Anisotropy of lattice vibrations of layered compounds

The cohesive energy of layered ionic compounds consists of contributions of the Madelung energy, favouring symmetric coordinations, a repulsive energy, and a polarization energy stabilizing an asymmetric coordination of the polarizable anions. This explains the occurrence of layer-type structures for compounds with highly polarizable anions and small cations. The anisotropy of the dielectric constant and of the optical lattice vibrations of layered compounds is discussed in terms of a simple model with polarizable ions. The observed anisotropies are attributed to the presence of large static dipoles at the anions. Many layered materials show lattice distortions at low temperatures. The different types of distortions, such as charge density waves, metal clustering due to metal-metal bonding, and ferroelectric distortions, are discussed