Polymorphism of solids under pressure

Abstract

The fundamental thermodynamic concepts involved in high pressure polymorphism of solids with reference to both the macroscopic and micro scopic aspects have been reviewed. A thermodynamic analysis involving a pressure invariable characteristic compressibility, has led to an expression for the high pressure transition pressure of solids entirely in terms of well known thermodynamic quantities. A further study of the concept of entropy, interpreted as the ‘degree of randomness’ of a system, indicates that a specific entropy change has to occur before transition from one stable phase to another can take place. Different structure calls for different values of entropy change. Structures of CN=6 involve a higher critical entropy change than those of CN=4. By correlating the radius ratio with the critical entropy change, the transition pressures of inorganic compounds which have the coordination number of four and six are predictable.