Anomalous mechanical behavior of the deltic, squaric and croconic cyclic oxocarbon acids

Abstract

The structural and mechanical properties of the deltic, squaric and croconic cyclic oxocarbon acids were obtained using theoretical solid-state methods based in Density Functional Theory employing very demanding calculation parameters in order to yield realistic theoretical descriptions of these materials. The computed lattice parameters, bond distances, angles, and x-ray powder diffraction patterns of these materials were in excellent agreement with their experimental counterparts. The crystal structures of these materials were found to be mechanically stable since the calculated stiffness tensors satisfy the Born mechanical stability conditions. Furthermore, the values of the bulk modulus and their pressure derivatives, shear and Young moduli, Poisson ratio, ductility and hardness indices, as well as mechanical anisotropy measures of these materials were reported. A complete review of the literature concerning the negative Poisson ratio and negative linear compressibility phenomena is given together with the theoretical study of the mechanical behavior of cyclic oxocarbon acid materials. The deltic, squaric, and croconic acids in the solid state are highly anisotropic materials characterized by low hardness and relatively low bulk moduli. The three materials display small negative Poisson ratios. The croconic acid displays the phenomenon of negative linear compressibility for applied pressures larger than ∼0.4 GPa directed along the direction of minimum Poisson ratio and undergoes a pressure induced phase transition at applied pressures larger than ∼1.0 GPa.