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1
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Dielectric and Dilatometric Properties of NH(CH_3)_3CuCl_3·2H_2O Low Dimensional Ferroics

100%
Korchak Y., Kapustianik V., Batiuk A., Girnyk I., Czapla Z., Dacko S.
Acta Physica Polonica A
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2004
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vol. 106
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issue 1
39-49
EN
On the basis of dilatometric and dielectric investigations of NH(CH_3)_3CuCl_3·2H_2O crystals the earlier unknown phase transition with considerable temperature hysteresis was found at T_1^c=198 K and T_1^h=223 K, respectively, in cooling and heating runs. Existence of the characteristic dielectric dispersion was revealed within the high-temperature phase. It corresponds to the single dielectric relaxator below 250 K as well as to co-existence of two relaxators above this temperature. The determined values of the activation energy and relaxation time are characteristic of the co-operative reorientation of the trimethylammonium cation (high-frequency relaxator) whereas the combined reorientation motions of the CuCl_2·2H_2O chains would be responsible for the low-frequency relaxation process. It was found that the above mentioned quasi-Debye type processes are involved into the mechanisms of the protonic conductivity.
2
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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.
Acta Physica Polonica A
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1999
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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.
3
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Investigations of High Temperature Phase Transitions in (NH_{3}c_{2}H_{5})_{2}CuCl_{4} Crystals

100%
Kapustianik V., Korchak Y., Sveleba S., Tchukvinskyi R., Girnyk I.
Acta Physica Polonica A
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1999
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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.
4
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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.
5
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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.
6
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On Temperature Dependence of Domain Structure in Cobalt

63%
Szmaja W., Polański K., Dolecki K.
Acta Physica Polonica A
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1997
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vol. 92
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issue 2
469-472
EN
The type-I magnetic contrast in a scanning electron microscope is used to study the domain behaviour on the basal planes of cobalt monocrystals of different thicknesses during a heating cycle. Digital image processing is applied to the original scanning electron microscope images for their restoration, enhancement and analysis. The main reasons for the application of digital image processing are: low level of type-I magnetic contrast, particularly used at the higher temperatures, and the complex character of the magnetic domain structure. The changes in both domain structures and type-I magnetic contrast are due to the strong temperature dependence of magnetocrystalline anisotropy energy for the hcp phase of cobalt, which implies the rotation of magnetic easy axis from the c-axis to the basal plane. The temperature of the magnetic phase transition between an open-flux and a closed-flux domain configuration was found to be dependent on the specimen thickness. The changes in domain structure during the heating cycle were reversible under the condition that the specimen was not carried through the hcp-to-fcc phase transition. Otherwise, they were partially or completely irreversible and were caused by crystal imperfections originating from the transition. The paper shows the great advantage of using digital image processing system for data restoration, enhancement and analysis.
7
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Thermal Study of the Phase Transitions in Potassium Hydrogen Sulphate KHSO_{4}

63%
Tahoon K. K., Abosehly A. M., Abdel-Kader M. M., El-Sharkawy A. A., El-Mansi F. M.
Acta Physica Polonica A
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1994
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vol. 86
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issue 3
349-355
EN
We report on some thermal properties (specific heat capacity C_{p}, thermal conductivity λ, and thermal diffusivity σ) in addition to the differential thermal analysis thermogram and thermogravimetric analysis for potassium hydrogen sulphate (KHS), KHSO_{4}. The accurate measurements of the above-mentioned parameters as functions of temperature revealed that the compound undergoes two structural phase transitions around 100°C and 175°C. It seems also likely that the compound behaves as a poorly conducting material and hence the maul mechanism of the heat transfer is due to phonons. The data are correlated to the crystal structure including the two different HSO¯_{4} ions.
8
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SnTe Phase Transition in Strained Superlattices PbTe/SnTe

63%
Mironov O. A., Makarovskii O. N., Nashchekina O. N., Shpakovskaya L. P., Litvinov V. I., Oszwałdowski M., Berus T.
Acta Physica Polonica A
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1995
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vol. 88
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issue 5
853-856
EN
An anomaly of the in-plane conductivity is observed in the superlattices PbTe/SnTe on (001)KCl in the temperature region of 60-130 K. The anomaly is caused by a structural phase transition in SnTe layer and as a result, the transition induced formation of defects. These defects are additional scattering centres which decrease the superlattice conductivity.
9
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Peculiarities of Dielectric Relaxation in DMAAS Ferroelectric Crystals

63%
Kapustianik V., Czapla Z., Czukwinski R.
Acta Physica Polonica A
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2002
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vol. 102
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issue 3
365-372
EN
This paper presents the results of investigations of the dielectric properties of NH_2(CH_3)_2Al(SO_4)_2×6H_2O crystals performed in a wide frequency range. In particular, the temperature and frequency dependences of the dielectric permittivity were measured in the region of ferroelectric phase at different rates of temperature change. The thermal dipole relaxation with a critical slowing-down at T_{c1} was observed at comparatively low frequencies of measuring field. The relaxation connected with the domain freezing takes place at still lower frequencies (below 100 kHz) and was found to be strongly dependent on the rate of the temperature change. It was shown that deuteration results in a noticeable change of the parameters characterising this process including the temperature of domain wall freezing.
10
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Evolution of Laser Light Induced Sodium Clusters

51%
Kolwas M., Kolwas K., Jakubczyk D.
Acta Physica Polonica A
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1994
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vol. 86
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issue 1
257-266
EN
We investigated the growing sodium clusters, induced by the laser light resonant with a sodium dimer transition. We observed the intensity of light scattered by clusters. The clusters were produced in the stainless steel cell containing sodium vapor and helium. We obtained data on cluster mean size and dynamics from fitting the theoretical formulas to the experimental data. The model of clusters' evolution basing on the thermodynamics of the vapor-liquid transition was applied.
11
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Monte Carlo Simulations of the Ashkin-Teller Model

51%
Dębski L.
Acta Physica Polonica A
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2000
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vol. 97
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issue 5
859-862
EN
Monte Carlo simulations are used to analyse the behaviour of the extended Ashkin-Teller model in two (2D) and three (3D) dimensions from the ratio of the square of the second moment of the order parameter to its fourth moment. The critical surface in three dimensional parameter space is calculated in regions where the Ising transitions are expected both for the ferro-and antiferromagnetic regions.
12
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Size Evolution of the Light Induced Sodium Clusters

51%
Hnat B., Jakubczyk D., Kolwas K., Kolwas M.
Acta Physica Polonica A
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1992
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vol. 81
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issue 6
629-637
EN
We report the observation of the size evolution of the laser light formed clusters. A cell containing sodium vapor and the buffer gas (He) under high pressure was used in the experiment. Changes of the radius and of the density of transient clusters were observed via the time evolution of the intensity of right-angle scattered light. The rate of the cluster radius growth was found to vary from 0.012 to 0.1 μm/s.
13
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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

51%
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}.
14
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Dielectric Study of Pressure Induced fcc-sc Phase Transition in Fullerene C_{60}

51%
Trybuła Z., Krupski M.
Acta Physica Polonica A
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2000
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vol. 98
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issue 4
389-392
EN
Phase transition in polycrystalline fullerene C_{60} from fcc phase to a simple cubic phase sc induced by hydrostatic pressure up to 0.5GPa was studied by dielectric method. Pressure coefficient dT_{c}/dp=144±8 K/GPa was determined. This value is consistent with pressure coefficients obtained with other experimental techniques.
15
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The Local Order in KTaO_3 Admixtured by the Ions of Li^+

45%
Kaszyńska K., Trybuła Z., Glinchuk M., Bykov I., Laguta V.
Acta Physica Polonica A
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2005
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vol. 108
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issue 2
379-383
EN
The low temperature behavior of the KTaO_3 type incipient ferroelectric crystals is of constant interest. The quantum fluctuations reduce the transition to the ferroelectric state in these crystals. It is possible that the small amount of the dopant Li^+ can lead, through the elastic interactions, to local glass-like short-range order or even to the relaxor ferroelectric order for x>2.6%. We presented that the low lithium doped x=0.005 KTaO_3 crystals exhibit the dielectric dispersion ofε andε", suggesting the glass-like behavior in the low temperature range.
16
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NMR study of heterogeneity in pyridine-N-oxide...HCl crystal

38%
Balevicius V., Gacesa A., Plavec J.
Open Physics
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2004
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vol. 2
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issue 1
120-131
EN
An origin of narrow 1H NMR signals in pyridine-N-oxide (PyO)...HCl crystal has been investigated by means of MAS, SPEDAS, NOESY and COSY techniques. Spectra of crystalline samples are compared with those of solid phase obtained from liquid PyO...HCl solutions (in acetonitile/H2O) after the heterogeneous phase separation. It has been concluded that partially resolved peaks in 1H NMR spectra of solids are related with heterogeneity of spin system and presence of different H-bond clusters of water molecules. NOESY spectra show no cross-peaks even at very long mixing time (500 ms). This indicates there is no exchange process between spins causing different peaks, and thus the corresponding molecular aggregates are captured in “islands of mobility8 without any channels sufficient for exchange. Appearance of MAS side bands as “pseudo8 cross-peaks in 2D NMR spectra using MAS/COSY technique is reported. In the case of accidental coincidence of spinning frequency (ωMAS) with spectral distances between some diagonal signals, intensive non-diagonal peaks are observed at the corresponding cross-positions. A misleading conclusion concerning spin coupling is easy to avoid using various ωMAS.
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