Dimensional Volume and Surface Characteristics Calculated for KDP-Type Crystals in the Lattice Gas Model with a Many-Particle Interaction Potential

An approach is formulated for calculating bulk and surface size characteristics of multidomain KDP-type crystals using the lattice gas model (LGM). Energy states of various proton configurations responsible for the development of spontaneous polarization of the crystal as the temperature is lowered are calculated nonempirically. With the use of statistical equivalence between orientations of pseudospins in KDP crystals and states of LGM site occupancy by the components of an A + B binary mixture, we derive equations describing size dependences of the critical ordering temperature, interfacial tension, and surface tension. Calculations are performed in the quasi-chemical approximation with allowance for direct correlations between neighboring A and B components. The critical ordering temperature, interfacial tension (IT), and surface tension (ST) are considered as functions of the linear size L of a cubic domain, when the contribution of the surface to thermodynamic functions of the substance becomes important. The size dependence of IT is calculated for a 180° orientation of neighboring phases. All three size dependences of the critical ordering temperature, IT, and ST in dimensionless form, normalized to the corresponding value for a macroscopic system, are shown to decrease with decreasing liner domain size L.