Theoretical studies of phase transitions in tetracyanoethylene

The equilibrium structure and harmonic lattice dynamics of monoclinic tetracyanoethylene under imposed symmetry-preserving strains are calculated from a 6-exp atom-atom potential function. Some lattice frequencies become imaginary for extensions and shears above ≈ 10%. Calculations of the eigenvalues of the dielectric function for the strained structures show the absence of dielectric singularities, but a dielectric mechanism for the observed transitions is not completely excluded.