Quantum-Chemical Modeling of Squaric Acid Ferroelectric Behavior

The properties of layered 2d-ferroelectric and at the same time 3d-antiferroelectric materials H(2)C(4)O(4)/D(2)C(4)O(4) are studied by quantum-chemical approach using pseudospin formalism in frames of Ising-type model Hamiltonian with tunneling terms. This Hamiltonian parameters (that are Omega - tunneling integral and J(ij)- Ising parameters) for both materials were obtained by means of RHF, MP2-MP4 and DFT/B3LYP calculations for different clusters which simulate characteristic fragments of materials crystalline structure. Resulting Omega and J(ij) values for crystals at hand were used in the limits of two mean field approximations that allow explaining main peculiarities of the observed order-disorder phase transition. The nature is discussed of the Bethe cluster lowest energy level splitting that has been introduced for correct description of this transition. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 2671-2676, 2011