Linear rate of the polymorphic transformations of sulphur

Abstract

Measurements of the linear rate of advance of the interface in the transformation of γ- to α-sulphur at low temperatures point to a close correspondence between the activation energy of the forward change and the heats of sublimation of the polymorphs, as has been found for other polymorphic transformations of molecular solids. Comparisons of the observed linear rates for the γ- to α-, β- to α-, α- to β-, and γ- to β-transformations of sulphur with those calculated for an evaporation-condensation mechanism show that all these transformations fall into following general pattern (allowing for the limitations of the range of supersaturation over which it has been possible to study each one) : (i) at very low supersaturations, i.e., near a transition point, there is near agreement between the observed and calculated values; (ii) at high supersaturations the observed rate is between 10³ and ca. 10⁴ times the calculated rate, and the ratio appears to become constant. A possible explanation of these results is discussed.