154 research outputs found
New, Lead Free, Perovskites With a Diffuse Phase Transition: NaNbO_{3} ~ Solid Solutions
Some of (1-x)NaNbO_3-(x)ABO_3 perovskite solid solutions exhibit a dramatic
diffusion of the dielectric permittivity epsilon' maximum and relaxor-type
behavior when the second component concentration exceeds a threshold value x_0.
The concentration phase transition to the relaxor-like phase is abrupt (of the
first order kind) that is seen from the step in the dependence of the
epsilon'(T) maximum temperature, T_m, on x. The precursor of this transition is
a giant (up to 100 K) temperature hysteresis of epsilon'(T). Some relaxor-like
properties appear even at x < x_0 in the course of cooling while disappear in
the course of heating. The experimental data obtained are qualitatively
described within a Landau-type phenomenological approach, assuming the
relaxor-type behavior to be local stress-induced.Comment: 5 figure
Dielectric Response in Microscopically Heterogeneous Dielectrics: Example of KTaO_3:Nb
New experimental data on solid solutions of quantum paraelectrics with
KTaO_3:Nb as an example are considered within a framework of a quantum theory
of ferroelectric phase transitions. In order to describe the effect of local
heterogeneities a percolation type theory together with a random field approach
were employed.Comment: 4 figure
First Principles Calculations of Ionic Vibrational Frequencies in PbMg1/3Nb2/3O3
Lattice dynamics for several ordered supercells with composition
PbMg1/3Nb2/3O (PMN) were calculated with first-principles frozen phonon
methods. Nominal symmetries of the supercells studied are reduced by lattice
instabilities. Lattice modes corresponding to these instabilities, equilibrium
ionic positions, and infrared (IR) reflectivity spectra are reported.Comment: 6 pages; Fundamental physics of Ferroelectrics 200
Diffusive phase transitions in ferroelectrics and antiferroelectrics
In this paper, we present a microscopic model for heterogeneous ferroelectric
and an order parameter for relaxor phase. We write a Landau theory based on
this model and its application to ferroelectric PbFeTaO
(PFT) and antiferroelectric NaNbO:Gd. We later discuss the coupling between
soft mode and domain walls, soft mode and quasi-local vibration and resulting
susceptibility function.Comment: 6 pages, Fundamental physics of ferroelectrics 200
Comment on ``Conduction states in oxide perovskites: Three manifestations of Ti Jahn-Teller polarons in barium titanate''
In this comment to [S. Lenjer, O. F. Schirmer, H. Hesse, and Th. W. Kool,
Phys. Rev. B {\bf 66}, 165106 (2002)] we discuss the electronic structure of
oxygen vacancies in perovskites. First principles computations are in favour of
rather deep levels in these vacancies, and Lenjer et al suggest that the
electrons' interaction energy is negative, but data on electroconductivity are
against.Comment: 2 pages, no figure
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