A ROLE OF MOLECULAR COMPRESSIBILITY IN TRANSFORMATION MECHANISM OF MIXED-STACK ORGANIC CHARGE-TRANSFER CRYSTALS

The neutral-ionic transformation in mixed-stack organic charge-transfer (CT) crystals has been analysed within a phenomenological theory. The theory uses two parameters: lattice distortion (the stack dimerization) which breaks inversion symmetry and molecular deformation (related to molecular ionicity) which preserves site symmetry. A coupling of totally symmetric molecular vibrations with a charge-transfer between donor and acceptor molecules allows to introduce the concept of molecular compressibility which includes both elastic and electronic contributions. The concept helps to understand discontinuity of the neutral-ionic transformation and can be usefull in the theory of solid state chemistry.