Anharmonic crystals

The study of anharmonic effects in crystals has received considerable attention in the last few years. As a result of this it has been shown that many properties can be understood by expanding the interatomic potential and keeping only the lowest-order terms in the anharmonicity. The theory of thermal expansion, elastic and dielectric properties, Raman and neutron scattering, and specific heat at low temperatures are all discussed from this point of view. The main lack in our understanding of these properties is uncertainty in the interatomic potentials, and some of the approximations used in calculations are discussed.