Studies of Thermal Reactions in the Solid State

Abstract

Abstract In surveying reactions of molecular crystals it is instructive to consider them in the context of two hypothetical mechanisms illustrated in Figure 1. The first type which may be designated the “homogeneous mechanism would involve conversion of “A” molecules of the starting material in the initial crystal lattice to “B” molecules, the B molecales remaining in place at the lattice sites previously occupied by the A molecules from which they were formed.1 A single crystal of A would thus in the process of conversion to B pass through an essentially continuous series of solid solutions, first of B in A and later of A in B. The reaction would be topotactic, that is, the crystal structure of the product single crystal would be related to that of the parent crystal from which it was formed. The second hypothetical mechanism, the “heterogeneous mechanism”, would involve initiation of reaction at a nucleation site (or sites) and spread of a reaction front (or fronts) through the crystal as A molecules were converted to B. There would be in this case no mixing of the A and B structures. The orientation of the lattice of the product crystal growing at the interface with the parent A crystal might be determined by the A lattice so that as with the homogeneous mechanism the reaction could be topotactic.