434 research outputs found
Size effects in the thin films of order - disorder ferroelectrics subject to the depolarization field
The films of order-disorder type ferroelectrics were considered in the mean
field approximation taking into account depolarization field. It was shown that
size effects in this system could be described on the base of bulk system
equation of state with Curie temperature dependent on the film thickness. The
critical size hc and critical temperature Tc of phase transition from
ferroelectric to paraelectric phase was calculated allowing for the
depolarization field contribution. The comparison of the polarization
dependence on the film thickness, temperature and electric field for the films
of order-disorder and displacement type ferroelectrics is performed. In
particular it was shown that all the dipoles become ordered at T=0
independently on the film thickness for h>hc contrary to the displacement type
ferroelectrics. Critical thickness appeared larger and polarization
distribution sharper for the displacement type ferroelectrics than for
order-disorder type ferroelectrics.Comment: 7 pages, 4 figures, 1 tabl
Size effects and depolarization field influence on the phase diagrams of cylindrical ferroelectric nanoparticles
Ferroelectric nanoparticles of different shape and their nanocomposites are
actively studied in modern physics. Because of their applications in many
fields of nanotechnology, the size effects and the possible disappearance of
ferroelectricity at a critical particle volume attract a growing scientific
interest. In this paper we study the size effects of the cylindrical
nanoparticle phase diagrams allowing for effective surface tension and
depolarization field influence. The Euler-Lagrange equations were solved by
direct variational method. The approximate analytical expression for the
paraelectric-ferroelectric transition temperature dependence on nanoparticle
sizes, polarization gradient coefficient, extrapolation length, effective
surface tension and electrostriction coefficient was derived. It was shown that
the transition temperature could be higher than the one of the bulk material
for nanorods and nanowires in contrast to nanodisks, where the decrease takes
place. The critical sizes and volume of ferroelectric-paraelectric phase
transition are calculated. We proved that among all cylindrical shapes a
nanobar reveals the minimal critical volume. We predicted the enhancement of
ferroelectric properties in nanorods and nanowires. Obtained results explain
the observed ferroelectricity enhancement in nanorods and could be very useful
for elaboration of modern nanocomposites with perfect polar properties.Comment: 22 pages, 7 figures, 1 tabl
Peculiarities of Raman Spectra Shape in the Disordered Ferroelectrics
Theory of the first order Raman scattering (FOR) line shape allowing for
nonlinear and correlation effects contribution to inhomogeneous broadening as
well as dynamic mechanisms of homogeneous broadening is developed. It is shown
that in general case FOR line contains two maxima, shape of low frequency one
being defined by homogeneous broadening. Our theory explains the peculiarity of
observed FOR scattering of TO_2 hard phonon in KTL and KTN at different
temperatures and concentrations of Li and Nb ions.Comment: 9 pages, 6 figure
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