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1
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Behaviour of the Optical Birefringence in Conditions of the "Viscous" Interaction

100%
Sveleba S., Zhmurko V., Pankivskyi Y.
Acta Physica Polonica A
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2000
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vol. 97
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issue 2
337-348
EN
The temperature dependences of optical birefringence δ(Δn) for the principal cuts of [N(CH_{3})_{4}]_{2}CuCl_{4} crystals are measured. On the basis of the obtained experimental and theoretical data the anomalous behavior of physical parameters in conditions of the "viscous" interaction of the modulated structure with mobile defects in the incommensurate phase is explained.
2
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Characteristic Features of the Modulated Phases in (MgGeF_6)·6H_2O Crystals

100%
Pankivskyi Y., Sveleba S., Hrabanski R.
Acta Physica Polonica A
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2004
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vol. 106
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issue 4
467-474
EN
The temperature dependences of the optical birefringenceδ(Δ n_c) in (MgGeF_6)·6H_2O crystal have been investigated. Specific properties observed in experiment such as global hysteresis, kinetic effect, thermooptic memory effect, localization of the incommensurability wave vector on the commensurate values of higher order, sensibility to the external mechanical stress confirm the existence of the incommensurate phase in current crystals.
3
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Manifestation of a Coexistence of the Long-Periodic Phase with the Incommensurate Phase

100%
Sveleba S., Polovinko I., Zhmurko V., Pankivskyi Y.
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vol. 96
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issue 3-4
437-444
EN
Electric and optical properties of the commensurate ferroelectric long-periodic phase in [N(CH_{3})_{4}]_{2}ZnCl_{4} and [N(CH_{3})_{4}]_{2}CoCl_{4} crystals were investigated to explain mechanism of the double hysteresis loops appearance.
4
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Influence of Mechanical Stress on the Birefringence Properties of (N(CH_{3})_{4})_{2}ZnCl_{4} Crystals

100%
Polovinko I., Sveleba S., Vlokh O., Zhmurko V.
Acta Physica Polonica A
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1992
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vol. 81
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issue 3
409-412
EN
The influence of applied mechanical stress on the birefringence of (N(CH_{3})_{4})_{2}ZnCl_{4} crystal in the temperature region of its incommensurate phase was investigated using Senarmonth's method (λ = 633 nm). The value of pinning force for the samples with different defectiveness was experimentally determined.
5
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Investigations of High Temperature Phase Transitions in (NH_{3}c_{2}H_{5})_{2}CuCl_{4} Crystals

88%
Kapustianik V., Korchak Y., Sveleba S., Tchukvinskyi R., Girnyk I.
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vol. 95
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issue 3
351-357
EN
On the basis of dilatometric, optical, and dielectric investigations of (NH_{3}C_{2}H_{5})_{2}CuCl_{4} crystals the existence of phase transitions at T_{1}=364 and T_{2}=356 K was confirmed. Anomalous behaviour of the thermal expansion coefficient and optical birefringence around T_{3}=330 K was related to earlier unknown phase transition. Besides, it was shown that these crystals are characterised by considerable protonic conductivity, the nature of which is fairly well explained in the framework of Grotthus mechanism of proton transport.
6
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The Stochastic Mode of the Modulated Structure in [N(CH_3)_4]_2MeCl_4 Dielectric Crystals

88%
Sveleba S., Semotyuk O., Katerynchuk I., Furgala Y., Pankivskyi Y.
Acta Physica Polonica A
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2006
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vol. 109
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issue 6
695-700
EN
Temperature behavior of the optical birefringence in the transitional regions of the incommensurate phase of [N(CH_3)]CuCl_4 was studied. Temperature dependencies of the modulated structure wave vector and of the specific heat in these regions were analyzed. The stochastic mode of the incommensurate modulated structure in [N(CH_3)]MeCl_4 (Me = Cu, Fe) dielectric crystals was discovered.
7
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Phase Transitions in [N(CH_3)_4]_2ZnCl_4 and [N(CH_3)_4]_2CuCl_4 Crystals Related with the Incommensurate Modulation Existence

76%
Sveleba S., Semotyuk O., Katerynchuk I., Kunyo I., Karpa I., Pankivskyi Y.
Acta Physica Polonica A
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2008
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vol. 114
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issue 4
791-806
EN
The continuous phase transitions are observed in the crystals with incommensurate phase. They are transitions of parent-incommensurate phases (T_i); transitions between metastable states; incommensurate-commensurate phases (T_c). It was set that phase transition between parent and incommensurate phase is a continuous second-order phase transition with a critical index β = 0.5. The transition between metastable states is a continuous phase transition through the intermediate temperature region - incommensurate phase. The wave vector changes with the temperature here and wave vectorwe q* = q₁ - q₂ appears, where q₁, q₂ denote commensurate values of incommensurability wave vector of neighboring metastable states. It was shown that the phase transition between incommensurate and commensurate phases is a continuous phase transition.
8
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Optical and Ultrasonic Investigations of Phase Transitions in Ferroelectric RbHSeO_{4} Single Crystal

76%
Sveleba S., Bublik M., Polovinko I., Kityk A., Sopronuk V., Czapla Z.
Acta Physica Polonica A
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1992
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vol. 82
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issue 3
481-486
EN
Birefringence and ultrasonic measurements were done for ferroelectric RbHSeO4 crystal. Influences of both stresses (σ_{i}) and pressure on the properties and phase transition were studied as well. The observed dependence of δ(Δn_{b}) = f(σ_{5}) shows hysteresis loop characteristic of ferroelectric phase whereas the dependence of δ(Δn_{c}) = f(σ_{6}) shows hysteresis loop characteristic of ferroelastic phase. Birefringence measurements prove the phase transition to ferroelectric phase at 373 K. Additionally, the phase transition was found at 398 K. The new phase may be regarded as an incommensurate one. On the grounds of the stresses influence the phase diagram is constructed in coordinates T, σ_{5}. The linear decrease in T_{c} with increase in pressure is observed in ultrasonic studies and it is similar to the results obtained in DSC measurements.
9
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Dielectric and Birefringent Properties of [(CH_3)_2NH_2]_5Cd_2CuCl_{11} Solid Solutions in the Region of Their Phase Transitions

76%
Eliyashevskyy Y., Kapustianyk V., Dacko S., Czapla Z., Batiuk A., Sveleba S.
Acta Physica Polonica A
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2006
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vol. 109
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issue 2
171-178
EN
On the basis of the optical and dielectric investigations of [(CH_3)_2NH_2]_5Cd_2CuCl_{11} solid solutions the existence of phase transitions at T_1=175 K and T_2=117.5 K was confirmed. Both phase transitions were found to be shifted toward lower temperatures with respect to the corresponding transitions in the "host" [(CH_3)_2NH_2]_5Cd_3Cl_{11} crystals. It was found that the proton conductivity in [(CH_3)_2NH_2 ]_5Cd_2CuCl_{11} crystals is realized through the Grotthus mechanism. The investigations of the birefringent properties confirmed existence of the structural changes at T_0=320 K connected with the complex co-operative effect involving weakening of the hydrogen bonds and modification of the Jahn-Teller distortion with temperature.
10
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Birefringent and Dielectric Properties of [N(C_{2}H_{5})_{4}]_{2}ZnBr_{4} and [N(C_{2}H_{5})_{4}]_{2}CdBr_{4} Crystals

64%
Sveleba S., Mokryi V., Polovinko I., Kapustyanik V., Trybuła Z., Petrenko P., Kiosse G., Kravtsov V.
Acta Physica Polonica A
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1993
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vol. 83
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issue 6
777-784
EN
Dielectric and birefringent properties of [N(C_{2}H_{5})_{4}]_{2}ZnBr_{4} and [N(C_{2}H_{5})_{4}]_{2}CdBr_{4} crystals were investigated. It has been ascertained that two phase transitions manifest themselves in the temperature dependencies є(T) for [N(C_{2}H_{5})_{4}]_{2}CdBr_{4} crystal - a second-order transition at T_{1} = 311 K and a first-order one - at T_{2} = 39 K (∆T_{2} = 11 K). Besides two earlier known phase transitions in [N(C_{2}H_{5})_{4}]_{2}ZnBr_{4} at T_{1} = 350 K and T^{cool}_{2} = 282.8 K (∆T_{2} ≈ 2.3 K), the additional one was found at T*_{2} = 287.0 K, both on the cooling and heating runs. On the basis of the data about the peculiarities of dielectric and optical properties in the vicinity of T_{2} one can made the conclusion about the ferroelectric character of the phase, situated between T*_{2} and T^{cool}_{2}.
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