Lattice Dynamics and Structural Instabilities in Biphenyl Sulfone Derivatives

Abstract

Bis(4-chlorophenyl)sulfone (BClPS) is one of the few organic molecular solids which exhibits a displacive normal-incommensurate phase transition involving spatial modulations of conformational parameters. In the present work, the room temperature phase of BClPS was taken as a prototype, in order to investigate the structural stability of a group of related solids. Lattice dynamics calculations were performed in order to detect possible soft mode instabilities in the phonon dispersion branches. An atomistic semiempirical approach was adopted in lattice calculations, in addition to a semirigid molecular model considering the possibility of coupling between lattice and low lying internal degrees of freedom. Also, free-molecule equilibrium structure and dynamics calculations were performed using ab initio 3-21G^* methods, to evaluate the extent of conformational distortions of the molecule in solid phase. The contribution of the internal potential and the molecular packing on the lattice instability of BClPS and related derivatives are discussed. The results suggest that a possible new group of unstable molecular solids with a common prototype packing at high temperature could be identified, in a situation resembling the A₂BX₄ family of ionic solids.