2010 Volume 59 Issue 2 Pages 117-125
Neutron and X-ray diffraction methods were complementary applied for the structure analyses of the hydrogen-bonded dielectric materials 5-R-9-hydroxyphenalenon(R=methyl group, bromine; MeHPLN, BrHPLN, respectively). The nuclear and electron density distributions(NDD and EDD)in the materials are derived from the neutron and X-ray data, respectively. The comparison of the NDD and EDD revealed that the centers of the NDD and EDD of the hydrogen atom in the hydrogen bond differ from each other. This indicates the existence of a local dipole moment in the hydrogen atom, of which magnitude is comparable to the one in the typical ferroelectric material BaTiO3. The dipole moment in the hydrogen atom is disordered at room temperature because of the disorder of the hydrogen atom. The antiferroelectricity in MeHPLN at low temperature was found to be due to the antiferroic ordering of the dipole moment accompanied by the ordering of the hydrogen atom. Meanwhile the hydrogen atom in BrHPLN does not exhibit the orderings. The absence of the ordering is indicative of the tunneling motion of the hydrogen atom.
The phase transition mechanisms of MeHPLN and BrHPLN were examined based on the tunneling model, using the experimentally obtained parameters. The calculation demonstrated that the phase diagrams of these materials can be well reproduced with this model.