Dielectric singularities and phase transitions in molecular crystals

In crystals composed of sufficiently polarizable molecules, the low-frequency dielectric screening can be singular. A self-consistent dynamical treatment using vibrationally averaged polarizabilities shows how the singularity can produce Kohn anomalies, soft modes and phase transitions. Dielectric screening by mobile conduction electrons need not to be invoked. It is shown that the singularity is suppressed by a periodic lattice distortion commensurate with the original lattice.