Study of the orientation and frequency of occurrence of elementary steps on the (010) cleavage face of potassium acid phthalate single crystals

Experimental observations of an atomic force microscopy study of the orientation and frequency of occurrence of elementary steps of height 1.386 nm (equal to the lattice parameter b) produced on the freshly cleaved (0 1 0) surfaces of potassium acid phthalate crystals grown from aqueous solutions are described and discussed. It was found that most of the elementary cleavage steps are oriented along low-index crystallographic directions. The relative frequencies of the experimentally observed elementary steps of different orientations were compared with their calculated energies. Van der Waals interactions between neighbouring molecules and Madelung summation of Coulomb interactions between an ion and the surrounding ions were considered to calculate the total surface energy or the total energy of a step of a given orientation. It is shown that: (1) in the case of crystals like potassium acid phthalate containing both Coulomb and Van der Waals bonds, the calculated values of surface and step energies may be incorrect when they are considered purely ionic, (2) the relatively low value of the surface energy, mainly due to Van der Waals interactions, is responsible for the perfect cleavage along the (0 1 0) plane, and (3) some of the observed cleavage steps have orientations that correspond to low step energies. (C) 2004 Elsevier B.V. All rights reserved.