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S. A. Prosandeev

Publications and source records attributed to S. A. Prosandeev.

18 recordsLinked to original sources
arXiv2015

Room-temperature paramagnetoelectric effect in magnetoelectric multiferroics Pb(Fe1/2Nb1/2)O3 and its solid solution with PbTiO3

We have observed the magnetoelectric response at room temperature and above in high-resistive ceramics made of multiferroic Pb(Fe1/2Nb1/2)O3 (PFN) and PFN-based solid solution 0.91PFN-0.09PbTiO3 (PFN-PT). The value of the paramagnetoelectric (PME) coefficient shows a pronounced maximum near the ferroelectric-to-paraelectric phase transition temperature, T_C, and then decreases sharply to zero for T>T_C. The maximal PME coefficient in PFN is about 4x10(-18) s/A. The theoretical description of the PME effect, within the framework of a Landau theory of phase transitions allowing for realistic temperature dependences of spontaneous polarization, dielectric and magnetic susceptibilities, qualitatively reproduces well the temperature dependence of the PME coefficient. In particular, the Landau theory predicts the significant increase of the PME effect at low temperatures and near the temperature of the paraelectric-to-ferroelectric phase transition, since the PME coefficient is equal to the product of the spontaneous polarization, dielectric permittivity, square of magnetic susceptibility and the coefficient quantifying the strength of the biquadratic magnetoelectric coupling

cond-mat.mtrl-sci↗
arXiv2005

Bias field effect on the temperature anomalies of dielectric permittivity in PbMg1/3Nb2/3O3- PbTiO3 single crystals

In contrast to ordinary ferroelectrics where the temperature, Tm, of the permittivity maximum monotonically increases with bias field, E, in (1-x)PbMg1/3Nb2/3O3-(x)PbTiO3 (0<x<0.35) single crystals, Tm was found to remain constant or decrease with E up to a certain threshold field, Et, above which Tm starts increasing. Et decreases with x and almost disappears at about x=0.4. We explain this field dependence by the quenched random fields and coupling between the orientable dipoles with average polarization. For crystals with 0.06<x<0.13, the T-E phase diagrams are constructed, which include the relaxor, ferroelectric, glass and mixed ferroelectric-glass phases. The boundary between the ferroelectric and relaxor phases corresponds to a first-order phase transition and is characterized by thermal hysteresis with a large curvature of the line obtained in the field cooling mode, and by the presence of the end-point above which there is no difference between the ferroelectric and relaxor phases. The glass phase has a nearly field-independent boundary in the vicinity of the Vogel-Fulcher temperature TVF. The coupling between the glass order parameter and polarization changes the ferroelectric boundary at temperatures lower than TV

cond-mat.mtrl-sci↗
arXiv2005

The Significance of the Burns Temperature in Relaxor PMN

We suggest considering the Burns temperature in lead containing relaxor ferroelectrics as the temperature, at which Pb becomes off-center and nearly randomly occupies positions around the center on a sphere with the radius, which increases with the temperature decrease. The significance of the Burns temperature, in this approach, is the crossover between the soft-mode and order-disorder dynamics. The lattice parameter, birefringence and neutron scattering experiments are analyzed in the light of this model.

cond-mat.dis-nn↗
arXiv2005

Experimental and theoretical study of the influence of disorder on diffuse first-order phase transitions: NaNbO3: Gd and KTaO3: Li as examples

We consider consequences of local disorder in systems experiencing first order phase transitions. Such systems can be of rather different nature. For example, manganates showing gigantic magnetoelectric effect, doped antiferroelectrics or biomembranes. Monte-Carlo computations performed have shown that the disorder increases the temperature interval where the high- and lowtemperature phases coexist and this provides thermodynamics in the disordered systems distinct from the thermodynamics of classical homogeneous systems.

cond-mat.mtrl-sci↗
arXiv2004

Nontrivial dependence of dielectric stiffness and SHG on dc bias in relaxors and dipole glasses

Dielectric permittivity and Second Harmonic Generation (SHG) studies in the field-cooled mode show a linear dependence of dielectric stiffness (inverse dielectric permittivity) on dc bias in PMN-PT crystals and SHG intensity in KTaO$_{3}$:Li at small Li concentrations. We explain this unusual result in the framework of a theory of transverse, hydrodynamic-type, instability of local polarization.

cond-mat.mtrl-sci↗
arXiv2004

First-order Raman spectra of double perovskites AB$'{1/2}B$''{1/2}O3

First principles computations of Raman intensities were performed for perovskite-family compound CaAl$_{1/2}$Nb$_{1/2}$O$_3$ (CAN). This compound features 1:1 (NaCl-type) ordering of Al and Nb superimposed onto the $b^-b^-c+$ octahedral tilting. Raman tensor for CAN was computed using the package for first-principles computations ABINIT (URL \underline {http://www.abinit.org}). Computations performed for both untilted cubic ($Fm\bar{3}m$) and tilted monoclinic ($P2_1/n$) CAN structures showed that the strongest Raman lines are associated with the ordering of Al and Nb. The computed spectrum agreed qualitatively with the experimental data measured on powder (CAN is available in polycrystalline form only). The effect of cation disorder on the Raman intensities was considered using phenomenological theory of light scattering in the vicinity of a phase transition. We suggest that, for certain modes, the corresponding Raman intensities depend primarily on the average long range order while, for other modes, the intensities are determined by fluctuations of the order parameter.

cond-mat.mtrl-sci↗
arXiv2004

Lattice Dynamics in PMN

Lattice dynamics for five ordered PMN supercells were calculated from first principles by the frozen phonon method. Maximal symmetries of all supercells are reduced by structural instabilities. Lattice modes corresponding to these instabilities, equilibrium ionic positions, and infrared reflectivity spectra were computed for all supercells. Results are compared with our experimental data for a chemically disordered PMN single crystal.

cond-mat.mtrl-sci↗
arXiv2004

Diffuse first-order phase transition in NaNbO$_3$:Gd (a report)

Dielectric permittivity measurements show that (1-x)NaNbO3-(x)Gd1/3NbO3 crystals, at $x=0.09$, exhibit a first order phase transition near 150 C which is diffuse over a wide temperature interval. The area of the ε(T) thermal hysteresis loop was found to depend both on the annealing and cooling temperatures. Optical studies in polarized light show the decrease of the domains sizes at this phase transition. The results obtained are fitted assuming the space distribution of Curie temperatures.

cond-mat.mtrl-sci↗
arXiv2003

The Energetics of Li Off-Centering in K$_{1-x}$Li$_x$TaO$_3$; First Principles Calculations

K$_{1-x}$Li$_{x}$TaO$_3$ (KLT) solid solutions exhibit a variety of interesting physical phenomena related to large displacements of Li-ions from ideal perovskite A-site positions. First-principles calculations for KLT supercells were used to investigate these phenomena. Lattice dynamics calculations for KLT exhibit a Li off-centering instability. The energetics of Li-displacements for isolated Li-ions and for Li-Li pairs up to 4th neighbors were calculated. Interactions between nearest neighbor Li-ions, in a Li-Li pair, strongly favor ferroelectric alignment along the pair axis. Such Li-Li pairs can be considered "seeds" for polar nanoclusters in KLT. Electrostriction, local oxygen relaxation, coupling to the KT soft-mode, and interactions with neighboring Li-ions all enhance the polarization from Li off-centering. Calculated hopping barriers for isolated Li-ions and for nearest neighbor Li-Li pairs are in good agreement with Arrhenius fits to experimental dielectric data.

cond-mat.mtrl-sci↗
arXiv2003

Comment on ``Conduction states in oxide perovskites: Three manifestations of Ti$^{3 +}$ 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.

cond-mat.mtrl-sci↗
arXiv2003

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.

cond-mat.mtrl-sci↗
arXiv2003

Random-Site Cation Ordering and Dielectric Properties of PbMg1/3Nb2/3o3-PbSc1/2Nb1/2o3

(1-x)PbMg1/3Nb2/3O3-(x)PbSc1/2Nb1/2O3 (PMN-PSN) solid solution crystals have been grown by the flux method in the whole concentration range. X-ray supercell reflections due to B-cation ordering were observed for as-grown crystals from the 0.1 < x < 0.65 compositional range. Though the ordered domains are rather large (~50 nm) the relaxor-like dielectric behavior is observed for compositions with x < 0.6. The diffusion of the dielectric permittivity maximum in as-grown crystals is the lowest at x = 0.6 and increases towards the end members of solid solution. Such behavior is explained within a Bragg-Williams approach by employing the random layer model. At x ~ 0.6 the excitation energy determined from the Vogel-Fulcher relation exhibits a jump which we regard to changing the kind of the polar regions from PbMg1/3Nb2/3O3 to PbSc1/2Nb1/2O3 related type.

cond-mat.mtrl-sci↗
arXiv2003

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 PbFe$_{1/2}$Ta$_{1/2}$O$_3$ (PFT) and antiferroelectric NaNbO$_3$:Gd. We later discuss the coupling between soft mode and domain walls, soft mode and quasi-local vibration and resulting susceptibility function.

cond-mat.mtrl-sci↗
arXiv2002

On the average charge of the oxygen vacancy in perovskites necessary for kinetic calculations

An analytical result has been obtained for the value of the dynamical charge necessary for calculations of oxygen vacancy kinetics in dielectric perovskite-type crystals. It is shown by using the Berry phase analysis that this charge equals the nominal charge of the vacancy: for example, for the double charged state, it is 2; a neutral vacancy has the zero charge.

cond-mat.mtrl-sci↗
arXiv2002

High Dielectric Permittivity in AFe$_{1 / 2}$B$_{1 / 2}$O$_{3}$ Nonferroelectric Perovskite Ceramics (A - Ba, Sr, Ca; B - Nb, Ta, Sb)

AFe$_{1 / 2}$B$_{1 / 2}$O$_{3}$(A- Ba, Sr, Ca; B-Nb, Ta, Sb) ceramics were synthesized and temperature dependencies of the dielectric permittivity were measured at different frequencies. The experimental data obtained show very high values of the dielectric permittivity in a wide temperature interval that is inherent to so-called high-k materials. The analyses of these data establish a Maxwell-Wagner mechanism as a main source for the phenomenon observed.

cond-mat.mtrl-sci↗
arXiv2002

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.

cond-mat.mtrl-sci↗
arXiv2002

Comparative Dielectric Behavior of PbFe$_{1 / 2}$Ta$_{1 / 2}$O$_{3}$ and NaNbO$_{3}$:Gd Relaxor-Like Crystals

Experimental data obtained for PbFe$_{1 / 2}$Ta$_{1 / 2}$O$_{3}$ (PFT) and NaNbO$_{3}$: Gd (NaNbGd) single crystals show a diffused dielectric permittivity peak that is inherent to relaxor ferroelectrics. However some deviations from the normal relaxor properties were also observed and are under discussion. One of the key features of the relaxors is the existence of the Burns temperature, at which the polar regions appear. We found out that PFT shows this feature but the NaNbGd dielectric behavior is different. We analyse these properties within a phenomenological theory and propose a microscopic model.

cond-mat.mtrl-sci↗