INVESTIGADORES
KOVAL Sergio Fabian
congresos y reuniones científicas
Título:
Phonons, tunneling, isotopic, geometric and pressure effects in KDP
Autor/es:
J. KOHANOFF; S. KOVAL; G. COLIZZI; J. LASAVE; R. MIGONI
Lugar:
Iguazú
Reunión:
Conferencia; 11th International Meeting on Ferroelectricity - IMF11; 2005
Resumen:
Abstract:By means of extensive first-principles calculations we studied the
ferroelectric phase transition and the associated isotope effect in KH2PO4
(KDP). Our calculations revealed that the spontaneous polarization of
the ferroelectric phase is due to electronic charge redistributions and
ionic displacements which are a consequence of proton ordering, and not
vice versa. The experimentally observed double-peaked proton
distribution in the paraelectric phase cannot be explained by a dynamics
of only protons. This requires, instead, collective displacements
within clusters that include also the heavier ions. These tunneling
clusters can explain the recent evidence of tunneling obtained from
Compton scattering measurements. The sole effect of mass change upon
deuteration is not sufficient to explain the huge isotope effect.
Instead, we find that structural modifications deeply connected with the
chemistry of the H bonds produce a feedback effect on tunneling that
strongly enhances the phenomenon. The resulting influence of the
geometric changes on the isotope effect agrees with experimental data
from neutron scattering. Calculations under pressure allowed us to
analyze the issue of universality in the disappearance of
ferroelectricity upon compression. Compressing DKDP so that the distance
between the two peaks in the deuteron distribution is the same as for
protons in KDP, corresponds to a modification of the underlying
double-well potential, which becomes 23 meV shallower. This energy
difference is what is required to modify the O-O distance in such a way
as to have the same distribution for protons and deuterons. At the high
pressures required experimentally, the above feedback mechanism is
crucial to explain the magnitude of the geometrical effect.