Two-dimensional C6B4 and C8B4 monolayers: a new family of non-classical systems with planar tetracoordinate carbon atoms and highly anisotropic mechanical properties

Crystalline forms of two-dimensional C₆B₄ and C₈B₄ monolayers designed on the basis of diboraspiropentadiene C₃B₂H₄ structural blocks with planar tetracoordinate carbon atoms were studied using DFT calculations. The calculations predict the structural and dynamic stability of isomeric forms of C₆B₄ and C₈B₄ monolayers, which exhibit pronounced in-plane anisotropy of mechanical properties with high values of the mechanical anisotropy coefficient (up to 6.6) and maximum values of the Poisson’s ratio (up to 1.5). Isomers containing a continuous six-membered carbon ring within the structural block have negative Poisson’s ratios for some directions and can be characterized as anisotropic auxetics.