Uzhgorod University

Uzhhorod, Ukraine

Uzhgorod University

Uzhhorod, Ukraine
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Oleaga A.,University of the Basque Country | Shvalya V.,University of the Basque Country | Shvalya V.,Uzhgorod University | Salazar A.,University of the Basque Country | And 2 more authors.
Journal of Alloys and Compounds | Year: 2017

A critical behavior study of Sn2P2(S1-xSex)6 has been undertaken for x = 0.28 (Lifshitz point) independently doped with Pb and Ge in search of the realization of a Tricritical Lifshitz point. The critical parameter α has been obtained from thermal diffusivity measurements performed with ac photopyroelectric calorimetry and its value compared with the different universality classes related to tricriticality and Lifshitz points. The evolution of the features of the experimental curves (position in temperature, shape, thermal hysteresis) has been compared with theoretical predictions related both to the stereoactivity of the metal cations in the studied samples and to the Blume-Emery-Griffith model with random field effects. A tricritical Lifhsitz point has been found for 5%Pb doping at TC = 259.12 K, for which the universality class Lifshitz Tricritical LT has been unambiguously assigned as the obtained critical exponent α = 0.64 is exactly the theoretical one. Higher dopant concentration (8%) induces a first order phase transition, further proving the presence of a tricritical point. The changes in the experimental curves agree with theoretical predictions. Impurity of Ge slightly increases the critical temperature from 281.31 K to 284 K due to the strengthening of the stereoactivity but changes neither the character of the transition nor the universality class, as it is still Lifshitz L, with critical exponent α = 0.25. © 2016 Elsevier B.V.

Oleaga A.,University of the Basque Country | Salazar A.,University of the Basque Country | Kohutych A.A.,Uzhgorod University | Vysochanskii Yu.M.,Uzhgorod University
Journal of Physics Condensed Matter | Year: 2011

The thermal diffusivity of the ferroelectric family Sn2P 2(SexS1-x)6 (0 ≤ x ≤ 1) has been measured by a high-resolution ac photopyroelectric technique, using single crystals, with the aim of studying the evolution of the ferroelectric transition with Se doping. Its change from second order character to first order while passing the Lifshitz point (x ≈ 0.28) has been evaluated, as well as the splitting of the transition at high Se concentrations. The critical behavior of the ferroelectric transition in terms of the different universality classes and their underlying physical dominant effects (tricriticality, long-range dipole interactions, Lifshitz point) has been discussed using thermal diffusivity measurements in the very close vicinity of the critical temperature. This study reveals that for Se concentrations around the Lifshitz point, long-range dipole interactions do not play a significant role and that the critical parameters are close to those predicted for the Lifshitz universality class. © 2011 IOP Publishing Ltd.

Salazar A.,University of the Basque Country | Oleaga A.,University of the Basque Country | Shvalya V.,University of the Basque Country | Shvalya V.,Uzhgorod University | Apinaniz E.,University of the Basque Country
International Journal of Thermal Sciences | Year: 2016

The photopyroelectric (PPE) technique in the front configuration consists of illuminating one surface of a pyroelectric slab while the other surface is in contact with the test sample. This method has been widely used to measure the thermal effusivity of liquids. However, in the case of solid samples the PPE signal is affected by the thin coupling fluid layer used to guarantee the thermal contact. Recently, a thickness scan of the coupling fluid layer has been proposed to extract the thermal effusivity of the solid. However, this method is delicate since it needs absolute values of the fluid thickness. In this work, we propose a classical frequency scan of the PPE signal in the front configuration, which only depends on two parameters: the effusivity of the solid and the thickness of the coupling fluid layer. It is demonstrated that both quantities are not correlated and that the sensitivity to the thermal effusivity of the solid is very high. PPE measurements performed on a set of calibrated solids covering a wide range of effusivity values (from polymers to metals) confirm the ability of the method to obtain the thermal effusivity of solids accurately. Moreover, this method is specially suited to measure the temperature dependence of the thermal effusivity across phase transitions. © 2015 Elsevier Masson SAS.

Kleemann W.,University of Duisburg - Essen | Shvartsman V.V.,University of Duisburg - Essen | Borisov P.,University of Duisburg - Essen | Borisov P.,University of Liverpool | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The lamellar antiferromagnet CuCr1-xInxP 2S6 (TN 32 K) rapidly loses magnetic percolation upon diamagnetically diluting the triangular Cu-Cr-P2 planar network with In3+ ions. For 0 ≤ x < 0.3 anti-ferroelectricity (Tc ≤ 150 K) and antiferromagnetic spin order (TN ≤ 32 K) coexist. Both orders are superposed by ferroic fluctuations. Dynamic polar clustering with glasslike polydispersive dielectric susceptibility emerges for x > 0 at T ≤ Tc, while for x < 0.3 pseudocritical planar 2D ferromagnetic fluctuations are encountered. They give rise to Langevin-type magnetization saturation and large quasimolecular magnetic anisotropy. Third-order magnetoelectric coupling is observed in collinear field configuration. © 2011 American Physical Society.

Kohutych A.,Uzhgorod University | Yevych R.,Uzhgorod University | Perechinskii S.,Uzhgorod University | Samulionis V.,Vilnius University | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The interaction between soft optic and acoustic phonons was investigated for Sn2 P2 (Se0.28 S0.72) 6 proper uniaxial ferroelectrics by Brillouin-scattering and ultrasonic pulse-echo techniques. The elastic softening of hypersound velocity of transverse-acoustic phonons and for both longitudinal and transverse ultrasound waves which propagate near direction of the modulation wave vector (in the incommensurate phase at x> xLP) was found at cooling to the Lifshitz point in the paraelectric phase. The strong increase in the ultrasound attenuation has also been observed. Such phenomena are related to the linear interaction of the soft optic and acoustic branches in the region of relatively short-range hypersound waves and to the strongly developed long-range order-parameter fluctuations in the ultrasound frequency range. The hypersound velocity temperature dependence was described within the Landau-Khalatnikov approximation for the ferroelectric phase. © 2010 The American Physical Society.

Morozovska A.N.,Institute of Semiconductor Physics of Ukraine | Eliseev E.A.,Institute of Semiconductor Physics of Ukraine | Eliseev E.A.,Ukrainian Academy of Sciences | Wang J.,Pennsylvania State University | And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We studied the phase diagram of thin ferroelectric films with incommensurate phases and semiconductor properties within the framework of Landau-Ginzburg-Devonshire theory. We performed both analytical calculations and phase-field modeling of the temperature and thickness dependencies of the period of incommensurate 180° -domain structures appeared in thin films covered with perfect electrodes. It is found that the transition temperature from the paraelectric into the incommensurate phase as well as the period of incommensurate domain structure strongly depend on the film thickness, depolarization field contribution, surface and gradient energy. The results may provide insight on the temperature dependence of domain structures in nanosized ferroics with inherent incommensurate phases. © 2010 The American Physical Society.

Yevych R.M.,Uzhgorod University | Vysochanskip Yu.M.,Uzhgorod University
Ferroelectrics | Year: 2011

A possibility of the existence of triple well potential for an order parameter of the second order phase transition in Sn2P 2S2 crystals is discussed. The statistical theory was applied for modelling system behaviour in such a case. The model parameters were estimated using available experimental data. The model can explain some peculiarities in temperature dependence ofheat capacity. Copyright © Taylor & Francis Group, LLC.

Macutkevic J.,Lithuanian Academy of Sciences | Banys J.,Vilnius University | Vysochanskii Y.,Uzhgorod University
Physica Status Solidi (B) Basic Research | Year: 2015

Investigations of the electrical conductivity of mixed CuInP2(SxSe1-x)6 crystals were carried out in the frequency range of 20-1MHz. Anomalies in dc conductivity were observed close to the ferroelectric phase-transition temperature. However, these anomalies are different in selenium- and sulfur-rich compounds. While in the selenium-rich crystals group the anomaly of conductivity is sharp, in the sulfur-rich crystals group it is less expressed. These results indicate that the copper ions contribution to the total conductivity is not vanishing down to the lowest temperatures in sulfur-rich crystals. Moreover, in paraelectric phase the dc conductivity activation energy correlate with the pre-exponential factor according to the Meyer-Neldel law. No conductivity anomaly was observed in mixed CuInP2(SxSe1-x)6 crystals with the dipolar glass or relaxor behavior. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Vysochanskii Yu.,Uzhgorod University | Glukhov K.,Uzhgorod University | Fedyo K.,Uzhgorod University | Yevych R.,Uzhgorod University
Ferroelectrics | Year: 2011

The first-principles calculations in LDA approach were performed for Sn2P2S6 ferroelectrics. The evolution of energy band structure and charge density distribution clearly illustrate the stereoactivity of the tin atoms lone pair electrons and their leading role in the phase transition nature which is described by three-well potential. Energy levels and charge density distribution were calculated for the tin vacancies acceptor states and for the sulfur vacancies donor states in relaxed structure of the paraelectric phase. Taking into account the named donors and acceptors, small electron polarons at Sn+ cations and small hole polarons at S- anions, the relationship of thermally induced lattice multistability with light induced optical bistability for Sn2P 2S6 ferroelectrics-semiconductors was established. Copyright © Taylor &Francis Group, LLC.

Vysochanskii Yu.,Uzhgorod University | Molnar A.,Uzhgorod University | Yevych R.,Uzhgorod University | Glukhov K.,Uzhgorod University | Medulych M.,Uzhgorod University
Ferroelectrics | Year: 2012

For Sn2P2S6 ferroelectric-semiconductor the relaxation processes at low temperatures in the ferroelectric phase were investigated by low frequency dielectric and Raman spectroscopies. Observed anomalies in dielectric losses and their change under illumination together with lattice anharmonicity appearance in Raman spectral bands shape are compared with previous data on photo- and thermoluminescence. The experimental data are analyzed with consideration of local three-well potential in ground state and explained by creation and annihilation of small hole and electron polarons with participation of tin vacancies acceptor and sulfur vacancies donor in gap energy states. Copyright © Taylor & Francis Group, LLC.

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