The stability of ionic crystal surfaces

When there is a dipole moment in the repeat unit perpendicular to the surface in an ionic crystal, lattice sums in the electrostatic energy diverge and the calculated surface energy is infinite. The cause of this divergence is demonstrated and the surfaces of any ionic or partly ionic material are classified into three types. Type 1 is neutral with equal numbers of anions and cations on each plane and type 2 is charged but there is no dipole moment perpendicular to the surface because of the symmetrical stacking sequence. Both these surfaces should have modest surface energies and may be stable with only limited relaxations of the ions in the surface region. The type 3 surface is charged and has a dipole moment in the repeat unit perpendicular to the surface. This surface can only be stabilised by substantial reconstruction. These conclusions are important for the analysis of the surface structure of ionic crystals.