Topology of P-T-X phase diagrams of binary systems. Polymorphism, metastable states and high-pressure equilibria

Abstract

Abstract A step-by-step geometrical approach is presented to the P-T-X phase equilibrium in binary systems with different types of polymorphism and metastable states, and the relationship between these two phenomena. Mutual solubility of the components in all the states of aggregation is taken into account, as well as formation of crystalline non-stoichiometric compounds. Special attention is given to equilibria at high pressures because until now, these types of phase diagrams have not been adequately discussed. At first, P-T relations in onecomponent systems are analysed and afterwards the third co-ordinate (composition X) is introduced for binary systems. The phase states of the systems are examined by means of P-T and T-X projections of the corresponding equilibria along with isobaric T-X sections of the representative and most complicated regions. All possible types of phase diagrams of binary systems with both parallel and non-parallel polymorphism of the components are discussed. Complete and limited solid state solubility is considered. Metastable states of the binary systems as well as different crystallization routes of non-stoichiometric compounds ( stable and metastable) are examined in connection with the polymorphism of the components. Certain general trends of metastable crystallization characteristic of all types of the phase diagrams are discussed