Pseudospin Hamiltonian parameters evaluated with regard to alkali metal ions in K3(H/D)(SO4)2 materials

Abstract

AbstractAn analysis of ferroelectric behavior of K3(H/D)(SO4)2 as members of the M3(H/D)(AO4)2 family is examined quantum chemically in the cluster approach using the structural models extended through the incorporation of K ions in an explicit form. We apply several structural models that describe the different types of (H/D)(SO4)2‐dimer surrounded by K ions, the geometry of the dimer being borrowed from the corrected diffraction data of the K3D(SO4)2 crystal in its ordered phase at the temperature below critical point Tc = 85 K. The calculations of the pseudospin Hamiltonian parameters—the Ising coupling parameters Jij and tunneling integrals Ω—were performed on the Hartree–Fock and second‐order Møller–Plesset perturbation theory levels with the 6‐31G–type basis sets. It is demonstrated that an effect of the K ions incorporation on the Jij values is minor (about several Kelvins). At the same time an influence of these ions on the double‐well potential characteristics (particularly, on the barrier heights) and thus on the tunneling integral value is more pronounced for both D‐ and H‐containing species. Despite the changes of the Hamiltonian parameters found, the major conclusion obtained earlier, referring to the suppression of the structural phase transition in H‐containing specimens due to proton quantum fluctuations, remains valid. The numerical estimations of the critical temperature Tc for completely deuterated materials that are obtained with and without regard to alkali metal ions are discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005