Study
of the orientation and frequency of occurrence of elementary steps on the (010)
cleavage face of potassium acid phthalate single crystals
Department
of Applied Physics, Lublin University of Technology,ul. Nadbystrzycka 38, 20-618
Lublin, Poland
Abstract
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 (010) 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 (010)
plane, and (3) some of the observed cleavage steps have orientations that correspond to low step energies.