In this work we have proposed novel carbon allotropes with tetragonal symmetry: C 9 and C 12 based on square carbon units and leading to mixed arrangements of corner and edge C4 tetrahedra. The initial structures, which were subjected to unconstrained geometry optimization within density functional theory, resulted in cohesive stable phases. C 9 and C 12 were found to be mechanically and dynamically close to diamond and to have similar thermal properties. The electronic band structures show insulating behavior with band gaps close to 5 eV, like diamond.