Ab Initio Studies of H-Bonded Systems: The Cases of Ferroelectric KH2PO4 and Antiferroelectric NH4H2PO4

S. KOVAL; J. LASAVE; R. L. MIGONI; J. KOHANOF; N. S. DALAL

Ferroelectrics - Characterization and Modeling

Intech

Lugar: Shanghai, China; Año: 2011; p. 411 - 436

A wide variety of molecular compounds are bound by Hydrogen bridges between the molecular units. In these compounds cooperative proton tunneling along the bridges plays an important role. However, it is apparent that not only the proton behavior is relevant but also that of their associatedmatrix, leading to a wide range of possible behaviors. We are thus faced with the consideration of two in principle coupled subsystems: the proton tunneling subunit and the host lattice. Ubbelhode noted, in 1939, that the nature of the H-bond changes upon substitution of Deuterium (D) for H. In addition, many H-bonded compounds show structural transitions that are strongly affected by deuteration.  The common assumption that proton tunneling completely dominates the transitional physics, in a chemically and structurally unchanged host, is an oversimplifiedmodel. Since the 1980’s, a number of authors have noted in pressure studies that the changes in transition temperatures correlate well with the H-bond parameters. Thus, the proton’s (deuteron’s) dynamics and the host are mutually determined. The host-and-tunneling system is not separable, and in this chapter the physics of the proton-tunneling systems  is revisited with the help of ab-initio tools.