Phase transitions

Many solids undergo transformations from one crystal structure to another as the temperature or pressure is varied and this phenomenon is popularly referred to as polymorphism. Whereas polymorphism normally refers to phase transitions involving changes in the atomic configurations in crystals, there are also transitions where the electronic or spin configuration undergoes changes. The subject of phase transitions is not only of great academic interest but also of technological importance. Phase transitions are exhibited by a wide variety of systems (Table 4.1) from simple metals and alloys to complex inorganic and organic materials. The subject has grown enormously in the last two decades with new types of transitions as well as new approaches to explain the phenomena. Traditionally, the subject has been of vital concern to metallurgists (Porter & Easterling, 1981) but there are many aspects of great importance to solid state chemistry (Rao, 1984). Varied aspects of phase transitions such as critical phenomena, soft modes, mechanisms and changes in properties at phase transitions have been treated in a unified manner by Rao & Rao (1978). We shall deal with some highlights of the subject and examine some classes of transitions in this chapter.