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

The Stability of Ionic Crystal Surfaces. P.W.Tasker: Journal of Physics C, 1979, 12[22], 4977-84