination with other remarkable properties of monolayer two-dimensional materials could lead to novel multi-functionalities.15224-enCC BYhttp://creativecommons.org/licenses/by/4.0/cond-mat.mtrl-scicond-mat.mtrl-sciArticle2021-01-22