The majority of observed phase transformations nucleate heterogeneously and in solids the tendency is greater still. The solid-solid transformation is accompanied by deformations and distortions that substantially augment the activation energy for nucleation. However, phase contours, grain contours, dislocation etc, all have their own deformation effects that can be cancelled out in part by nucleation of a new phase. Along with this, atomic diffusion will happen more quickly along the grain contours than in the crystal and because of this reduce the energy barrier ∆G*. Consequently, nucleation of solids is invariably heterogeneous. Figure 2.1.14 is a typical case of heterogeneous nucleation in solids in which the new phase first appears in the contours in the grain. Figure 2.1.14 is an animation of the heterogeneous nucleation process of a particle in the contour of a grain of a phase. One of the start conditions for nucleation from a solid phase to another solid phase is the equilibrium of interfacial tension.
\(\gamma_{\alpha\alpha}\) and \(\gamma_{\alpha\beta}\).
Figure 2.1.14 – Nucleation and growth of a new phase on the grain boundaries.
Figure 2.1.15 – Animation of surface tension equilibrium \(\gamma_{\alpha\alpha}\) and \(\gamma_{\alpha\beta}\) in the process of heterogenous nucleation of a solid particle \(\beta\) on a grain boundary of a solid phase \(\alpha\).
This is because the molecules in the crystal can form links with those of the substrate solids stronger than solvate bonds. The process is observed, for example, in the dolomitization of biomineralized structures from calcium carbonate (like shells) during fossil formation on the seabed; it is a geochemical process where magnesium ions (Mg2+) substitute calcium ions (Ca2+) in crystalline polymorphs of calcium carbonate (CaCO3), creating dolomite (CaCO3.MgCO3). Because the enthalpy contribution of free energy comes principally from chemical bonds, stronger links provide smaller interfacial free energy. This could be the key physical phenomenon that permits organisms to control the location and orientation of crystals. The force of the interfacial bonds is dependent on the structure and chemical composition of the substrates involved. If the atomic structure of the surface of the solid substrate approaches a particular nucleating phase plane in a way that encourages preferential determined crystalline inheritance, nucleation will occur in a preferential crystal plane.
The format of observed crystals is that which minimizes total surface free energy ∆G. For non-spherical geometry, the faces that grow slowly turn out to be larger and those that grow quickly are smaller or disappear altogether. The faces that have less free energy are also those that grow more slowly.
