CO Physisorbed on Ionic Crystals: An Ising Model Study of Adsorbed Dipolar Molecules
The 2D dielectric phases and phase transitions of adsorbed dipolar molecules are studied using a dilute spin-one Ising model. The spin-one formulation assigns a spin Si=±1 to a (up/down) dipole occupying a lattice site I adsorbed perpendicular to the substrate surface and Si=0 to unoccupied sites. We relate the mean field expression for the spin-model interaction energy to a more detailed microscopic model involving dipolar, quadrupolar and repulsive interactions beyond nearest neighbor. Analytic solutions in the mean field approximation are discussed for dipole-orientation order–disorder transitions and ferroelectric-to-antiferroelectric transitions as a function of temperature and coverage. The model is applied to two prototypical dipolar physisorbed systems, CO on MgO(100) and CO on NaCl(100), using previous experimental and theoretical studies to determine the interaction energy parameters. We find for most of these model parameters that only temperature-dependent ferroelectric dipole order–disorder phase transitions can occur for these two systems. However, we find that phase transitions between the two ferroelectric states, driven by changes in coverage, should also be observed for some predicted values of the parameters. Phase diagrams are presented. Application of the model to other related adsorbed dipolar systems is also discussed.
T. E. Burns and JR Dennison, "CO Physisorbed on Ionic Crystals: An Ising Model Study of Adsorbed Dipolar Molecules," Surface Science, 395, 46-59 (1998)
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