This chapter focuses on the role played by the periodic bond chain (PBC) theory in predicting the structural morphology, in particular of ionic structures, illustrated by theoretical and experimental research, past and present. Case studies on ammonium dihydroxy-phosphate (NH4H2PO4) or ADP-type structures and fractal gel-grown ammonium chloride are treated in detail. Future prospects are outlined.
The ultimate goal in computational prediction of crystal morphology, when a crystal, molecular or ionic, grows according to a layer-by-layer mechanism, is twofold. First, to estimate the relative growth rates, that is, the ratios Rhikili/Rhjkjlj, for each Pair of faces (hikili) and (hjkjlj) on the growth form. Rhkl is the rate with which (hkl) grows in the normal direction. Second, to describe adequately and for all types of crystal structures, the roughening transition that the growth fronts can undergo; that is, the transition from the ordered, flat-face mode to the disordered, rough-face mode.
The PBC theory (Section 5.2) offers a realistic and well founded simplified description of the complex physical processes occurring in nature, and has in many cases enjoyed gratifying confirmation from experiment. The theory considers the actually observed morphology to be a superposition of: first, a basic generic morphology caused by internal or structural factors and called the theoretical or structural morphology; and second, a habit modification effect caused by external factors, notably the solid–fluid interaction, since in general the fluid will interact differently with the different faces.