ORIENTATIONAL ORDERING IN MOLECULAR-CRYSTALS

The general phenomenon of orientational ordering in molecular crystals is reviewed. Some of the basic physical questions which arise in conn ection with orientational ordering in molecular crystals are enumerate d and experimental techniques used to address them are described. The description of orientational ordering in terms of order parameters and the resulting form of the Landau expansion is discussed for several t ypical cases. The relation to the modern theory of critical phenomena and notions of elementary excitations are also discussed. Analogous th eories of dynamics are briefly mentioned.