Quantum-chemical modeling of squaric acid ferroelectric behavior

The properties of layered 2d-ferroelectric and at the same time 3d-antiferroelectric materials H2C4O4/D 2C4O4 are studied by quantum-chemical approach using pseudospin formalism in frames of Ising-type model Hamiltonian with tunneling terms. This Hamiltonian parameters (that are Ω - tunneling integral and Jij- 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 Ω and Jij 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. Copyright © 2010 Wiley Periodicals, Inc.