Sub-crystalline Changes of Structure Accompanying Thermal Transitions in Rochelle Salt, and in Potassium Dihydrogen Orthophosphate

Abstract

ACCORDING to the classical derivation of the Phase Rule, a transition point in a one-component system involves the co-existence of three independent phases, with the same thermodynamic potential of the component in three independent equations of state. For equilibria involving two solid and one vapour phase, the requirement that the phases shall be 'independent' can be given a definite interpretation in terms of crystal structure. For example, if we start with a single crystal of rhombic sulphur, and gradually heat it to above the transition point with monoclinic sulphur, the crystal may appear to preserve its external form, but X-ray photographs would show that actually it breaks up into a mass of smaller crystals of the monoclinic variety. These would give powder rings with the monoclinic spacings, insofar as they are independent of the original rhombic lattice. Corresponding with this rearrangement of the molecules to form a new crystal structure independent of the original arrangement, there are finite changes of volume and heat content at the transition temperature between the two crystal forms.