(703c) Towards a Mechanistic Growth Model for Ionic Crystals

The relative surface areas and shapes of the various faces of ionic crystals significantly affect their functionality in areas like catalysis, photovoltaics, zeolites, etc. The layered growth rates of the crystal faces determine the steady-state shape of the crystal. For crystals grown from solution, the growth rate depends on the rates of attachment and detachment of solute units on to and from the crystal surfaces. These rates are determined by the solid-solid as well as the solid-solvent interactions. A mechanistic understanding of these interactions is instrumental in engineering the shapes of ionic crystals to achieve better functionality. The Periodic Bond Chain (PBC) theory has been used here to study the solid-state interactions in ionic crystals by identifying directions of strong bonding. The PBCs are parallel to the edges of the growth spirals and their identification provides insight into the spiral growth mechanism of crystals. Bond chain networks that are stoichiometric and do not have any out-of-plane dipole moment have been identified for anatase (TiO₂) and calcite (CaCO₃) crystals using the crystal structure as input. The electrostatic interactions in the PBCs were calculated by a simple columbic potential and the Madelung approach for a 1D ionic chain. The bond chain energies for all the PBCs in anatase have been calculated and the lattice energy has been accurately predicted. The kink and edge energies will be calculated for the spiral growth on these crystal faces to estimate the relative growth rates and eventually predict steady-state shapes of ionic crystals.